UPS Battery Size Calculator (Ah, Wh, kWh)

This calculator estimates the battery energy and amp-hour size needed for a UPS backup target. Example: a 1 kW load for 2 hours needs more than 2 kWh of nominal battery storage after efficiency and usable-capacity adjustments.

The calculation begins with the load’s energy demand in watt-hours. It then increases that demand to account for losses in converting stored DC battery energy to usable AC load energy and for the portion of a battery’s rated capacity that is intended to be usable. Finally, it converts required watt-hours into ampere-hours using the selected nominal Battery voltage.

The required battery capacity result does not size branch-circuit conductors, feeder ampacity, raceway fill, or overcurrent protection. Those installation decisions require the actual UPS equipment ratings, battery-bank configuration, charger characteristics, available fault current, conductor terminations, voltage-drop review, and applicable AHJ requirements.

Load Energy Requirement

The calculator’s Load power (W) is the power the UPS must supply to the connected equipment. Target runtime (hours) is the length of time that load must remain energized from battery power.

Load energy is:

\(\displaystyle \text{Load energy (Wh)} = \text{Load power (W)} \times \text{Target runtime (hours)}\)

A 500 W load operating for 2 hours requires 1,000 Wh of delivered load energy:

\(\displaystyle 500\text{ W} \times 2\text{ h} = 1{,}000\text{ Wh}\)

This is the energy required at the UPS output. The battery bank must store more than this amount because conversion losses and usable-capacity limits reduce the energy available to the load.

Battery Energy Adjustment

Conversion efficiency (%) represents the battery-to-load conversion efficiency. A 90% value is entered as an efficiency factor of 0.90. Lower conversion efficiency requires more stored battery energy for the same connected load and runtime.

Usable battery capacity (%) represents the fraction of rated battery energy treated as available for the intended calculation. An 80% value is entered as a usable-capacity factor of 0.80.

The calculator determines required battery energy as:

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

This produces the preliminary stored-energy requirement for the battery system.

Calculator input or result	Electrical meaning
Load power (W)	UPS output load that must be supported
Target runtime (hours)	Required duration of battery operation
Conversion efficiency (%)	Battery-to-load energy conversion factor
Usable battery capacity (%)	Portion of nominal battery energy treated as usable
Load energy	Energy delivered to the connected load
Required battery energy	Stored battery energy required after adjustment factors
Battery voltage (V)	Nominal DC voltage used to convert Wh to Ah
Required battery capacity	Preliminary nominal battery capacity in Ah

Ampere-Hour Conversion

Battery systems are commonly reviewed in both watt-hours and ampere-hours. Watt-hours describe energy; ampere-hours describe nominal capacity at a stated battery voltage.

The calculator uses Battery voltage (V) to convert the required battery energy into required battery capacity:

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

The entered voltage must represent the nominal DC battery-bank voltage used for the proposed UPS arrangement. A 48 V battery system and a 24 V battery system can require the same watt-hour capacity while producing different ampere-hour values.

For the same required energy, a lower nominal battery voltage produces a higher Ah requirement:

\(\displaystyle \text{Ah} = \frac{\text{Wh}}{\text{V}}\)

Battery voltage also affects the DC current associated with the battery system, which can affect conductor sizing, overcurrent protection, terminations, voltage drop, and equipment layout. Those items require a separate design review using the actual UPS and battery equipment data.

Calculation Example

For the following inputs:

• Load power: 500 W
• Target runtime: 2 hours
• Conversion efficiency: 90%
• Usable battery capacity: 80%
• Battery voltage: 48 V

The calculator produces the following result.

1. Load Energy

\(\displaystyle 500\text{ W} \times 2\text{ h} = 1{,}000\text{ Wh}\)

\(\displaystyle \text{Efficiency factor} = 90\% = 0.9\)

\(\displaystyle \text{Usable capacity factor} = 80\% = 0.8\)

\(\displaystyle \text{Required battery energy} = \frac{1{,}000\text{ Wh}}{0.9 \times 0.8} = 1{,}388.8889\text{ Wh}\)

\(\displaystyle \frac{1{,}388.8889\text{ Wh}}{48\text{ V}} = 28.9352\text{ Ah}\)

The preliminary result is 1,388.8889 Wh of required battery energy, equivalent to 28.9352 Ah at 48 V.

Battery Bank and UPS Selection

The required battery energy and required battery capacity provide an initial target for evaluating a proposed UPS battery bank. The final battery selection must match the actual UPS configuration and manufacturer runtime information.

A nominal Ah rating by itself does not establish that a battery will deliver the calculated runtime. Actual available capacity can vary with battery condition, temperature, discharge rate, battery age, series and parallel configuration, and the UPS operating characteristics. The battery model, UPS model, charger, and approved battery arrangement must be verified against manufacturer data.

Where the result is used to plan a battery installation, perform separate reviews for:

• Battery-bank nominal voltage and series/parallel arrangement
• Manufacturer runtime tables or approved runtime configuration
• DC conductor ampacity, terminal ratings, and overcurrent protection
• Battery circuit voltage drop and connection resistance
• Charger capacity and recharge requirements
• Equipment listing, installation instructions, ventilation, clearances, and AHJ requirements

Field Verification

This worksheet performs UPS battery sizing arithmetic only. It does not select a battery model or confirm battery aging, temperature effects, discharge-rate behavior, charger capacity, listing, equipment compatibility, or manufacturer-approved runtime.

Use the calculated Wh and Ah values as a preliminary electrical load-and-runtime target, then verify the installed UPS and battery system against the manufacturer’s runtime and battery data before final equipment selection or installation.