How to size a backup power system
Almost every bad backup-power purchase comes from starting at the wrong end: picking an inverter off a shop shelf, then hoping it carries the house. It works the other way around. You start with what you actually run, and every other number follows from it. That is the order these tools are built in.
1. Work out what you run
Add up the running watts of everything that has to be on at the same time. That total is your continuous load, and it sets the size of the inverter. Separately, add up how many hours each appliance runs in a day to get your daily energy in kilowatt-hours, which is what sizes a battery bank and any solar. Two different numbers, two different jobs, and mixing them up is the most common sizing mistake there is.
2. Size the inverter for the surge, not just the load
Anything with a motor, a fridge, a water pump, an air conditioner, pulls three to five times its running watts for a second or two when it starts. An inverter sized only for the running total will trip every time that compressor kicks in. You also have to convert watts to VA, the unit inverters are actually sold in, by dividing by a power factor of about 0.8. A 1,000 VA inverter delivers roughly 800 usable watts, not 1,000.
3. Size the battery for the hours you need
The inverter decides how much you can run. The battery decides how long. The catch is that you never get the number on the label: lead-acid and tubular banks give up about half their capacity safely, lithium around 90 percent, and the inverter loses a little more in conversion. Real backup time is calculated from usable capacity, not rated capacity.
4. Get the cable and the protection right
The battery-to-inverter cable is the highest-current run in the system and the one most often undersized. It has to satisfy two separate checks, carrying 125 percent of the continuous current and holding voltage drop under about 2 percent over its length, and you take whichever demands the thicker copper. Add a correctly rated DC fuse at the battery, and never tie an inverter into house wiring without a proper transfer switch.
Every figure on this site comes from that same arithmetic, applied the way a licensed engineer would apply it on a paid job. The calculators do the math; the guides explain why it works; and the field notes show it on real installations, with the actual hardware named.
