Where every number comes from

Arbitrage value of one cycle

Shifted energy is the smaller of the pack’s usable capacity and the household consumption inside the expensive window. Charging energy is shifted energy divided by the round-trip efficiency — the loss is bought, at the charging price, on the charging side. Day value is shifted energy times the peak price, less charging energy times the cheaper of the grid’s lowest published block and the export credit given up.

Break-even peak price

Charging price divided by round-trip efficiency. Below that price a cycle loses money regardless of capacity, cost or incentive.

Annual value

Useful cycle days are split between the seasons in proportion to the months each tariff assigns to summer, then each season’s day value is multiplied by its own day count and the two are added.

Net cost

Quoted price, less any rebate at the stated dollars per usable kilowatt-hour, less the federal credit computed on that already-reduced expense. A rebate tied to the cost of the equipment is a purchase-price adjustment; an ongoing export credit is not, and does not touch this figure.

Capacity in year N

One minus cumulative cycles times the assumed loss per cycle, floored at zero. Cycles accumulate at the rate the load and capacity imply rather than at a fixed one per day.

Payback

Each year’s benefit is the first-year arbitrage value scaled by that year’s retention, plus any outage value you priced yourself. The running total is compared against net cost and the crossing point is interpolated inside the year it happens.

Lifetime arbitrage ceiling

Warranted cycles times usable capacity times the widest per-kilowatt-hour margin the tariff offers, where that margin is the highest peak price less the cheapest charging price divided by efficiency. It is an upper bound assuming perfect dispatch, so the real figure is always below it.

Backup runtime

Usable energy divided by average draw, where average draw is each load’s running watts times its duty cycle. Reported only after the simultaneous running load and the largest start-up surge have both been checked against continuous power.

Capacity fade per cycle

Anchored to a thirty percent loss over the full warranted cycle count and applied in a straight line. Real cells fade fastest early and then flatten, so this understates year-one capacity slightly and overstates late-life capacity slightly. Anchored so that a pack cycled to its full warranted count ends at 70 percent of day-one capacity. The manufacturer pages we retrieved publish a term and a cycle count but no retention curve, so the shape is ours.

Depth of discharge for a nameplate figure

Applies only to a nameplate figure. Every battery listed on this site publishes usable energy directly, and for those this number is never used.

Evening share of daily consumption

A starting point, not a measurement. Your utility can show you the real number: an hourly interval download for one week settles it in minutes and usually moves the payback figure more than any other single input.

Useful cycle days per year

Two hundred and fifty weekdays less twenty for travel, weather and reserve holds. Raise it if you are on a tariff whose peak also runs at weekends.

Installed cost. There is no price anywhere in the data module. The national dataset that tracks installed system prices did not resolve from the machine this site was built on, the laboratory’s own report page returns an HTTP 403 to non-browser clients, and the remaining candidates are commercial pages citing each other. So the calculator asks for your quote. That is a fact you hold; an average we invented would silently determine every figure downstream of it.

Export credit rates. We can quote the regulatory basis for how an export credit is set, but not a number, because it varies by utility, by hour and by season. The calculator takes yours.

Outage frequency and value. Both default to zero. Your outage exposure depends on your feeder and your weather, and what an interrupted evening is worth depends on what is in your fridge and whether anyone in the house depends on a machine.

Incentives outside the one programme we could quote. Rather than assembling a fifty-state table from secondary sources of unknown freshness, the calculator accepts whatever rebate you have actually been offered.