Solar Panel System GuideBattery specs confuse people because capacity and power sound like the same thing but are not. One tells you how long the battery lasts, the other tells you how many appliances it can run at the same moment. Getting the two straight is the whole game when sizing backup. Facts here come from EnergySage and from the Tesla Powerwall 3 datasheet.
Capacity versus power
EnergySage draws the line clearly. A battery's capacity is the total amount of electricity it can store, measured in kilowatt-hours (kWh). A battery's power tells you the amount of electricity that it can deliver at one point in time, measured in kilowatts (kW).
Those two numbers pull in different directions. EnergySage explains that a battery with high capacity but low power can only provide a small amount of electricity at one time, but can store enough energy to keep providing that electricity for long periods of time.
In plain terms, capacity is how big the tank is and power is how wide the pipe is. A big tank with a narrow pipe runs a few things for a long time; a wide pipe with a small tank runs many things briefly.
Usable capacity and round-trip efficiency
The headline capacity is not always the number you can draw. EnergySage lists batteries by their usable capacity, the portion of the total you can actually access, because some batteries reserve a share that cannot be used.
Some energy is also lost in the store-and-release cycle. EnergySage defines a battery's round-trip efficiency as how much energy can be used as a percentage of the amount of energy that it took to store it, and notes typical losses around 5 percent.
Manufacturer numbers reflect this. Tesla rates the Powerwall 3 with a solar-to-home-or-grid efficiency of 97.5 percent, tested using CEC weighted efficiency methodology.
What the ratings mean for backup: a worked example
A real product shows how the two ratings work together. Tesla lists the Powerwall 3 with 13.5 kWh of nominal battery energy and up to 11.5 kW of continuous power per unit. The energy number sets how long it can run, and the power number sets how much it can run at once.
Starting large motor loads takes a burst above the steady rating, which is a separate spec. Tesla states the Powerwall 3 has a load start capability of 185 LRA, or locked rotor amps, meaning a single unit can support the power needs of most homes.
When one battery is not enough, capacity and power both scale. Tesla notes that up to 4 Powerwall 3 units are supported, with up to 3 Expansion units for a maximum total of 7 units.