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.

Warranted-cycle arbitrage ceiling

Warranted cycles times day-one usable capacity times the widest per-kilowatt-hour margin on the tariff’s published prices, where that margin is the highest peak price less the cheapest charging price divided by efficiency. It assumes perfect dispatch, no fade and unchanged prices, so the model’s own year-by-year arbitrage never exceeds it within the warranted cycles. It is a bound of this model rather than of the hardware: it counts no cycles after the warranty ends, when a pack may keep working at reduced capacity without cover.

Backup runtime

Usable energy at the chosen state of charge divided by average draw, where average draw is each load’s running watts times its duty cycle; nothing is added for standby draw or conversion losses. Two power checks run beside it, both against the pack’s continuous rating only: the running watts of every selected load added together, and the largest single start-up surge. When the running total is above the rating, the worksheet headline says so instead of giving hours; a surge above it is flagged as something to check against the pack’s short-term peak rating, which the worksheet does not model.

Capacity fade per cycle

Anchored to a thirty percent loss over the full warranted cycle count and applied in a straight line. The real shape of the fade for these packs is not published on the pages we cite, so the capacity shown for any single year may be above or below what a particular pack keeps. Anchored so that a pack cycled to its full warranted count ends at 70 percent of day-one capacity; that anchor is our own assumption, not a warranty term. The manufacturer pages we retrieved publish a term and a cycle count but no retention curve, so the shape is ours too.

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 may offer an hourly interval download; one week of it shows what share of your own day falls inside the evening window, and you can enter that share in place of this default.

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.