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Inverter Surge Calculator: What Size Inverter Do You Actually Need?

What inverter rating survives my largest motor load?

1 · The bankGlobal inputs

Drives every DC-side figure.

Sets the loaded terminal voltage used in the DC current calculation.

Seeds the discharge limits below.

100 A per 100 Ah LiFePO4. Editable.

200 A per 100 Ah for 5 s. This decides whether the surge is possible at all.

Advanced inverter efficiency 88% · 5 ft cable run

Passed to the wire gauge calculator, not solved here.

2 · Simultaneous loads

Tick the loads that may be running at the same time — not your daily list. The two get conflated constantly, and the conflation always sizes the inverter too small.

Load Running W Qty Start × Hold
Rooftop air conditioner, no soft start 13,500 BTU class ×5.0 3 s
Rooftop air conditioner, soft start fitted Same unit ×1.5 1 s
Rooftop air conditioner, variable-speed compressor Inverter-driven unit ×1.2 1 s
Domestic fridge or chest freezer Residential compressor ×5.0 3 s
Submersible well pump, ½ hp ×6.0 3 s
Air compressor Pancake or twin-stack ×6.0 3 s
AC water pump Shallow-well jet pump ×4.5 1 s
Washing machine Drum motor ×3.5 1 s
Vacuum or hand tools Universal motor — drill, vacuum ×3.0 0.5 s
Microwave 900W output ≈ 1,300W input ×1.5 0.1 s
Induction hob Single or twin zone ×1.0
Coffee maker Resistive ×1.0
Kettle, toaster or resistance heater Resistive ×1.0
Laptop, TV, chargers Inrush is capacitive — ignore it ×1.0
12V DC air conditioner Never touches the inverter

The ×5.0 on a rooftop unit is not conservative padding. A 13,500 BTU rooftop air conditioner running at 11–16 A commonly carries a locked-rotor rating of 45–62 A. A soft start ramps the compressor rather than slamming it and cuts that inrush by roughly 65–75%, which is where the ×1.5 comes from — not from the marketing.

Result

Inverter required

0W continuous

The surge, and how long it must hold

DC current at the battery

Largest start event

Show your working
This tool sizes an inverter. It does not design an electrical installation. DC overcurrent protection between battery and inverter, conductor ampacity, terminal torque, ventilation, and neutral-to-ground bonding and transfer switching are installation matters, and lithium banks in particular can deliver fault currents that ordinary automotive fuses are not rated to interrupt. Have the installation designed and inspected by someone qualified for it. Nothing here implies compliance with any code or standard.

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