An inverter is sized from the alternating-current loads it must supply, not from the watts printed on the solar panels. Add the loads that will run at the same time. Then look separately at anything with a motor, because the moment it starts can ask for several times its running watts for a short time. The inverter’s continuous rating has to cover the simultaneous running load. Its surge rating has to cover the start, for the duration the inverter actually allows.
A hypothetical list, not a house
Suppose, only as arithmetic, a fridge nameplate of 150 W running, lights of 80 W, a router and a laptop of 100 W, and a well pump of 800 W running with a start the pump maker lists at 2,400 W. The simultaneous running load without the pump is 330 W. With the pump running it is 1,130 W. The start is 2,400 W plus the other 330 W if those stay on. None of these figures is a survey, a code minimum, or a quote. Your nameplates replace them.
If the inverter surge is “2,400 W for 5 seconds” and the pump needs longer, the pump does not start, even though the continuous number looked fine. How those two ratings get misread is in solar inverter overload. Off-grid sizing, where there is no utility to hide behind, is in how to size an off-grid inverter.
| Add this | Do not add this |
|---|---|
| Nameplate running watts of loads that overlap | The solar array’s STC watts, as if they were the inverter size |
| The highest start among motors that might start together | A forum multiple such as “times three” when the motor sheet is silent |
| A margin the inverter manual asks for | Every appliance in the house if a backup design only transferred some of them |
Backup and a whole house are different sums
A backup system supplies the circuits on the load side of the transfer. The electric range stays in the sum only if it was transferred. The method for that list is in sizing backup for critical loads. A grid-tied inverter that cannot form a supply does not get sized by this backup sum at all. It has to follow the utility, and the utility is the voltage source.
Diversity is real: people do not run every load every minute. Diversity is also how undersized inverters get sold. Write the overlap you actually refuse to shed. If you will not turn the pump off while the kettle boils, both watts belong in the same minute.
Nameplates beat app guesses
A sticker on the appliance is stronger than a memory of what a similar fridge used, and stronger than a phone app that never saw this motor. If the nameplate is gone, the manufacturer’s sheet for that model is the next source. A multiple such as “times three for start” is a last resort, and it is not a measurement. Write “estimated” next to that line so the next person does not treat it as a reading.
Inverter size and array size remain different sums. A larger inverter does not make the roof produce more energy. How to choose between two kilowatt ratings, once this list exists, is in how to choose between inverter sizes. If the list is still a wish, do not pick a favorite kilowatt yet.
