Battery Energy Density Calculator Guide: Watt-Hours, mAh, Specific Energy, Wh/kg, and Cell-vs-Pack Comparisons
Battery energy density describes how much electrical energy a battery can store relative to a physical constraint such as mass or volume. Two different metrics are commonly used and should be distinguished carefully.
The U.S. Department of Energy distinguishes specific, or gravimetric, energy from volumetric energy density. Gravimetric specific energy is stored or delivered energy per unit mass and is commonly expressed in watt-hours per kilogram, Wh/kg. Volumetric energy density is energy per unit volume and is commonly expressed in Wh/L.
The existing calculator implements the gravimetric form only. It accepts nominal voltage, rated capacity in milliampere-hours, and battery mass, calculates nominal stored energy in watt-hours, and then divides that energy by mass in kilograms.
Battery capacity in mAh is not itself an energy value. Milliampere-hours measure electric charge capacity. To estimate energy, the capacity in ampere-hours must be multiplied by an appropriate battery voltage.
For a simple nominal estimate, energy in watt-hours is E = V_nominal × Ah. If capacity is entered in mAh, divide by 1,000 first: E = V × mAh / 1,000.
For example, a 3.7 V cell rated at 2,500 mAh has nominal energy 3.7 × 2.5 = 9.25 Wh. If that cell weighs 45 g, or 0.045 kg, its nominal gravimetric specific energy is approximately 205.56 Wh/kg. This is the same example implemented by the current calculator.
The word nominal is important. Real battery voltage changes during charge and discharge, so multiplying rated capacity by one nominal voltage is an approximation to the energy available over an actual discharge cycle.
Engineering measurement of delivered battery energy is more rigorously obtained by integrating voltage multiplied by current over time during a defined discharge test. The result depends on discharge rate, temperature, cutoff voltage, state of health, cell chemistry, and test protocol.
DOE battery-performance programs therefore define gravimetric and volumetric energy density under specified test conditions rather than treating every nameplate voltage-capacity product as a directly comparable laboratory measurement. One ARPA-E battery definition, for example, specifies energy delivered per kilogram at a defined discharge rate and temperature.
The calculator should consequently describe its output as nominal specific energy rather than measured usable or delivered specific energy.
System boundary is equally important. Cell-level Wh/kg can be substantially higher than module- or pack-level Wh/kg because a pack adds enclosure, cooling equipment, busbars, wiring, sensors, control electronics, protection devices, structural elements, and other non-cell mass.
This means two values are comparable only when they refer to equivalent boundaries. A cell-level value should not be presented as directly equivalent to a complete vehicle battery-pack value.
Specific energy also does not describe how rapidly the battery can deliver energy. DOE separately distinguishes specific power in W/kg from specific energy in Wh/kg. Energy is analogous to how much energy is available, while power describes the rate at which energy can be delivered.
A battery can therefore have high specific energy but modest power capability, or high specific power but lower energy capacity. Runtime, acceleration performance, charge rate, thermal behavior, cycle life, safety, cost, and degradation require additional metrics.
This calculator is best used for nominal cell and pack comparisons, preliminary mass budgeting, educational calculations, and sanity checking. Detailed battery-system design should use measured discharge energy and test conditions appropriate to the intended application.
How to Calculate Battery Stored Energy and Gravimetric Energy Density
- Choose a consistent battery boundary: Decide whether the input represents a cell, module, or complete battery pack and use voltage, capacity, and mass from that same boundary.
- Enter nominal voltage: Use the manufacturer-rated nominal or representative voltage rather than maximum charge voltage unless a different calculation is explicitly intended.
- Enter rated capacity: Provide capacity in mAh as specified for the battery under its rated test conditions.
- Enter battery mass: Use the mass corresponding to the same cell, module, or pack whose voltage and capacity were entered.
- Calculate nominal stored energy: The calculator converts mAh to Ah and multiplies by nominal voltage to estimate Wh.
- Calculate gravimetric specific energy: The calculator converts mass from grams to kilograms and divides nominal Wh by kg.
- Interpret the result as nominal: Do not treat the result as measured usable discharge energy unless the input values were obtained from an appropriate measured-energy test.
- Compare like with like: Compare cell with cell or pack with pack under similar test and state-of-health conditions whenever possible.
Formula and variables
Capacity in milliampere-hours is first converted to ampere-hours. Multiplying nominal voltage by ampere-hours gives a nominal watt-hour estimate. Dividing that nominal energy by battery mass in kilograms gives nominal gravimetric specific energy in Wh/kg.
