How much storage does your evening actually call for?

In this site’s model a battery earns only when it is discharging into the expensive window. Once it has supplied everything the house draws during that window, the model gives the remainder of its charge nowhere profitable to go: discharged later, into hours that were already cheap, it would displace energy that cost very little, and the model counts the saving on those kilowatt-hours as nothing.

So, in this model, useful capacity is bounded by the window’s consumption. A household drawing four kilowatt-hours between four and nine gets essentially the same modelled annual saving from a four kilowatt-hour pack as from a ten, while paying for six kilowatt-hours the model gives nothing to sell. The worksheet prints that ceiling explicitly, and the payback tool warns when your selected pack is above it.

That is why both packs are listed here, a factor of two apart: 5.0 kWh ^[1] against 10.0 kWh ^[2]. For a small evening the smaller one is not a compromise; on this model it is the better fit.

Capacity in kilowatt-hours says how long. Continuous power in kilowatts says how much the pack is rated to carry for as long as that energy lasts. A pack rated 3.84 kW ^[3] is not rated to carry a five-kilowatt load continuously, however full it is. Its maker also publishes higher peak output that it can hold for a few seconds, quoted below, so a brief motor start-up above the continuous rating is a separate test from the running load. The larger unit at 7.08 kW ^[4] has correspondingly more continuous headroom; its product page lists no separate peak figure.

On the sizing worksheet’s opening case the evening draw averages about 1.78 kW across the expensive window, inside both packs’ continuous ratings, but the arbitrage and payback worksheets do not check power at all, so a household whose evening peaks above the rating should check that separately. For backup it matters more, which is why the outage tool on this site checks the running draw and the largest start-up surge against the continuous rating beside its runtime figure, and flags a surge above it as something to check against the peak rating rather than as a verdict.

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.

If your utility’s account portal offers hourly interval data, 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.

Our own default of 30 percent ^[5] is a starting point offered on the control, never a silent constant baked into the maths. So is our assumed depth of discharge of 90 percent ^[6], which applies only when a quote gives a nameplate figure instead of usable energy.