Solar Battery Sizing Guide for Home Backup
Size a home solar battery from critical loads, runtime, backup days, usable capacity, inverter efficiency, and reserve margin.
Quick answer
A solar battery should be sized from the loads you want to run, how long you want them to run, usable battery capacity, inverter efficiency, and reserve margin. Do not start by trying to back up the whole house unless that is truly the goal.
For a quick estimate, list critical loads first and run them through the Battery Sizer. Then compare the result with battery quotes.
kW and kWh are different
Battery energy capacity is measured in kWh. It tells you how much energy can be stored. Battery power output is measured in kW. It tells you how much load the battery can serve at one time.
- A refrigerator and lights may need modest kW but many hours of kWh.
- Air conditioning, electric ovens, dryers, and well pumps can require high surge or running power.
- A battery can have enough energy but still fail to start a large load if power output is too low.
Critical-load sizing workflow
- List the loads you truly need during an outage.
- Estimate each load's running watts and daily runtime.
- Convert watt-hours to kWh and add the loads together.
- Adjust for usable capacity, inverter efficiency, and reserve margin.
- Check whether the battery inverter can start the largest loads.
Battery sizing formula
A planning formula is: required battery kWh = daily critical-load kWh x backup days / usable depth of discharge / inverter efficiency + reserve margin.
For example, if critical loads use 8 kWh per day and you want one day of backup, a battery with 90% usable depth of discharge and 90% inverter efficiency would need more than 9.9 kWh before adding reserve margin.
Whole-home backup caveats
Whole-home backup is possible, but it can require multiple batteries, load management, or a larger inverter. It is usually better to define critical loads first, then decide which comfort loads are worth adding.
- Separate must-run loads from nice-to-have loads.
- Ask whether the proposal includes a critical-loads panel or automatic load management.
- Confirm battery warranty terms, cycling assumptions, and whether solar can recharge the battery during outages.
What to calculate next
After sizing backup, compare the battery decision with the Solar Batteries Guide and Solar Battery Buying Guide. If savings are the main goal, check the solar-only case in the Solar ROI Calculator before adding storage cost.
Evidence
Sources and methodology
Draft prepared from SolarPel editorial rules with official-source context, clear limitations, internal links, and no universal savings promises. Re-check sources on publication day.
formula
A critical-load battery-sizing formula using kWh, backup days, usable depth of discharge, inverter efficiency, and reserve margin.
checklist
A step-by-step critical-load workflow for separating must-run loads from whole-home backup wants.
mistake warning
Explains the difference between kW and kWh so readers do not buy a battery with enough energy but insufficient power output.
Article FAQ
Common questions
How many kWh of battery backup do I need?
It depends on the loads you want to run and for how long. Start with critical-load kWh per day, then adjust for usable capacity, efficiency, and reserve margin.
Can a solar battery run air conditioning?
Some systems can, but air conditioning often needs high power output and substantial energy capacity. Confirm both running load and startup requirements.
What is critical-load backup?
Critical-load backup powers selected circuits such as refrigeration, lights, internet, medical equipment, or a few outlets instead of backing up the whole house.
Can solar recharge a battery during an outage?
Some solar-plus-storage systems can recharge during outages, but the design must support islanding and safe operation. Confirm this with the installer.
Written by
Firoz Ahmed
SolarPel Editorial Lead
Firoz Ahmed writes SolarPel's solar calculators, planning guides, and technical explainers with a focus on practical home-energy decisions, transparent assumptions, and source-backed solar research.