Nominal energy: E_Wh = V_nom × (mAh/1000); Gravimetric specific energy: E_g = E_Wh / (mass_g/1000)- E_Wh — Nominal stored energy
- Estimated battery energy based on nominal voltage and rated charge capacity, expressed in watt-hours.
- V_nom — Nominal voltage
- Representative rated battery voltage used for the simplified energy estimate.
- mAh — Rated charge capacity
- Battery charge capacity in milliampere-hours.
- Ah — Ampere-hour capacity
- Charge capacity after converting mAh to Ah by dividing by 1,000.
- mass_g — Battery mass
- Mass of the cell, module, or complete pack in grams.
- E_g — Gravimetric specific energy
- Nominal battery energy divided by mass, expressed in Wh/kg.
Scenario 1: 3.7 V, 2,500 mAh Lithium-Ion Cell Weighing 45 g
A battery cell has nominal voltage 3.7 V, rated charge capacity 2,500 mAh, and mass 45 g.
- Nominal voltage
- 3.7 V
- Rated capacity
- 2,500 mAh
- Mass
- 45 g
- Convert 2,500 mAh to ampere-hours.
- 2,500 mAh = 2.5 Ah.
- Nominal energy = 3.7 V × 2.5 Ah.
- Energy = 9.25 Wh.
- Convert mass: 45 g = 0.045 kg.
- Specific energy = 9.25 Wh / 0.045 kg.
- Specific energy ≈ 205.56 Wh/kg.
Result: Nominal stored energy = 9.25 Wh; nominal gravimetric specific energy ≈ 205.56 Wh/kg.
This is a nominal cell-level calculation. Actual delivered energy can differ because battery voltage changes during discharge and performance depends on current, temperature, aging, cutoff voltage, and other operating conditions.
Understanding your results
Nominal stored energy
This is the simplified voltage-times-capacity estimate in Wh.
It is not automatically equal to measured usable energy under a real discharge profile.
Gravimetric specific energy
This is nominal energy divided by battery mass.
A larger Wh/kg value means more nominal energy for a given mass under the same comparison boundary.
Cell-level result
A cell result includes only the mass represented by the entered cell.
It should not be compared uncritically with a full battery pack.
Pack-level result
A pack result should include enclosure, cooling, electrical interconnects, electronics, structural hardware, and other mass included in the actual pack boundary.
This generally lowers Wh/kg relative to the cells alone.
Usable energy
Actual usable energy depends on the voltage profile, permitted state-of-charge range, cutoff voltage, load, temperature, efficiency, and battery condition.
The calculator does not determine that value directly.
Assumptions
- Nominal voltage is a reasonable representative voltage for the simplified energy estimate.
- Rated mAh capacity corresponds to the battery and operating boundary being analyzed.
- Voltage, capacity, and mass refer to the same cell, module, or pack.
- Capacity is converted from mAh to Ah using 1 Ah = 1,000 mAh.
- Mass is converted from grams to kilograms using 1 kg = 1,000 g.
- The voltage-times-capacity product is treated as nominal stored energy.
- No discharge-curve integration is performed.
- No load-, temperature-, age-, or cutoff-specific derating is included.
- The battery mass is positive.
- Nominal voltage and rated capacity are nonnegative.
Limitations
- The calculation uses one nominal voltage rather than integrating the actual voltage-discharge curve.
- Actual delivered energy can differ from nominal V × Ah energy.
- Delivered capacity can depend on discharge rate, temperature, voltage limits, battery chemistry, state of charge, and state of health.
- The calculator does not calculate volumetric energy density in Wh/L.
- DOE distinguishes gravimetric specific energy in Wh/kg from volumetric energy density in Wh/L; they answer different design questions.
- The calculator does not calculate specific power or power density.
- It does not calculate peak current, continuous-current capability, C-rate, internal resistance, voltage sag, or thermal behavior.
- It does not calculate remaining runtime under a variable load.
- It does not calculate charging time.
- It does not account for battery-management-system reserve, state-of-charge windows, or depth-of-discharge restrictions.
- It does not account for inverter, converter, wiring, or other system losses.
- Cell-level and pack-level specific energy should not be compared without clearly stating the boundary.
- A nominal Wh/kg number alone does not describe cycle life, calendar life, safety, cost, charge acceptance, or power capability.
