Watt-hours needed--
Amp-hours needed--
Battery Bank Size Calculator for Backup Power
Plan battery storage for backup power, camping, RVs, and small off-grid loads.
Updated May 2026No signup requiredBuilt for mobile
Formula or method
Watt-hours = load x hours / efficiency.
Amp-hours = watt-hours / battery voltage / usable fraction.
Worked examples
Basic planning example
Enter realistic values in the Battery Bank Size Calculator to get a quick planning estimate.
Round-up example
Increase the reserve or waste input to see how the result changes before buying or relying on the estimate.
Practical use cases
- quick planning
- buying supplies
- checking a rough budget
- comparing scenarios
- avoiding shortfalls
Common mistakes
- Using guessed inputs
- forgetting waste or reserve
- mixing units
- treating an estimate as a final quote
FAQ
Battery Bank Size Calculator questions
How accurate is this calculator?
It is a planning estimate based on the values you enter. Real-world conditions can change the result.
Why do results vary?
Overhead, rounding, equipment limits, supplier units, network conditions, and user behavior can all affect the final number.
Should I round up?
Round conservatively when running short would interrupt a project, backup, stream, trip, or outage plan.
What should I do next?
Use the result card and checklist, then compare related calculators or guides before making a final decision.
Does this replace official documentation?
No. Use manufacturer documentation, platform guidance, or professional advice for critical decisions.
How to use this estimate
The battery bank size calculator estimates storage capacity for backup power, cabins, RVs, solar projects, and essential outage loads. It turns watt-hours, days of autonomy, depth of discharge, and inverter losses into a more realistic battery capacity target.
Inputs that matter most
- Daily energy use in watt-hours or kilowatt-hours for the loads you plan to support.
- Days of autonomy before recharging from solar, generator, grid, or vehicle charging.
- Battery chemistry and usable depth of discharge recommended by the manufacturer.
- Inverter efficiency, wiring losses, cold weather effects, and aging reserve.
Formula and method
Battery bank capacity = daily energy use x days of autonomy, divided by usable depth of discharge and efficiency. Convert watt-hours to amp-hours only after choosing system voltage, because the same energy requires different amp-hours at 12, 24, and 48 volts.
A larger result often means the load list needs trimming. Refrigeration, heating appliances, pumps, and inverters can dominate the bank size. Compare the output with charging ability so the bank can actually be refilled.
Worked example
Example: a small cabin needs 2,000 watt-hours per day and wants two days of autonomy. The raw energy target is 4,000 watt-hours. If the battery should only be discharged to 80 percent and inverter efficiency is 90 percent, the planned bank needs about 5,556 watt-hours before adding cold-weather or aging reserve.
Common planning mistakes
- Sizing batteries without measuring daily load first.
- Ignoring inverter losses and battery depth-of-discharge limits.
- Mixing amp-hours across different system voltages.
- Forgetting cold weather, battery age, cable sizing, fuses, and manufacturer limits.
Safety and disclaimer note
Battery bank planning involves fire, shock, ventilation, and equipment risks. Follow manufacturer specs, battery chemistry requirements, local electrical code, fuse and cable guidance, and consult a qualified electrician or solar professional for permanent systems and safety-critical installations.
FAQ
Practical questions
Is watt-hours better than amp-hours?
Watt-hours are easier for comparing energy. Amp-hours only make sense after the system voltage is known.
How many days of autonomy should I use?
Backup systems often use one to three days, but cabins, storms, and off-grid plans may need more depending on recharge options.
Can I mix old and new batteries?
Manufacturer guidance should control that decision. Mixing batteries can reduce performance and create safety concerns.
Do lithium and lead-acid batteries size the same way?
No. Usable capacity, discharge limits, charging behavior, temperature limits, and safety requirements differ by chemistry.
Should I add solar before increasing batteries?
Sometimes. A balanced system needs enough storage and enough charging capacity to refill that storage.