Backup energy required for an outage

The first figure is the bill: power times duration is the energy the outage will draw through the meter that no longer runs. It is deliberately indifferent to WHERE that energy comes from — battery, generator, or a neighbour’s extension lead — because the demand exists before any supply is chosen, and mixing the two questions is how backup plans flatter themselves.

The second figure is the verdict. Subtracting the bill from the usable capacity on hand returns a margin in kilowatt-hours that keeps its sign: positive means the stored energy outlasts the interruption with something to spare, negative means the lights go out before the grid comes back, and the pack ships a declared warning that fires on exactly that shortfall rather than leaving it to be noticed.

The capacity term must be USABLE energy — the nameplate already discounted by the depth of discharge, as this pack’s battery cluster computes it. Entering the label figure here overstates the margin by the held-back fraction, which for some chemistries is half the battery, and turns a failing plan into a passing one on paper alone.

Read the shape of the relation: the bill grows linearly in both load and hours, so a plan has two equally priced levers. Shedding half the circuit buys the same hours as doubling the storage, and it is usually cheaper. The declared reverse workflow works the other lever for you — fix the energy available and it returns the outage duration that budget can ride through.

The most useful discipline in outage planning is to refuse, for one calculation, to think about hardware at all. What does the essential circuit draw, and for how many hours must it keep drawing? Multiply the two and the interruption has a price in kilowatt-hours — a price that is deliberately indifferent to where the energy will come from. Battery, generator, a neighbour’s extension lead: the demand is the same for all of them, and mixing the demand question with the supply question is precisely how backup plans come to flatter themselves.

The load figure deserves the same scepticism a solar sizing gives its meter reading. The honest entry is the time-averaged draw of the circuit that will actually run — from a meter, an energy monitor, or a careful census of what stays on — not the sum of every rating plate in the house. A rating plate describes an appliance at full song; an outage evening mostly contains appliances idling, cycling, or switched off because somebody decided the toaster could wait.

The hours figure has an honest source too, and it is not optimism. Utility outage history — how long interruptions in this place actually last — beats a guess in either direction. Planning for a shorter outage than the grid delivers is the obvious failure; planning for a much longer one buys storage that spends its life waiting for a storm that never comes.

Setting the bill against the energy on hand gives the calculation its verdict, and the verdict is deliberately signed. A positive margin is slack: room for the outage to run long, the load to creep, the bank to be colder than its rating assumed. A negative margin is a schedule for the lights going out — the bank empties before the grid returns, and everything after that moment runs on nothing. The pack ships a declared warning that fires on exactly that shortfall, because a failure discovered on paper is a failure that can still be fixed.

The sign matters more than the size, and it is the part intuition rounds away. A plan that “roughly covers” an outage is a plan whose margin nobody computed; the whole point of doing the multiplication is that the answer is allowed to come back negative, and a negative answer is not a defeat but a diagnosis. It says, in energy terms, exactly how far short the plan falls — which is the number a generator, a bigger bank, or a leaner circuit must make up.

A thin positive margin deserves almost as much respect as a negative one. Everything in this calculation leans slightly kind: the load is assumed constant, nothing derates the bank, and the conversion losses of the discharge path are carried by a sibling calculator rather than deducted here. A margin that barely clears zero on kind arithmetic is a plan holding its breath.

The capacity term is where passing plans are quietly manufactured. A battery’s label states what the cells hold; what the plan may spend is that figure discounted by the depth of discharge — the fraction the chemistry tolerates giving up without paying for it in lifetime. Entering the label figure overstates the margin by the whole held-back fraction, which for some chemistries is half the battery, and it turns a failing plan into a passing one without changing anything in the real world.

This pack’s battery cluster computes the usable figure properly, from the nameplate and the chemistry’s depth of discharge, and that output is exactly what belongs here. The flattery is not always deliberate — a label is simply the number closest to hand — but the margin is judged against whatever is entered, so any kindness in the capacity term propagates straight through to the verdict.

The bill grows in a straight line with both of its inputs, and that shape is worth internalising because it hands the planner two levers of exactly equal power. Halving the circuit buys the same hours as doubling the storage — and shedding load is usually the cheaper move by a wide margin, because a leaner circuit is decided once at the breaker panel while a larger bank is bought, installed, and eventually replaced.

The calculator makes the comparison honest by being rerunnable: price the outage with the full essential circuit, then again with each candidate for shedding, and watch the margin change sign. The declared reverse workflow works the other lever for you — fix the energy available and it returns the outage duration that budget can ride through, which is often the more natural question when the bank is already bought and bolted to the wall.

The calculator below is the same certified engine the calculator page runs — fetched, verified and mounted mid-lesson. It arrives pre-filled with the pack’s own anchor: a modest essential circuit — refrigeration, a router, some lights — riding through an overnight outage with a bank standing behind it. Change the load to your own circuit’s draw and the hours to your utility’s honest history, and watch the margin move; then push the load or the hours until the sign flips, and note how little flattery that takes.

Read the bill as the demand the outage presents regardless of supply, and the margin as the verdict — its sign first, its size second. Every figure shown is computed by the verified engine as you type; nothing on this page stores an answer.