How Much Home Battery Do I Need

Battery sizing rests on two numbers: usable energy in kilowatt-hours, and the load in kilowatts times the hours you need it. EnergySage reports that a typical home installs about 13.5 kWh to keep essential devices running during outages, against an average daily use near 30 kWh. Anker's runtime formula shows how to convert stored energy into hours for a given load.

Start from usable kWh, loads, and hours

The core sizing question is how much energy your chosen loads consume over the time you want backup. Anker frames this as watt-hours being the size of the tank and watts being the flow, and gives the runtime formula of watt-hours multiplied by 0.85 to 0.90 for inverter losses, divided by the appliance watts.

Anker's example makes the math concrete: a 1,000 watt-hour store running a 100 watt load lasts roughly 8.5 to 9 hours. Scaled up, this is how you convert a battery's usable kilowatt-hours into backup time. List the loads you want, estimate their watts, decide the hours, and the required usable capacity falls out of loads times hours plus a conversion margin.

Sizing to essentials versus the whole day

Most homeowners size to essentials first. EnergySage reports that the roughly 13.5 kilowatt-hours a typical home installs is meant to keep essential devices running during outages, not to cover everything at once.

Context for that number comes from daily consumption. EnergySage states the average American home uses approximately 30 kilowatt-hours of electricity each day, so a single 13.5 kilowatt-hour unit covers under half a normal day of total use but stretches much longer when limited to a refrigerator, lights, and internet. Deciding between essentials-only and full-day coverage is the biggest driver of how much capacity you buy.

Account for the biggest loads and their surge

The refrigerator usually anchors an essentials list, and its numbers show why nameplate wattage overstates the load. EnergySage explains that a fridge cycles on and off, so dividing rated wattage by three approximates running draw, putting a 500 watt refrigerator near 167 running watts and about 4 kilowatt-hours over a day.

Power, not just energy, can also cap your plan. A single Tesla Powerwall 3 delivers up to 11.5 kW of continuous AC output per its datasheet, which sets how many appliances can run at once from one unit regardless of stored energy. If your simultaneous load exceeds one unit's output, you size up on power as well as capacity.

Whole-home and off-grid sizing

Bigger goals need more capacity and stack multiple units. Tesla's Powerwall 3 datasheet supports up to 4 units and up to 7 total units with Expansions, which is how installers reach whole-home energy targets from a modular product.

EnergySage shows the cost consequence of scaling up: whole-home backup runs about $34,000, and covering off-grid needs runs over $115,000, because an off-grid home must store enough to bridge days without the grid. Sizing therefore ends where budget meets the loads and hours you truly need, which is why most homeowners start with essentials and add capacity deliberately.