- Nameplate capacity and mass can differ from measured values and can change with battery aging.
Common mistakes
- Treating mAh as though it were watt-hours.
- Forgetting to multiply capacity by voltage.
- Using mAh directly without dividing by 1,000.
- Dividing Wh by grams instead of kilograms while labeling the result Wh/kg.
- Using maximum charge voltage instead of nominal voltage for a nominal energy estimate.
- Comparing cell-level Wh/kg with pack-level Wh/kg.
- Calling Wh/kg volumetric energy density.
- Confusing Wh/kg with W/kg.
- Assuming rated capacity is identical at every discharge rate and temperature.
- Treating nominal stored energy as guaranteed usable energy.
- Ignoring BMS reserve and cutoff voltage.
- Comparing batteries with different system boundaries or test conditions as though the values were directly equivalent.
Practical use cases
Scenario 2: Compare two cells by mass
Two cells provide similar nominal Wh but have different masses.
The lighter cell has higher nominal Wh/kg if the same comparison boundary is used.
Scenario 3: Portable electronics
A designer has a strict battery mass budget.
Specific energy helps estimate how much nominal energy can be carried for that mass.
Scenario 4: Cell versus pack
Calculate Wh/kg once from cells alone and again using complete pack mass.
The pack-level value will generally be lower because non-cell components add mass.
Scenario 5: Convert mAh to Wh
A 7.4 V battery rated at 5,000 mAh has nominal capacity 5 Ah.
Nominal energy = 7.4 × 5 = 37 Wh.
Scenario 6: Mass budgeting
A battery needs to provide 500 Wh nominal energy at a target 250 Wh/kg.
An idealized minimum battery mass implied by those two values is 2 kg before considering whether that specific energy is achievable at the desired system boundary.
Planning and decision guide
mAh measures charge capacity, not energy
Ampere-hours describe electric charge transferred over time.
Energy also depends on the voltage through which that charge moves.
One ampere-hour equals 3,600 coulombs
An ampere is one coulomb per second.
Therefore 1 Ah represents 3,600 C of electric charge.
Voltage converts charge capacity into an energy estimate
Electrical energy can be expressed as voltage multiplied by charge.
In battery rating units, V × Ah gives Wh.
Scenario 7: Equal mAh, different voltage
A 2,000 mAh battery at 3.7 V has nominal energy 7.4 Wh.
A 2,000 mAh battery at 7.4 V has nominal energy 14.8 Wh.
This is why comparing batteries by mAh alone can be misleading
Capacity in mAh does not incorporate voltage.
Wh is the more appropriate simple energy comparison when voltage differs.
Convert mAh to Ah before multiplying
Ah = mAh/1,000.
The conversion is exact by unit definition.
Scenario 8: 4,500 mAh
4,500 mAh = 4.5 Ah.
At 11.1 V, nominal energy = 49.95 Wh.
A watt-hour is a unit of energy
One watt-hour represents one watt delivered for one hour.
It is not a unit of power.
Power and energy answer different questions
Energy describes how much can be delivered in total.
Power describes how quickly it can be delivered.
DOE explicitly separates specific energy and specific power
Specific energy is commonly expressed in Wh/kg.
Specific power is commonly expressed in W/kg.
Scenario 9: Same Wh/kg, different power capability
Two battery technologies can store the same energy per kilogram.
One may nevertheless support much higher discharge power because internal resistance and cell design differ.
Gravimetric energy density is often called specific energy
DOE uses “specific, or gravimetric, energy” for energy per unit mass.
Wh/kg is therefore more precisely described as specific energy, although “gravimetric energy density” is widely used.
Volumetric energy density uses Wh/L
This metric asks how much battery energy fits into a given volume.
DOE defines it separately from Wh/kg.
Scenario 10: Light versus compact
Battery A can have better Wh/kg while Battery B has better Wh/L.
The preferred design depends on whether mass or available volume is more restrictive.
The existing calculator should not add Wh/L without a volume input
Its current inputs contain mass but not battery volume.
Volumetric energy density should therefore remain a future separate mode or require an explicit volume field.
Nominal battery voltage is a representative value
Electrochemical cell voltage varies over state of charge.
A single nominal value approximates that changing voltage for rating and comparison purposes.
Maximum charge voltage is not nominal voltage
A battery can briefly reach a higher charging voltage than its nominal rating.
Multiplying capacity by that maximum voltage overstates the ordinary nominal energy estimate.
