Does a home battery pay for itself?

There is no installed price anywhere in our data. Not a national average, not a range, not a dollars-per-kilowatt-hour rule of thumb. We went looking for one: the national laboratory dataset that tracks installed prices would not resolve from this machine at all, the laboratory’s public report page refuses non-browser clients, and what is left is a field of marketing pages quoting each other in a circle.

So the box at the top of the worksheet is empty until you fill it. That is a deliberate choice about which direction to fail in. A cost anchor invented to make a page feel complete would silently determine every figure below it, and the reader would have no way to know. A missing number is visible; a fabricated one is not.

The distinction that matters here: asking for the price is not the same as asking for the answer. The price is a fact you already hold, printed on a piece of paper an installer gave you. The payback period is what you came to find out, and a calculator that asks for it and multiplies has done nothing at all.

Your rate plan. A battery does not generate energy; it moves energy from one hour to another and charges you a loss for the trip. So the entire revenue side is the gap between two prices on the same day. Among the three published tariffs on this site that gap runs from 26 to 38 cents a kilowatt-hour on a summer weekday. On the worksheet’s defaults the same battery, in the same house, earns about $969 a year from cycling on TOU-D 4 PM to 9 PM and $1,126 on TOU-D 5 PM to 8 PM.

What an exported kilowatt-hour earns. If an export is credited at the same price as the evening kilowatt-hour it would otherwise offset, as it is under retail netting on a rate with the same price at every hour, the grid is already storing your surplus for free and a battery can only subtract the round-trip loss. If exports earn less than that evening price, as a net billing export rate or a midday time-of-use credit can, holding energy back can be worth real money. Same hardware, opposite verdict, decided by how your exports are credited.

How much your evening actually uses. The pack can only sell what the house buys during the expensive window. A 10 kilowatt-hour battery ^[4] in a home that draws four kilowatt-hours between four and nine has six kilowatt-hours that this model gives no evening work.

The physics, which is small but never zero. A pack rated 90 percent round trip ^[5] has to absorb about eleven percent more energy than it returns, and you buy that eleven percent at the charging price.

The tax position, which just changed. The federal credit was 30 percent ^[1] of the installed cost with no dollar limit, on any pack of at least 3 kilowatt-hours ^[3]. For a system placed in service today it is 0 percent ^[2]. On the worksheet’s defaults and an illustrative $10,000 quote, that single change moves the payback from 8.0 years to 11.6.

A battery’s warranty names a number of cycles — 6,000 for the pack above ^[6]. Multiply that by the usable capacity and by the widest margin on your tariff’s published prices, and you have the most that pack can earn on arbitrage within its warranted cycles, assuming perfect dispatch every single day and prices that stay where they are.

That figure is this calculator’s ceiling, and it is worth computing before anything else. If the net price after incentives is above it, no dispatch strategy repays it from bill savings within the warranted cycles at those prices. It is not a physical limit: a pack can keep cycling after its warranty ends, at reduced capacity and without cover, and prices can change, and the model counts neither. The worksheet prints the ceiling next to your net cost so the comparison is unavoidable. Where the ceiling is the binding constraint, the honest remaining question is not how long payback takes — it is whether you value keeping the lights on enough to buy the thing anyway.

On the default plan here, energy in the expensive window is worth 61¢/kWh ^[7] against 26¢/kWh ^[8] outside it. The worksheet charges the pack at the season’s cheapest grid price or, if you have solar, at the export credit you enter when that is lower. The utility describes that plan like this:

Several figures on this site work the same way. A published document states something narrow and checkable; the useful work is deciding which side of your arithmetic it lands on. A daily charge and a peak price both appear on the same tariff sheet, and only one of them is a battery’s business.