Inverter and battery: the runtime arithmetic the sticker prices never do for you

The energy conversation in most homes is conducted in the wrong unit: people buy batteries by amp-hours, quote inverter capacity by watts, and then run an arithmetic they were never shown - which is exactly why the setup that was 'supposed to last till midnight' died at ten. Every number you need is printed on the equipment itself, and once the units click, sizing a backup system stops being a faith purchase and becomes a three-line division.

The units, held for one paragraph

Watts are the flow - what each device drinks per hour. Watt-hours are the volume - what the battery actually holds, and they are made from the sticker numbers: volts multiplied by amp-hours (a 12-volt, 100-amp-hour battery is a 1,200 watt-hour tank; the maths is that simple and the sticker is that readable). A household's evening - fans, lights, the router, the television, charging three phones and a laptop - is the same multiplication run backwards: watts of each device times hours of use, summed, and that daily figure is the only honest starting point for buying anything. The desk's standing order: write the load list first, with the label numbers, before the salesman's numbers ever enter the house.

The two percentages nobody states at the shop

The first is the battery's own usable slice. Lead-acid families - tubulars, flooded, AGM - are built to live at shallow drinks: regularly drawing past the half-empty mark is the fast road to plates that no longer charge back, which is why the trade's line is roughly half the sticker capacity for the life you were promised. Lithium (the iron-phosphate class most home inverters now run) tolerates deep draws - its datasheet's percentage, commonly in the eighties and nineties, is the number to use - and it costs the difference in a way that pays in cycles rather than words. Take whichever the spec sheet states and multiply the tank by it: 1,200 amp-hours on paper is 600 in the lead-acid world, and midnight budgeting against the sticker is how batteries die young and owners call the installer a thief. The second percentage is the inverter's appetite for doing nothing: every inverter burns standby watts with no load at all - the no-load figure sits in the manual, and it is a real nightly tax that small systems feel first; switching the inverter off with the last light is the free kilowatt-hour the paperwork already gave you.

The division, with its two honest taxes

Runtime hours = usable watt-hours, times the conversion losses the inverter eats between battery and plug - call the interval ten to fifteen percent, the class every manufacturer's efficiency curve lives in - divided by your load in watts. The fridge paragraph before the verdict: a compressor does not draw its label watts steadily; it runs in bursts with a starting surge several times its running figure, and every inverter carries two numbers - continuous output and surge - with the surge being the one the morning start tests. Anything on the circuit that turns a motor needs the pair of figures checked against the manual's pair, not the salesman's loudest.

The oversize trap and the recharge truth

A tank twice the charger's reach is worse than a tank the right size: charge current is a rate, and a big battery fed by a small charger spends its life half-full - lead-acid left uncharged is the slow death the trade calls sulfation, and the genset-or-solar hours to top a giant bank arrive in the same invoice as the purchase. Size the battery to what one real charging day refills - the alternator, the mains window, the sun - and let a second battery be an expansion you can actually pay in hours. And the solar line the inverter salesmen skip: panels refill the same tank through the same arithmetic; a system sized to the night it must survive needs its daytime source checked against the same watt-hours, or the brochure is describing weather.

The verdict, as a rule

Write the load list, read the four stickers - volts-amp-hours, the usable percentage, the inverter's no-load watts, its continuous and surge pair - do the division, and buy one size larger than the answer only when the charger can top it inside the day you actually have. The homes that 'own the blackout' did exactly this with a pencil; the ones arguing with installers never saw the arithmetic, because nobody was ever allowed to show them the stickers.