How the calculations work
Every number this site shows comes from one of five formulas below. No hidden fudge factors — where we simplify, it's stated plainly, with the same worked example running through all five so you can see how one household's numbers flow from page to page.
The running example
Three appliances, used throughout this page: refrigerator (700W running, 2,200W starting, runs 24h/day, battery-suitable), LED TV (100W running, 100W starting, runs 4h/day, battery-suitable), and sump pump (1,050W running, 2,150W starting, runs 2h/day, notbattery-suitable — pumps need surge capacity most inverters don't have).
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.
Surge: fridge 2,200−700=1,500 · TV 100−100=0 · pump 2,150−1,050=1,100 → largest is the fridge's 1,500W
Peak = 1,850 + 1,500 = 3,350 W
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.
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.
TV: 100 × 4h = 400 Wh
Pump: 1,050 × 2h = 2,100 Wh
Total = 16,800 + 400 + 2,100 = 19,300 Wh (19.3 kWh/day)
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.
Capacity for a 12-hour outage = 800 × 12 = 9,600 Wh (9.6 kWh)
Recommended, with 20% buffer: 9,600 × 1.2 = 11,520 Wh (11.52 kWh)
Shown on Battery as “Battery capacity,” with a second “Recommended, with buffer” figure underneath — the raw number above stays honest math; 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.
At 4.5 peak sun hours/day: 17,200 ÷ (4.5 × 0.75) = 17,200 ÷ 3.375 = 5,096W
Rounded up to the nearest 10W = 5,100 W (5.1 kW)
Recommended, with 20% buffer: 5,100 × 1.2 = 6,120W, already a multiple of 10 = 6,120 W (6.12 kW)
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 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.
Peak-hour energy = 19,300 × 0.208 = 4,021 Wh · Off-peak energy = 19,300 × 0.792 = 15,279 Wh
With sliders at 0% / 0% (grid only):
Cost = (4,021 ÷ 1000 × $0.32) + (15,279 ÷ 1000 × $0.14) = $1.29 + $2.14 = $3.43/day
With 30% solar/battery coverage and 50% peak shaving:
Self-supplied = 30% of both buckets → 1,206 Wh (peak) + 4,584 Wh (off-peak) = 5,790 Wh, free
Remaining peak = 4,021 − 1,206 = 2,815 Wh → 50% shifted to battery = 1,407 Wh (billed off-peak), 1,407 Wh stays at the peak rate
Cost = (1,407 ÷ 1000 × $0.32) + ((15,279 − 4,584 + 1,407) ÷ 1000 × $0.14) = $0.45 + $1.69 = $2.14/day
Savings = $1.28/day (~$38/month) — using the unrounded figures above, not $3.43 minus $2.14, since rounding each first and then subtracting loses a little precision.
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.