Home Battery Backup Calculator: Runtime With and Without Solar

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

1. Enter the battery’s rated capacity in kilowatt-hours from the spec sheet.
2. Set the usable share. Many systems publish usable capacity directly; otherwise 90 percent is a reasonable starting point for lithium iron phosphate.
3. Enter your average essential load in watts. An energy monitor or plug-in meter gives the best number.
4. 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

How many batteries do I need to power a house?

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.

Does inverter efficiency matter?

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.

Can solar keep a battery charged during a long outage?

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.

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.