Scenario 11: Lithium-ion voltage confusion
A cell may be labeled around 3.6 or 3.7 V nominal while reaching a higher voltage when fully charged.
Use the manufacturer’s nominal voltage for the simple nameplate-energy calculation.
Actual energy requires integrating voltage over discharge
Instantaneous electrical power is P = VI.
Delivered energy is the time integral of power.
General measured-energy relationship
E = ∫V(t)I(t)dt.
For constant current this can also be viewed as integrating voltage over delivered charge.
Nominal V × Ah approximates this integral
It replaces the changing discharge voltage by one representative nominal value.
That simplification explains both its convenience and its limitation.
Scenario 12: Flat versus sloped discharge curves
Two cells can have identical nominal voltage and Ah ratings but different voltage curves under load.
Their measured delivered Wh can therefore differ.
Discharge rate affects measured performance
Battery losses and electrochemical limitations can reduce delivered capacity or voltage at higher current.
DOE performance definitions often specify the discharge rate when quoting measured energy density.
C-rate expresses current relative to battery capacity
A 1C discharge nominally corresponds to discharging rated capacity in approximately one hour.
Actual energy behavior still depends on the cell and test conditions.
The current calculator does not need C-rate to calculate nameplate Wh/kg
Its calculation is purely nominal.
C-rate becomes relevant when moving from rating arithmetic to measured performance.
Temperature influences usable battery energy
Electrochemical kinetics, internal resistance, and accessible capacity change with temperature.
A nominal rating cannot encode every operating temperature.
Scenario 13: Cold operation
A battery can have the same nameplate V and mAh rating at room temperature.
Its usable energy under cold high-load operation can nevertheless be lower.
Battery aging also changes usable energy
Capacity generally changes as a rechargeable battery ages.
Using original rated mAh on an aged battery can overestimate current stored or deliverable energy.
State of health matters
A battery at 80% of original capacity no longer delivers the same Ah under equivalent conditions.
Its effective specific energy can therefore decline if mass remains essentially unchanged.
Scenario 14: Capacity fade
A 10 Ah pack declines to 8 Ah while mass remains unchanged.
At the same representative voltage, nominal effective energy based on measured current capacity falls by 20%.
Usable state-of-charge window can reduce operational energy
Battery-management systems may prevent operation at the electrochemical extremes.
The full nominal Wh value may therefore be larger than routinely accessible energy.
Scenario 15: 80% usable window
A nominal 100 kWh pack restricted to an 80% usable window provides at most about 80 kWh before other losses under that simplified assumption.
The exact system behavior depends on manufacturer controls and operating conditions.
Do not build a universal derating percentage into this calculator
Usable fraction differs by chemistry, BMS configuration, temperature, aging, and application.
The existing calculator appropriately reports nominal energy instead.
Cell versus pack boundary strongly affects Wh/kg
DOE energy-density discussions distinguish battery-system mass and volume because packaging and support components matter in real applications.
The current calculator already advises users to compare like with like.
Scenario 16: Cell-to-pack penalty
Cells provide 10 kWh and weigh 40 kg, giving 250 Wh/kg at cell aggregate level.
If the complete pack weighs 55 kg, pack-level nominal specific energy becomes about 181.8 Wh/kg.
The energy did not disappear
The denominator increased because support hardware was added.
This is why system boundary must accompany every specific-energy claim.
Pack mass includes more than cells
Examples include enclosure, wiring, busbars, thermal management, contactors, sensing, protection, and battery-management electronics.
Structural integration can add additional mass.
Module-level values create another boundary
A module can sit between individual cells and the complete pack.
Its Wh/kg should not be labeled simply as “battery energy density” without boundary context.
The calculator should display the selected boundary
A simple optional label such as Cell, Module, or Pack would improve result interpretation.
It need not alter the mathematics.
Recommended result wording
Nominal cell specific energy: 205.6 Wh/kg.
or Nominal pack specific energy: 165.2 Wh/kg.
Battery chemistry cannot be inferred from Wh/kg alone
Different chemistries overlap in practical performance ranges.
Specific energy is one characteristic, not a chemical identifier.
DOE comparisons show chemistry ranges rather than one universal value
Battery technologies occupy ranges of gravimetric and volumetric energy density rather than fixed single values.
Cell format and engineering implementation also matter.
Avoid embedding stale chemistry benchmarks in the calculator
Commercial cell performance changes over time and depends on cell type.
