Continuous vs. surge watts: how to size an inverter correctly
Every inverter has two power ratings, and sizing against only one of them is one of the most common ways a system fails in practice — not from a lack of capacity, but a lack of headroom for the first half-second a motor runs.
Continuous watts is what an inverter can sustain indefinitely; surge watts is the much higher power it can supply for a few seconds when a motor starts. Size the continuous rating to your combined running load with headroom, and the surge rating to your largest motor's startup draw.
Continuous rating: what it can run indefinitely
The continuous (or "running") wattage is what the inverter can sustain for as long as needed without overheating or shutting down. Add up the simultaneous AC wattage of everything that might run at once — not everything you own, just what's plausibly on together — and the inverter's continuous rating needs headroom above that total, commonly around 20%, to avoid running it at its thermal edge constantly.
Surge rating: what it can handle for a few seconds
Motors — compressors (fridges, air conditioners), pumps, power tools — draw a brief but large surge at startup, often 2–7x their running wattage, lasting anywhere from a fraction of a second to a couple of seconds while the motor spins up. The inverter's surge rating is what it can supply during that window without faulting. An inverter with plenty of continuous headroom but an undersized surge rating will still shut down or trip the instant that compressor kicks on — a confusing failure because the inverter "should" have enough power on paper.
Which loads actually surge
Anything with a motor: refrigerator and freezer compressors, well pumps, sump pumps, air conditioners, power tools, some microwaves. Resistive loads — heaters, incandescent and most LED lighting, electronics chargers — draw essentially flat power with no meaningful surge. If your load list has more than one motor that could start at the same moment (a fridge compressor cycling on while a well pump is also running, for instance), size surge against the sum of the worst plausible overlap, not just the single largest motor.
Sizing it
List every AC load's continuous watts and, for motor loads specifically, its surge/starting watts (often on the nameplate or datasheet as "LRA" — locked rotor amps — for compressors). Sum continuous watts for everything that runs together for the continuous rating; take the largest single surge event (or worst-case overlap) for the surge rating. The inverter sizing calculator does both calculations from your device list.
Related tools
Sources & standards
- UL 1741 — inverter safety and interconnection standard.
- NFPA 70 (NEC) Articles 690 & 706 — PV source circuits and energy storage systems.
- IEEE 1013 — Recommended Practice for Sizing Lead-Acid Batteries for Stand-Alone Photovoltaic Systems — general sizing-practice reference.
Educational content, not an electrical design. Verify with your equipment's datasheet and a licensed installer for anything permanently wired. See the methodology.