How the calculators work together
Sizing a backup power system is a few questions in a row, and each of these tools answers one of them. Used in order, they take you from a pile of appliances to a specific inverter, battery bank, expected runtime, and the cable to wire it, all from your own numbers rather than a salesperson's.
1. Find your load and energy
Start with the appliance wattage calculator if you are not sure what your appliances draw. Set how many of each you run and for how long, and it returns two numbers that drive everything else: your total load in watts (what has to be on at once) and your daily energy use in kilowatt-hours (how much you consume over a day). The first sizes the inverter; the second sizes the battery and any solar.
2. Size the inverter
Take your load into the inverter size calculator. It adds the startup surge of your largest motor, converts watts to the VA rating inverters are actually sold in, applies a safety margin, and returns a recommended inverter size and the battery voltage (12V, 24V, or 48V) that suits it. This is the step that stops an inverter from tripping every time the fridge or pump kicks in.
3. Check how long the battery lasts
Finally, the battery backup time calculator takes a battery bank and your load and returns a realistic runtime, after accounting for the depth of discharge you can safely use and normal inverter losses. If the hours fall short, you add capacity or trim the load, not the inverter.
4. Size the DC cable
Once you know your system voltage and current, the DC cable size calculator works out the copper cable between the battery and inverter, checking both the 125% ampacity rule and a 2% voltage drop over your run length, then recommending the larger of the two. That battery cable is the highest-current, highest-risk run in the whole system, so it is worth getting right.
Every tool here is free, runs entirely in your browser, and stores nothing. The figures are solid planning estimates built on standard engineering math, but they are not a substitute for the rating plate on your own equipment or a licensed electrician's inspection before you buy or install anything.