The tool should calculate from entered data rather than declare a chemistry good or bad based on a fixed historical range.
Power density should remain a different metric
Wh/kg describes energy capacity relative to mass.
W/kg describes power output relative to mass.
Scenario 17: EV design
Higher Wh/kg can help reduce battery mass for a target energy capacity.
Acceleration and fast-power delivery depend additionally on power capability.
Runtime cannot be determined from Wh alone without load
A nominal 100 Wh battery does not imply a fixed operating time.
Runtime depends on the load power and how battery voltage and capacity respond during use.
Simple constant-load runtime estimate
Idealized runtime ≈ usable Wh / load W.
The current calculator intentionally does not perform this because usable energy and load behavior require additional assumptions.
Scenario 18: Why nominal runtime can overstate reality
A 100 Wh nominal battery powering a 20 W load might suggest 5 hours ideally.
Real runtime can differ because of usable-energy limits, conversion efficiency, temperature, and load response.
Energy efficiency is also separate
Charging 100 Wh into a battery does not necessarily make 100 Wh available at the output later.
Round-trip efficiency and conversion losses require separate measurements.
Battery voltage should match the capacity definition
Pack capacity and cell capacity behave differently under series and parallel connections.
Use the voltage and Ah rating corresponding to the assembled boundary being evaluated.
Series connection increases voltage
For identical cells connected ideally in series, pack voltage adds while Ah capacity remains the same.
Total Wh therefore adds across the series string.
Scenario 19: Two cells in series
Each cell is 3.7 V and 2.5 Ah.
Ideal series pack is 7.4 V and 2.5 Ah.
Energy = 18.5 Wh, twice one cell’s 9.25 Wh.
Parallel connection increases Ah capacity
For identical cells connected ideally in parallel, voltage remains the same while Ah capacity adds.
Total Wh again adds.
Scenario 20: Two cells in parallel
Each cell is 3.7 V and 2.5 Ah.
Ideal parallel pack is 3.7 V and 5 Ah.
Energy = 18.5 Wh.
Ideal series and parallel arrangements preserve summed cell energy
The voltage-capacity split changes.
The total ideal V×Ah energy is the sum of the component-cell energies.
Real packs add non-cell mass
Even if electrical energy sums ideally, Wh/kg decreases when pack support mass is included.
This is another reason cell and pack specific energy differ.
The current calculator does not need series/parallel topology
If the assembled pack nominal voltage and mAh rating are already known, those values can be entered directly.
A future pack-builder calculator could derive them from cell configuration.
Mass must use the same boundary as energy
Do not enter full pack voltage and capacity while entering only cell mass.
That artificially inflates Wh/kg.
Scenario 21: Boundary mismatch
Pack energy = 1,000 Wh.
Using only 3 kg of cell mass instead of the actual 5 kg complete pack mass reports 333 Wh/kg instead of the correct pack-level 200 Wh/kg.
This boundary mismatch is one of the most important errors to prevent
The arithmetic is internally correct but scientifically misleading.
A boundary selector or warning can prevent it.
The numerator can also use the wrong boundary
Cell rated energy should not be divided by pack mass unless the intended metric is explicitly defined that way.
Use matched numerator and denominator boundaries.
Wh/kg is a ratio and should be interpreted accordingly
High Wh/kg can result from greater stored energy, lower mass, or both.
It does not directly reveal which design change created the improvement.
Scenario 22: Same energy, different mass
Two batteries each store 100 Wh nominally.
One weighs 0.5 kg and the other 1 kg.
Specific energies are 200 and 100 Wh/kg respectively.
Scenario 23: Same mass, different energy
Two 1 kg batteries store 100 Wh and 200 Wh.
Specific energies are 100 and 200 Wh/kg.
Specific energy can estimate mass for a target energy
Rearrange m = E / specific energy.
This is useful for preliminary mass budgeting.
Scenario 24: Target 5 kWh at 250 Wh/kg
5,000 Wh / 250 Wh/kg = 20 kg.
This is an idealized mass at the same system boundary represented by the specific-energy figure.
Do not use cell Wh/kg to predict pack mass directly
Pack overhead reduces system-level specific energy.
Use pack-level data for pack-level mass estimation.
Volumetric calculation would require battery volume
Volumetric energy density = Wh / L.
Without volume input, it cannot be derived from V, mAh, and mass alone.
A future mode could add volume
Then the same nominal Wh numerator could produce Wh/L.
