Enter the connected load
Add the total continuous power of all appliances and equipment that will operate from the battery.
Battery Sizing Tool
Battery Capacity Calculator
Calculate the battery capacity required to power a load for a selected amount of time. Get results in amp-hours, watt-hours, kilowatt-hours, battery quantity, and estimated installed runtime.
Capacity in Ah, Wh, and kWh
Lead-acid and lithium presets
Series and parallel battery layout
Battery sizing inputs
Calculate required battery capacity
Enter your load power, required runtime, battery voltage, efficiency, discharge limit, battery condition, and reserve.
Capacity sizing
Ah, Wh, and kWh
Runtime based
Minutes or hours
Battery arrangement
Series and parallel
Realistic factors
Losses and reserve
How it works
Calculate battery capacity in four steps
The calculator converts your connected load and desired runtime into a practical battery-bank recommendation.
Select the required runtime
Enter how many minutes or hours the battery system should operate the connected load.
Apply battery factors
Add efficiency, depth of discharge, battery condition, and a reserve margin.
Review the battery bank
View required Ah, energy capacity, total batteries, wiring arrangement, and estimated runtime.
How battery capacity calculation works
Battery capacity describes how much electrical energy a battery or battery bank can store. Amp-hours describe charge capacity at a specific voltage, while watt-hours describe total stored energy.
The calculation begins by multiplying the connected load by the required runtime. This produces the energy that must reach the load. A reserve margin is added for uncertain consumption, load growth, and other operating conditions.
The calculator then adjusts the result for inverter efficiency, usable depth of discharge, and battery condition. These factors make the nominal battery capacity larger than the energy directly consumed by the load.
Battery capacity formulas
Load energy
Load watts × runtime hours
Energy with reserve
Load energy × (1 + reserve percentage)
Nominal battery energy
Design energy ÷
(efficiency × DoD × battery condition)
Required battery capacity
Nominal watt-hours ÷ battery-bank voltage
Why nominal capacity is higher
A battery should not normally be discharged to zero, and an inverter cannot convert every watt-hour without losses. Battery ageing and temperature can also reduce available energy. The calculator accounts for these conditions to provide a more practical capacity estimate.
Practical applications
When this battery calculator is useful
Use the calculator for early planning of common backup, mobile, solar, and DC battery systems.
Router and internet backup
Calculate battery capacity for routers, modems, switches, and small communication devices.
Home backup system
Size batteries for lights, fans, refrigeration, internet, and selected household loads.
Solar battery storage
Estimate the battery capacity needed to operate loads during the evening or limited solar production.
UPS and server backup
Plan battery banks for servers, network equipment, security systems, and office electronics.
RV and mobile power
Estimate storage for lighting, appliances, electronics, pumps, and mobile equipment.
Remote monitoring
Calculate battery capacity for cameras, sensors, radios, telemetry, and control equipment.
Frequently asked questions
Battery Capacity Calculator FAQs
Answers to common questions about amp-hours, watt-hours, runtime, discharge limits, and battery-bank wiring.
How do I calculate required battery capacity?
Multiply the connected load in watts by the required runtime in hours. Add the reserve margin, then divide by inverter efficiency, usable depth of discharge, and battery condition. Divide the resulting watt-hours by system voltage to calculate amp-hours.
What is the difference between Ah and Wh?
Amp-hours measure electrical charge, while watt-hours measure stored energy. Watt-hours are calculated by multiplying battery voltage by amp-hours. A 12-volt 100 Ah battery contains approximately 1,200 nominal watt-hours.
What depth of discharge should I use?
A planning value of around 50 percent is commonly used for many lead-acid batteries. Lithium iron phosphate batteries may permit 80 to 90 percent. Always follow the battery manufacturer specifications and warranty conditions.
Why does the calculator include inverter efficiency?
An inverter consumes some energy while converting battery DC power into AC power. Including efficiency increases the required battery capacity so the connected load can run for the selected time despite conversion losses.
What does battery condition mean?
Battery condition represents the usable capacity remaining after ageing, temperature effects, discharge rate, maintenance, and normal degradation. A battery operating at 80 percent condition stores less usable energy than a new battery.
How many batteries do I need?
The calculator first determines how many batteries are required in series to reach the selected bank voltage. It then calculates how many parallel strings are required to provide the necessary amp-hour capacity.
Can I connect batteries of different capacities together?
Mixing batteries with different voltages, capacities, chemistries, ages, or conditions is generally not recommended. Mismatched batteries can charge and discharge unevenly. Use matching batteries and follow manufacturer wiring instructions.
Is the calculated battery capacity exact?
The result is a planning estimate. Actual runtime can change because of battery chemistry, temperature, discharge rate, inverter standby use, cable losses, battery management limits, load variation, and battery condition.
Important battery-system notice
This calculator provides a planning estimate. Final
system design should verify maximum discharge current,
inverter specifications, battery management limits,
cable size, overcurrent protection, ventilation,
temperature, manufacturer requirements, and applicable
electrical regulations.