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Inverter sizing

Inverter continuous rating and surge rating for your loads. Sized wrong, you trip the inverter or burn it out.

Inverter sizing
The most watts you'll have running at once. Add up the AC loads that could overlap (grow lights + pump + heater + ...).
Used to calculate startup surge. Skip if you have no pumps or motors.
How much current the motor pulls at startup vs running. The label on a soft-start pump tells you. Worst-case is 5-7x for old induction pumps.
Continuous rating (min) -- watts
Surge rating (min) -- watts
Practical inverter size --
Notes --
Buy pure sine wave, not modified sine wave. The price gap has closed and modified sine wave damages motors, pumps, and sensitive electronics over time.

Two numbers matter

Inverters have two power ratings:

  • Continuous is how much they can run forever. This needs to cover the sum of everything that might be on at once, with margin.
  • Surge is how much they can handle for a few seconds during startup spikes. Critical for pumps, compressors, and anything with an electric motor that has to overcome inertia.

A 1000W continuous / 2000W surge inverter can run 800W of steady load all day but won't survive starting a 1500W well pump because the pump pulls 4500W for a half-second on startup.

Why pumps matter so much

Induction motor pumps (deep well pumps, refrigerator compressors, some pond pumps) draw 3-7x their running wattage for the first half-second of startup. A 100W pump can spike to 400-700W. Most off-grid inverter failures are surge-related, not continuous-load related.

Brushless DC pumps and modern soft-start variable-speed pumps don't have this problem. If you have the option, pick the soft-start version.

The calc

Continuous rating sized for the sum of all simultaneous loads plus 25% margin:

continuous_rating = sum_of_simultaneous_W × 1.25

Surge rating sized for the biggest motor load times its surge multiplier:

surge_rating = biggest_motor_W × surge_multiplier

Required inverter is the larger of the two. Round up to the nearest practical inverter size. The smallest practical pure sine wave inverters are around 500W; cheaper than that and quality drops fast.

Pure sine wave vs modified sine wave

Buy pure sine wave. Modified sine wave (cheaper) damages sensitive electronics, makes pumps and fans run hot and noisy, and shortens the life of anything with a transformer or motor. The price gap has closed enough that there's no reason to save 30% on the inverter and pay it back in dead equipment.

What this doesn't cover

  • 240V split-phase inverters for whole-home loads (different beast, usually permanent installs)
  • Inverter/charger units (handles both directions, used in hybrid systems)
  • High-frequency vs low-frequency designs (low-frequency tolerates surge better, costs more, weighs more)

Further reading