The right home battery size depends on what you want the battery to accomplish. A battery designed to reduce evening grid purchases can be much smaller than one intended to keep an entire home operating through a long outage.

Start with two different numbers: energy and power

Battery energy capacity is usually measured in kilowatt-hours (kWh). It describes how much energy can be stored. Battery power capacity is measured in kilowatts (kW) and describes how much power the system can deliver at a given moment.

The U.S. Department of Energy explains that storage systems need both capacity and power specifications because a battery can store a large amount of energy without necessarily being able to deliver every high-power load simultaneously.

Estimate the loads you actually want to back up

Make a list of the appliances and circuits you would want during an outage. Typical priorities might include refrigeration, lighting, internet equipment, selected outlets, a furnace or heat-pump system, medical equipment, or other critical loads.

For a rough energy estimate:

Energy required (kWh) ≈ appliance power (kW) × hours of use

Repeat that calculation for the loads you want to support, then add them together.

Why “whole-home backup” can require much more

Large heating and cooling equipment, electric water heaters, electric ranges, well pumps and EV charging can consume substantial power. If several high-power loads operate at the same time, the battery’s power rating and inverter capacity become just as important as its kWh capacity.

Solar + battery is not automatically the same as backup

A battery can store solar energy for later use, but backup operation also depends on the system’s inverter architecture, electrical configuration, critical-load panel or equivalent equipment, utility requirements and installation design.

Battery sizing example

Suppose the loads you want to support average 1.5 kW while operating and you expect to need them for four hours. A simplified energy calculation would be 1.5 × 4 = 6 kWh. In practice, you would need to account for usable capacity, conversion losses, battery operating limits, reserve settings and the possibility that loads vary over time.

Use the battery for the job you actually want

• Bill savings: Focus on when your utility charges more for electricity and when solar production is available.
• Short outages: Size around critical circuits and the expected outage duration.
• Long outages: Consider how the battery will be recharged and whether solar can replenish it.
• Whole-home backup: Pay close attention to inverter power, starting loads and electrical-panel configuration.

Bottom line

Battery sizing is not simply “How many kWh should I buy?” Start by identifying the loads you care about, estimate both their energy use and simultaneous power demand, then evaluate usable battery capacity and inverter limits.

Sources: U.S. Department of Energy — Solar Energy and Storage Basics; U.S. Department of Energy — Solar-Plus-Storage 101.