Home Battery System GuideA home battery is rated by both energy capacity and power. Energy capacity, measured in kilowatt-hours, sets how long the battery can run your loads. Power, measured in kilowatts, sets how many appliances can run at the same time. Whole-home backup is only possible when both numbers are large enough for your house.
Capacity and power are two different ratings
The U.S. Department of Energy separates the two measurements directly. Energy capacity is the total amount of energy that can be stored, usually in kilowatt-hours or megawatt-hours, while power capacity is the amount of energy that can be released at a given time, usually in kilowatts or megawatts. A battery can have a large store of energy but a small power rating, which limits how many appliances it can run at once.
EnergySage uses a water pipe analogy to make the difference concrete. The usable energy capacity is the amount of water available to push through the pipe, while power is the size of the pipe itself. A high power rating lets the battery deliver more electricity at one moment, but running at that maximum also drains the stored energy faster.
For whole-home backup you have to read both numbers together. Enough kilowatt-hours to last the outage means little if the kilowatt rating cannot start and carry the appliances you need at the same time.
Continuous power versus startup (peak) power
Power itself has two sub-ratings. EnergySage defines peak power as the amount of power a battery can push out over a very short period to turn on more power-hungry appliances, and continuous power as the amount of power a battery can supply to keep a device running after it has already started. Motor-driven loads such as HVAC systems draw a surge on startup before settling into steady-state operation.
The numbers vary widely by product. EnergySage cites a 5 kW power rating as a common benchmark for units such as the LG Chem RESU 10H and the Tesla Powerwall 2, notes a Tesla Powerwall+ rating of 10 kW peak and 7 kW continuous, and lists the Homegrid Stack'd 9.6 at 12 kW peak and 9.6 kW continuous.
For context on individual loads, EnergySage notes a clothes dryer can draw upwards of 4 kW while a refrigerator uses around 200 watts. Whole-home backup means the continuous power rating has to cover the sum of everything running at once, with peak headroom for motor starts.
Usable capacity versus total capacity
The kilowatt-hour number on a spec sheet is not always the number you can actually use. EnergySage explains that batteries have internal power requirements, so not all the electricity stored in a battery is available for consumption. Usable capacity is the amount of electricity stored in the battery that is available for consumption, while total capacity is how much is stored in total when fully charged.
The gap shows up on real products. EnergySage lists the Tesla Powerwall at 13.3 kWh usable against 14 kWh total, and the Enphase IQ Battery at 10.08 kWh usable against 10.5 kWh total. When sizing whole-home backup, the usable figure is the honest one to plan around.
Stacking units is the usual path to whole-home coverage. More usable kilowatt-hours means more devices powered for longer, and higher combined power ratings mean more of them can run at the same time.
Sizing for a whole house
A typical home storage system falls in a broad range. Department of Energy material referenced through NYSERDA notes a typical residential system size is around 10 to 20 kWh, while most residential properties consume between 20 and 30 kilowatt-hours daily. That means a single battery often covers essential loads rather than the entire daily load of a house.
This is why whole-home products carry both high capacity and high power. Real examples from manufacturers include the FranklinWH aPower 2 at 15 kWh usable and 10 kW continuous, and the Tesla Powerwall 3 at 13.5 kWh with 11.5 kW continuous, each designed to carry heavy household loads on a single unit.
The planning method is the same regardless of brand. Add up the running wattage of everything you want backed up, confirm the continuous power rating covers it, confirm peak headroom for motor starts, then size usable kilowatt-hours for how many hours you want that to last.