How much storage does your evening actually call for?

A battery earns only when it is discharging into an expensive hour. Once it has supplied everything the house draws during the expensive window, the remainder of its charge has nowhere profitable to go: it will discharge later, into hours that were already cheap, displacing energy that cost very little. The saving on those kilowatt-hours is roughly nothing.

So the useful capacity has a hard upper bound equal to the window’s consumption. A household drawing four kilowatt-hours between four and nine gets essentially the same annual saving from a four kilowatt-hour pack as from a ten, while paying for six kilowatt-hours it can never sell. The worksheet prints that ceiling explicitly, and the payback tool warns when your selected pack is above it.

This is the opposite of how storage is usually sold, and it is why the two packs listed here differ by a factor of two: 5.0 kWh ^[1] against 10.0 kWh ^[2]. For a small evening the smaller one is not a compromise, it is the correct answer.

Capacity in kilowatt-hours says how long. Continuous power in kilowatts says whether at all. A pack rated 3.84 kW ^[3] cannot supply a five-kilowatt load for one second, however full it is; the larger unit at 7.08 kW ^[4] has correspondingly more headroom.

For arbitrage this rarely binds, because household evening draw is spread across several hours. For backup it binds constantly, which is why the outage tool on this site checks power before it reports a runtime at all.

The distinction between kVA and kW is not pedantry here. Apparent power counts the current the inverter has to supply, real power counts the work that current does, and motors and compressors pull the two apart. For an evening of lights and cooking they are close enough to ignore. For a well pump they are not, and the gap is on the side that costs you.

The single input that moves this page most is the share of your daily consumption that falls inside the expensive hours, and it is also the one people are worst at estimating. A household that cooks at six and runs air conditioning until nine is a different machine from one that leaves for the evening at five.

Nearly every utility with a time-of-use plan publishes hourly interval data in the account portal. Download one week, sum the hours inside the window, divide by the week’s total, and the assumption becomes a measurement. It takes about ten minutes and it is worth more than every other refinement on this site combined.

Our own default of thirty percent ^[5] — like the depth-of-discharge figure cited beside it — is a starting point offered on the control, never a silent constant baked into the maths.