Home Battery Backup Calculator: Runtime With and Without Solar
How long a home battery or power station runs your essential loads, and how much solar recharging extends it.
Estimate how long a home battery or large power station will keep your essentials running, and whether daily solar recharging can carry you through a multi-day outage. The calculator accounts for usable capacity and inverter losses.
How to use this tool
- Enter the battery’s rated capacity in kilowatt-hours from the spec sheet.
- Set the usable share. Many systems publish usable capacity directly; otherwise 90 percent is a reasonable starting point for lithium iron phosphate.
- Enter your average essential load in watts. An energy monitor or plug-in meter gives the best number.
- Add expected solar recharge per day if you have panels, using a conservative figure for the season.
Worked example
A 13.5 kWh battery with 90 percent usable gives 12.15 kWh. An average essential load of 900 watts through a 90 percent efficient inverter draws 1,000 watts from the battery, so the runtime is about 12.2 hours. That load uses 24 kWh per day, so solar recharging of 12 kWh a day would roughly double the runtime, and 24 kWh or more would cover the load on a typical sunny day.
Understanding your results
Runtime is usable energy divided by battery draw. Because the average load matters so much, the easiest way to extend runtime is to trim it: skip electric heat and cooking, run the fridge and internet, and charge devices in the daytime. Also check that the battery’s continuous output covers your largest loads running together.
Common mistakes to avoid
- Using peak load instead of average load, which understates runtime.
- Using rated capacity instead of usable capacity, which overstates it.
- Assuming summer solar in winter. Plan with the season you care about.
- Forgetting cold weather, which temporarily reduces available capacity.
Next steps
Size a portable unit with the Home Power Station Selector or read the Home Batteries and Power Stations hub.
Frequently asked questions
Divide the energy you want to cover, in kilowatt-hours, by the usable capacity of one battery. Covering essentials for a day often takes one or two typical home battery units, while whole-home coverage with heating or cooling takes considerably more. Yes. Converting battery power to household AC loses some energy, commonly around 5 to 15 percent, so the battery drains faster than the appliance wattage alone suggests. It can if daily solar production exceeds your daily use, which is more likely in summer with a modest load. In winter or with heavy loads, solar slows the drain rather than stopping it.How many batteries do I need to power a house?
Does inverter efficiency matter?
Can solar keep a battery charged during a long outage?
This tool is for planning and education only. It does not store your entries. Work on home wiring must be done by a licensed electrician. Read our disclaimer.
When this tool helps most
Use it when comparing home batteries or large power stations. It estimates runtime from your loads, with and without solar recharging.
Common mistakes to avoid
- Using the label capacity instead of usable capacity.
- Forgetting inverter losses and a reserve.
- Counting resistive heaters or central air on a small battery.
Understanding your result
Runtime is usable watt-hours divided by your average load. Solar recharging extends it on sunny days but cannot be counted on during storms.
How to get the most accurate result
- Use numbers from your own appliance labels, manuals, bills and quotes instead of averages.
- Change one input at a time to see which assumption matters most.
- Save or print the result and bring it to your electrician or installer.
- Re-run the tool when your rates, appliances or plans change.
Step-by-step guides that use this tool
These guides explain the decisions behind the numbers, so you can act on your result with confidence.