The UI should clearly distinguish “Specific Energy — Wh/kg” from “Volumetric Energy Density — Wh/L.”
DOE uses both metrics because different applications face different constraints
Mass can dominate aircraft or vehicle design.
Volume can dominate tightly packaged electronics or vehicle architectures.
The calculator title can remain Battery Energy Density
That is the common search phrase.
But the result itself should say “Gravimetric specific energy (Wh/kg)” for scientific precision.
The current page already uses both terms
It labels the result gravimetric energy density and explains that it is often called specific energy.
The expanded content should preserve that clarification.
Do not use a universal “good Wh/kg” threshold
Acceptable performance depends on chemistry, safety requirements, power demand, cycle life, packaging, cost, and application.
Battery technology also evolves over time.
Historical DOE data are useful context, not grading thresholds
DOE has documented large improvements in lithium-ion volumetric pack energy density over time.
Those historical values should not be embedded as permanent pass/fail criteria.
Battery energy is chemical energy converted through electrochemistry
DOE explains that rechargeable batteries store energy in chemical potential and exchange electrons and ions during charge and discharge.
The electrical Wh rating describes the external electrical-energy scale, not the complete microscopic chemical-energy inventory.
The calculator should support scientific notation for extreme values
Small laboratory cells and very large battery systems span many orders of magnitude.
Formatting should not lose significant information.
Input validation should reject zero mass
Specific energy divides by mass.
A zero-mass input would produce an undefined result.
Negative voltage, capacity, or mass are not ordinary battery-rating inputs
Direction and polarity are relevant electrically, but nameplate stored-energy magnitude should use the intended positive rated values.
The calculator should validate the physical-rating context.
Round only after unit conversion and division
Use full internal precision for Ah, Wh, kg, and Wh/kg.
Round the displayed result according to useful engineering precision.
Scenario 25: Avoid intermediate rounding
Converting 45 g to 0.05 kg instead of 0.045 kg before division would materially distort the specific-energy result.
Keep exact converted values internally.
The strongest result should show the complete unit chain
2,500 mAh → 2.5 Ah.
3.7 V × 2.5 Ah → 9.25 Wh.
45 g → 0.045 kg.
9.25 Wh / 0.045 kg → 205.56 Wh/kg.
This makes mAh-to-Wh errors visible immediately
Many users know battery capacity only in mAh.
Showing the intermediate Ah and Wh values improves both education and auditability.
The strongest result also states the system boundary
Nominal cell specific energy.
Nominal module specific energy.
or Nominal pack specific energy.
The final warning should remain concise
Nominal estimate only.
Usable energy varies with load, temperature, aging, cutoff voltage, discharge profile, and system losses.
Frequently asked questions
What does this Battery Energy Density Calculator calculate?
It calculates nominal stored energy in Wh and gravimetric specific energy in Wh/kg from nominal voltage, rated mAh capacity, and battery mass.
What is battery specific energy?
DOE defines specific or gravimetric energy as stored energy per unit mass, commonly expressed in Wh/kg.
What is battery energy density?
The phrase can refer broadly to energy relative to mass or volume, but DOE distinguishes gravimetric specific energy in Wh/kg from volumetric energy density in Wh/L.
What formula does this calculator use?
Nominal Wh = V × mAh / 1,000, then specific energy = Wh / mass in kilograms.
How do I convert mAh to Wh?
Multiply mAh by nominal voltage and divide by 1,000.
Is mAh the same as Wh?
No. mAh measures charge capacity, while Wh measures energy.
Why is voltage needed to convert mAh to Wh?
Electrical energy depends on both charge and voltage. Batteries with the same mAh capacity can store different nominal energy if their voltages differ.
What is 2,500 mAh at 3.7 V in Wh?
3.7 × 2,500 / 1,000 = 9.25 Wh.
How do I calculate Wh/kg?
Divide battery energy in Wh by battery mass in kilograms.
Why must grams be converted to kilograms?
Wh/kg uses kilograms in the denominator. Divide grams by 1,000 before calculating.
What is 9.25 Wh in a 45 g battery in Wh/kg?
45 g = 0.045 kg, so 9.25/0.045 ≈ 205.56 Wh/kg.
Is Wh/kg the same as Wh/L?
No. Wh/kg is gravimetric specific energy; Wh/L is volumetric energy density.
Does this calculator calculate Wh/L?
