our local library has a kill a watt to checkout. don’t have to buy. most inverters even the small alligator clip to a batter twelve volt have in and out power reading. you can run one item at a time record the in and out. paying attention to you start-up draw and regular run loads and do the math.
another huge variable in calculation system requirements is capacitors. in well switches and if your using a transformer to flip 120 to 240. usually a capacitor will hold its charge for 2-3 days so to be accurate flip the breaker servicing that load for a couple of days to get a more accurate start up that includes the capacitor’s load too.
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