Battery Capacity Calculator (Wh, kWh, Ah)

This calculator estimates battery capacity from load power, runtime, system voltage, depth of discharge, and efficiency. Example: a 500 W load for 4 hours needs 2,000 Wh of usable energy before depth of discharge and efficiency losses.

A battery capacity calculation starts with the load energy required during an outage, then increases that energy requirement to account for the portion of battery capacity that will actually be used and the losses between the battery and the supported load.

This Battery Capacity Calculator produces three planning values:

• Load energy in watt-hours (Wh)
• Required battery capacity in watt-hours (Wh) and kilowatt-hours (kWh)
• Required battery capacity in ampere-hours (Ah) at the selected Battery system voltage

The result supports early backup-runtime planning for a battery-backed branch circuit, critical-load panel, communications equipment, controls, lighting, networking equipment, or other defined load. It establishes an energy and nominal battery-capacity target before equipment selection and installation design.

Load Energy and Required Capacity

The calculator first determines the energy required by the connected load:

\(\displaystyle \text{Load energy (Wh)} = \text{Load (W)} \times \text{Runtime (h)}\)

The battery bank must contain more energy than the load consumes because the calculation applies two usable-energy limits:

• Usable depth of discharge (%) limits the portion of nominal battery capacity intended for discharge.
• System efficiency (%) accounts for conversion and system losses between stored DC energy and delivered load energy.

The calculator combines these values as the Usable factor:

\(\displaystyle \text{Usable factor} = \left(\frac{\text{Usable depth of discharge}}{100}\right) \times \left(\frac{\text{System efficiency}}{100}\right)\)

Required nominal battery capacity is then:

\(\displaystyle \text{Required battery capacity (Wh)} = \frac{\text{Load energy (Wh)}}{\text{Usable factor}}\)

The ampere-hour result converts that nominal stored energy to capacity at the entered battery-system voltage:

\(\displaystyle \text{Required battery capacity (Ah)} = \frac{\text{Required battery capacity (Wh)}}{\text{Battery system voltage (V)}}\)

Because ampere-hours depend on voltage, an Ah value cannot be compared directly between a 12 V, 24 V, and 48 V battery system. Watt-hours or kilowatt-hours provide the more direct stored-energy comparison.

Inputs Used in the Calculation

Field	Electrical purpose
Load (W)	The power demand to be supported. Use the actual planned load rather than the rating of an entire panel or service unless the whole panel is intended to operate on battery power.
Runtime (h)	The required duration of battery support in hours.
Battery system voltage (V)	The nominal DC voltage of the battery system used to express the required capacity in Ah.
Usable depth of discharge (%)	The usable portion of nominal battery capacity assumed for the runtime calculation.
System efficiency (%)	The assumed percentage of battery energy delivered to the load after conversion and system losses.

The Load (W) should reflect the load expected during the backup period. For a mixed critical-load panel, that may require a load review rather than adding every branch-circuit breaker handle rating. Breaker ratings, conductor ampacity, and connected equipment nameplates are not interchangeable with actual operating watts.

Calculation Example

For a 500 W load requiring four hours of backup on a 24 V battery system, with 80% usable depth of discharge and 90% system efficiency:

Input	Value
Load (W)	500 W
Runtime (h)	4 h
Battery system voltage (V)	24 V
Usable depth of discharge (%)	80%
System efficiency (%)	90%

First, calculate the supported load energy:

\(\displaystyle 500\text{ W} \times 4\text{ h} = 2{,}000\text{ Wh}\)

Then calculate the usable factor:

\(\displaystyle 0.80 \times 0.90 = 0.72\)

The battery capacity required is:

\(\displaystyle \frac{2{,}000\text{ Wh}}{0.72} = 2{,}777.7778\text{ Wh}\)

\(\displaystyle 2{,}777.7778\text{ Wh} \div 1{,}000 = 2.7778\text{ kWh}\)

At a 24 V nominal battery system:

\(\displaystyle 2{,}777.7778\text{ Wh} \div 24\text{ V} = 115.7407\text{ Ah}\)

The calculated result is therefore:

• Load energy: 2,000 Wh
• Required battery capacity: 2,777.7778 Wh
• Required battery capacity: 2.7778 kWh
• Required battery capacity: 115.7407 Ah
• Usable factor: 0.72 x

The calculation identifies the nominal battery energy required to deliver 2,000 Wh to the load under the stated depth-of-discharge and efficiency assumptions.

Relationship to Electrical Design

Battery capacity is an energy calculation, not a conductor-sizing calculation. It can be used early in a backup-power design to define the approximate energy storage requirement and to support later review of the DC and AC electrical system.

The resulting battery capacity may inform subsequent work such as:

• Reviewing the expected load served by an inverter output, critical-load panel, branch circuit, or feeder.
• Evaluating DC current associated with the selected battery-system voltage.
• Sizing battery conductors, overcurrent protection, disconnecting means, terminals, raceways, and equipment space after actual listed equipment is selected.
• Checking voltage drop on DC battery conductors and AC output conductors.
• Coordinating battery system voltage with inverter input requirements and available battery equipment.
• Planning physical routing, raceway fill, conductor bending space, equipment clearances, and installation layout.

A lower battery-system voltage generally requires higher DC current for the same power transfer. That can affect conductor AWG or kcmil selection, overcurrent protection, terminal rating, voltage drop, raceway fill, and the physical size of the DC installation. Those decisions require actual equipment ratings and installation conditions; they do not follow from battery capacity in Ah alone.

Equipment and Field Limits

The calculation estimates battery capacity from Load (W), Runtime (h), Battery system voltage (V), Usable depth of discharge (%), and System efficiency (%). It does not select a battery chemistry, battery management system, inverter, charger, listed energy-storage system, or equipment configuration.

Field design requires separate verification of actual battery and inverter performance, including temperature, discharge rate, load profile, starting or surge loads, charging behavior, wiring losses, terminal limitations, and manufacturer installation instructions. A motor, compressor, pump, or transformer load may have starting characteristics that exceed its steady-state watts even when its runtime energy is within the calculated battery capacity.

Code and installation decisions must also be made separately with the applicable requirements, listed equipment documentation, utility requirements where applicable, and the authority having jurisdiction (AHJ). The calculator does not provide a ventilation, fire, utility-interconnection, listing, protection, conductor ampacity, overcurrent device, voltage-drop, or code-compliance result.