No. The current calculator has no battery-volume input and calculates gravimetric Wh/kg only.
What is the difference between energy density and power density?
Energy density or specific energy describes how much energy is available relative to mass or volume. Power density describes how quickly energy can be delivered relative to mass or volume. DOE treats these as separate performance metrics.
Is Wh/kg the same as W/kg?
No. Wh/kg is specific energy; W/kg is specific power.
Should I use nominal voltage or maximum charge voltage?
For this nominal energy estimate, use the manufacturer-rated nominal voltage rather than maximum charge voltage.
Why does battery voltage change during discharge?
Battery terminal voltage depends on electrochemical state, current, internal resistance, temperature, and state of charge, so one nominal value is only a representative rating.
Is V × Ah the exact usable battery energy?
No. It is a nominal estimate. Measured delivered energy is more rigorously obtained from the voltage-current discharge profile.
How is actual battery energy measured?
Electrical energy can be calculated by integrating V(t)I(t) over the discharge interval under specified test conditions.
Why can usable energy be lower than nominal energy?
Load, cutoff voltage, temperature, state-of-charge limits, battery aging, internal losses, and system conversion losses can reduce usable delivered energy.
Does discharge rate affect energy?
It can. DOE battery-performance definitions commonly specify discharge rate and temperature because measured delivered energy depends on test conditions.
Does temperature affect battery energy?
Yes. Battery electrochemistry and internal resistance are temperature-dependent, so usable capacity and voltage behavior can change.
Does battery aging affect Wh/kg?
Yes. If usable capacity falls while mass remains essentially unchanged, effective delivered specific energy decreases.
Why is pack energy density lower than cell energy density?
A complete pack includes enclosure, cooling, wiring, busbars, electronics, protection, sensing, and structural mass in addition to cells. The current page already highlights this boundary distinction.
Can I compare a cell Wh/kg value with an EV pack Wh/kg value?
Not directly without clearly accounting for the different system boundaries.
What is cell-level energy density?
It uses the cell’s energy and cell mass or volume only.
What is pack-level specific energy?
It uses the complete pack energy and complete pack mass.
What happens to Wh/kg when extra packaging mass is added?
If stored energy remains the same while mass increases, Wh/kg decreases.
Can two batteries with the same Wh have different Wh/kg?
Yes. The lighter battery has higher specific energy.
Can two batteries with the same mAh have different Wh?
Yes. Their nominal voltages may differ.
How do series-connected cells affect energy?
For ideal identical cells in series, voltage adds while Ah remains the same, so total Wh is the sum of the cell energies.
How do parallel-connected cells affect energy?
For ideal identical cells in parallel, Ah capacity adds while voltage remains the same, so total Wh again equals the sum of cell energies.
Does higher Wh/kg mean a better battery?
Not universally. Safety, power capability, cycle life, cost, temperature behavior, charging performance, and application requirements also matter.
Can this calculator tell me battery runtime?
No. Runtime additionally depends on usable battery energy, load power, conversion efficiency, discharge rate, and operating conditions.
Can this calculator tell me charging time?
No. Charging time requires charge-current, charging-profile, efficiency, and battery-management information.
Can I use measured discharge Wh instead of nominal V × Ah?
For engineering comparison, measured delivered Wh under relevant test conditions can provide a more realistic numerator. The current calculator, however, derives nominal Wh from voltage and rated capacity.
What system boundary should I use?
Use the boundary relevant to the decision and keep voltage, capacity, energy, and mass consistent—cell with cell, module with module, or pack with pack.
How accurate is a battery energy density calculator?
The nominal arithmetic can be calculated accurately from the entered ratings. Real usable-energy accuracy depends on the discharge profile, load, temperature, cutoff voltage, battery condition, measurement method, and system boundary.
Sources and review
- Energy Density — Specific and Volumetric Battery Energy Metrics — U.S. Department of Energy. Accessed 2026-09-02.
- DOE Explains...Batteries — U.S. Department of Energy. Accessed 2026-09-02.
- Volumetric Energy Density of Lithium-Ion Batteries — U.S. Department of Energy. Accessed 2026-09-02.
- Battery Performance Metrics — Gravimetric and Volumetric Energy Density — Advanced Research Projects Agency–Energy. Accessed 2026-09-02.
- Battery Performance Metrics for Electric Vehicles — U.S. Department of Energy. Accessed 2026-09-02.
Reviewed 2026-09-02 by Dr Akawak Ejigu, DBA.