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, by a wide margin. 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 24 to 40 cents a kilowatt-hour on a summer weekday — the same battery, in the same house, earning nearly double on one plan versus the other.

What an exported kilowatt-hour earns. If your utility credits exports at the full retail price, the grid is already storing your surplus for free and a battery can only subtract the round-trip loss from that arrangement. If exports earn a fraction of retail, holding energy back is suddenly worth real money. Same hardware, opposite verdict, decided by a clause in your interconnection agreement.

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 doing nothing every single evening.

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]. That single change moves a typical payback figure by years.

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 your tariff will ever offer, and you have the absolute most that pack can earn on arbitrage across its entire covered life, assuming perfect dispatch every single day.

That figure is a hard ceiling, and it is worth computing before anything else. If the net price after incentives is above it, the battery cannot repay you from bill savings under any dispatch strategy or amount of patience. 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 59¢/kWh ^[7] against 26¢/kWh ^[8] outside it. Everything the calculator does downstream is built on that pair of prices, and the utility describes that plan like this:

That is the shape of nearly every figure on this site. 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.