Three ways the answer is no

This is the most fundamental failure and it has nothing to do with price, size or brand. Moving a kilowatt-hour through a pack rated 90 percent ^[3] means buying about eleven percent more energy than you get back. If the expensive hour is not priced above the cheap hour by at least that margin, every cycle ends behind where it began.

On a genuinely flat tariff the margin is zero and the loss is certain. On a shallow time-of-use plan it can be positive but too small to matter. In both cases the correct output is a sentence, not a large number of years, and the only remedy is a different tariff. Our arbitrage tool prints the break-even peak price so this can be tested in about ten seconds against your own bill.

It is worth being explicit about what this means geographically, because average prices get quoted constantly in this context and they answer a different question. Here are the five dearest and five cheapest jurisdictions for residential electricity, with the median for scale:

The median jurisdiction sits at 16.3¢/kWh ^[6]. Now notice what this table cannot tell you: not one of these figures is a spread. A household in the dearest jurisdiction on a flat rate has no arbitrage opportunity whatsoever, while a household in a mid-priced one with a steep evening peak has a good one. Average price is the wrong axis, and a calculator that sizes savings from it is measuring the wrong thing.

That is not a criticism of the source. The federal table is doing exactly what its own title scopes it to do: one price, for one state, for one customer sector, for a named month, in cents per kilowatt-hour. Every one of those qualifiers is load-bearing and most people quoting the figure drop at least two of them. What the table cannot offer, because averaging a month of days removes it, is the gap between two hours of the same day — and that gap is the only quantity a battery converts into money. No amount of care with a state figure recovers a number that was taken out before publication.

Suppose the tariff does work. There is still a hard limit on what the pack can ever earn, and it comes from the warranty. 6,000 covered cycles ^[1] at 10 usable kilowatt-hours ^[2] is a fixed quantity of energy, and that energy has a maximum value on your tariff — the widest margin you will ever see, on every single cycle, with perfect dispatch and no missed days.

Multiply those together and compare against the net price. If the price is higher, the battery cannot repay it from bill savings under any circumstances. That is not a forecast or a conservative assumption; it is an upper bound, and the real number will be well below it because nobody dispatches perfectly for six thousand consecutive cycles.

The payback worksheet prints this ceiling next to your net cost for exactly this reason. It is the fastest way to discover that a quote is not merely expensive but outside what the equipment can ever return, and it takes one multiplication.

That inverts the usual advice about buying bigger. Both packs listed here carry 6,000 covered cycles — the smaller at 5.0 kWh usable ^[5] and the larger at 10.0 kWh. Lifetime throughput therefore scales with capacity while the cycle budget does not, which sounds like an argument for the larger unit and is one only if the evening consumes it.

Capacity above the window adds to the price and to the theoretical ceiling without adding a single earned kilowatt-hour, so it raises the bar and the cost at the same time. That is the specific mechanism by which an oversized pack can fail this test while a smaller one on the same tariff, in the same house, passes it.

This is the only one of the three that is genuinely a judgement call. The arithmetic clears, the cumulative saving does eventually overtake the cost, and the question is whether the date is one you are prepared to underwrite.

Two things push it later than most projections show. Capacity fades, so a flat annual saving overstates the later years. And the warranty ends — on a term, or on a cycle count, whichever comes first — so a payback landing after that point is being paid by an uncovered asset that may need replacing before it gets there.

Our rule of thumb, stated as ours: a payback date beyond the warranty is not a payback. If the arithmetic only clears in year eighteen on a fifteen-year warranty, the honest reading is that the savings case has failed and the purchase needs a different justification.

Quite a lot, and it is worth saying so plainly rather than leaving the impression that storage is a mistake. A battery keeps a freezer cold and a medical device running through an outage, silently and with no fuel to store. It removes the exposure to evening price rises that a household on a steep peak otherwise carries indefinitely. It allows an all-electric home to ride through the hours when importing is most expensive.

None of those show up in a payback figure, and none of them should be smuggled into one as a fudge factor. Price them separately, decide separately, and be honest with yourself about which argument is actually carrying the purchase.