How the calculations work

1. Running & peak watts

Running wattsis the sum of every selected appliance's continuous draw × quantity — it assumes everything could be running at the same time, which is the safe-side number for sizing an inverter or generator. Peak wattsadds the single largest startup surge on top (starting watts − running watts, for whichever appliance has the biggest gap). Surges aren't summed across appliances, because in practice motors don't all start in the same instant — only one typically does.

Used directly on Generators, Battery, and Solar as “Running/Continuous” and “Peak (surge).” The generator page also shows a shop-forfigure: running watts ÷ 80%, rounded up to the nearest 50W — installers size to about 80% of a generator's rating so it isn't run flat-out, which sags voltage and shortens its life. That's already a built-in ~25% safety margin, which is why generator sizing doesn't get the separate 20% buffer described in sections 3 and 4 below — it would just be a second, differently-derived number competing with the first.

The generator and battery pages each have a soft starter installed?checkbox for every AC/heat-pump compressor and well/sump pump in your current selection — the single-phase motor loads these devices actually target — right next to the real soft-starter products you could buy. Checking it for a specific unit cuts that unit's own surge by 67%, the midpoint of the ~65-70% reduction soft-start devices like the Micro-Air EasyStart and SoftStartRV commonly claim, before the largest-surge math runs, and shows it side by side with a without-your-soft-starters baseline, never in place of it — the baseline still matters for whichever units don't have one installed.

Starting-watts figures for compressor/pump/motor-class appliances now come from real published or professionally-cited nameplate locked-rotor-amp (LRA) ratings where a manufacturer or spec sheet actually publishes one — a central AC compressor and submersible well/sump pump motors are the clearest cases, and the real numbers run far higher than a simple running-watts multiplier ever suggested. Where no solid LRA data exists for a class (window ACs, inverter-driven mini-splits, several small pump types), the figure stays an estimate and says so plainly on that appliance's own page rather than implying precision the math doesn't have. The measured figures, the multiplier they replace, and how to read LRA off your own nameplate are all laid out on the starting-watts reference.

2. Daily energy (kWh)

Running and peak watts assume everything runs simultaneously — useful for sizing an inverter, but wrong for estimating actual daily energy use, since a fridge running 24 hours a day and a garage door opener running a few minutes a day aren't the same load over a day. Daily energy uses each appliance's own hours/day instead: running watts × quantity × hours/day, summed across everything selected.

This is what feeds the battery capacity and solar array formulas below — and, on the House calculator, the electricity cost estimate.

3. Battery capacity

Battery sizing answers a different question than daily energy: not “how much do I use in a day,” but “how much do I need stored for a specific outage of length D.” It's running watts (battery-suitable items only) × the backup duration you choose — assuming a steady draw for the whole outage, which is conservative, since real cyclic loads like a fridge compressor don't run 100% of the time.

Shown on Battery as “Battery capacity,” with a second “Recommended, with buffer” figure underneath — the raw number above is the math with no buffer applied; the buffered one is what to actually shop for, since real capacity fades with battery age and cold weather, and most power stations shouldn't be run to empty. Note this uses running watts, not daily energy — an outage duration isn't the same thing as an appliance's normal daily runtime.

4. Solar array sizing

The array has to generate a full day's energy within its sun-hours window, whether that energy is used directly during the day or drawn from the battery after dark — so the input is daily energy (kWh), not just an overnight recharge figure. Formula: daily energy ÷ (peak sun hours × 75% system efficiency), rounded up to the nearest 10W. The 75% factor covers panel angle, temperature derating, and inverter/ MPPT conversion losses between a panel's rated wattage and what actually reaches the load.

Shown on Solar as “Solar array you need,” with a second “Recommended, with buffer” figure underneath, same reasoning as battery capacity above — panels degrade a little every year, real installs rarely land at the ideal angle, and dust/ soiling costs output between cleanings. Peak sun hours come from NASA POWER climatology data per country, with Annual/Winter/Summer variants — winter is the conservative sizing target at higher latitudes, where the seasonal swing can be 5x.

5. Electricity cost & the solar/battery mix

The House calculator estimates a daily/monthly electricity bill from your daily energy, a peak and off-peak $/kWh rate, and how many of the 24 hours are billed at the peak rate. Two sliders let you model self-generation: solar/battery coverage assumes that % of your daily energy is free, covered directly by solar or a battery instead of bought from the grid — applied evenly to both peak- and off-peak-hour usage. Peak shaving via battery then takes whatever's left in the peak-hour bucket after self-supply, and shifts that % of it onto a battery that was charged during cheaper off-peak hours — so it bills at the off-peak rate (or the night rate, see below) instead of the peak rate. Both are simplifications (real usage isn't spread evenly across the day, and self-supply isn't literally a flat share of every hour), stated here rather than hidden as false precision. An optional night ratecarves a third, usually-cheaper window out of the off-peak hours — when it's enabled, its own $/kWh rate and hours/day work exactly like the off-peak bucket above, just billed separately, and peak-shifted energy bills at whichever of the two is cheaper to charge from (the night rate, since that's the point of having one). Once the night rate is on, each appliance also gets a Runs at nightcheckbox — checking it pulls up to a night window's worth of that appliance's daily energy out of the flat spread above and bills it at the night rate, for things actually scheduled overnight (an EV charger, a delay-start dishwasher). If the appliance runs more hours/day than the night window is long — a fridge running 24h/day, say — only the hours that fit get the night rate; the rest still goes through the normal spread, so a 24h/day appliance isn't treated as if it ran only at night.

Why generator, battery, and solar show different numbers

All three read the same appliance list from the House calculator, but each applies its own filter before running the formulas above. Every appliance carries a fixed tier (Required, Recommended, or Luxury — a fridge is always Required, a table saw is always Luxury) and a fixed battery-suitableflag. The generator page counts all three tiers by default, since generators have the most headroom. Battery and solar default to Required + Recommended, and always drop anything that isn't realistic to run off an inverter — pumps, central AC, other big motors — regardless of tier. Each page shows toggleable chips (with an eligible-item count on each) so you can widen or narrow that filter without leaving the page.

That's why a battery/solar page might say “7 of 11 House appliances count here” instead of matching your full House total — the gap is always spelled out right below it.