⚙️ Inverter Size Calculator
Enter the total watts of everything running at once and your largest startup surge to size the inverter's continuous and surge ratings.
Minimum continuous rating
1,875 W
Minimum surge rating
3,500 W
The inverter's continuous rating must cover all loads running at once, plus headroom (~25%) so it isn't maxed out; its surge (peak) rating must cover the brief startup spike of your largest motor. Motors — fridges, pumps, power tools — draw roughly 2–3× their running watts for a moment on startup (some compressors more), so check that figure on the nameplate. Add up only what runs simultaneously, not everything you own. 🔒 In your browser.
How the inverter size calculator works
An inverter turns battery DC into household AC, and it needs two ratings big enough for your loads: a continuous rating for everything running at the same time (plus headroom so it isn't constantly maxed out), and a surge rating for the brief spike when a motor starts. The tool multiplies your running watts by a headroom factor for the continuous size and takes the larger of that or your surge figure for the peak.
Add up only what actually runs simultaneously, not every device you own — an inverter sized for the whole house when you never run it all at once just wastes money. The surge matters for motors: fridges, pumps, compressors and power tools draw roughly 2–3× their running watts for a fraction of a second on startup, and the inverter's surge rating must cover the biggest one while other loads run.
Frequently asked questions
What size inverter do I need?
A continuous rating at least 25% above the total watts of everything running at once, and a surge rating that covers your largest motor's startup. For 1,500 W of simultaneous load that's about a 1,875 W continuous inverter, with surge headroom above that.
What is inverter surge?
The brief peak power an inverter can supply for a moment — needed because motors draw far more than their running watts when they start (roughly 2–3×). A fridge that runs at 150 W might surge to 400–600 W for a fraction of a second.
Do I add up all my appliances?
Only the ones that run at the same time. Size the inverter for your realistic simultaneous load, not the sum of everything you own — you rarely run the microwave, kettle and vacuum together, and sizing for that is wasteful.
Why include headroom?
Running an inverter permanently near its limit stresses it and cuts efficiency and lifespan. A ~25% margin lets it run comfortably and leaves room for a small extra load, without oversizing (which wastes idle power).
What happens if the inverter is too small?
It overloads and shuts down (or trips) when demand exceeds its rating — often right when a motor starts and the surge exceeds its peak. Sizing the continuous and surge ratings correctly avoids nuisance shutdowns.