Key takeaways
- Continuous watts needed = running watts × (1 + margin ÷ 100).
- Surge rating must cover your largest motor's startup, not just the running load.
- Add a ~20–25% safety margin so the inverter never runs at its limit.
- 1,500 W running + 25% = 1,875 W continuous → a 2,000 W inverter, with surge ≥ 2,200 W.
How to size an inverter
An inverter has two numbers that matter: continuous watts, which it can deliver all day, and surge (peak) watts, a brief burst for motors that spike on startup. Size the continuous rating to your steady load plus a margin, then make sure the surge rating covers your single worst startup. Picking only on continuous watts is the most common mistake — a pump or AC can stall an undersized inverter the instant its motor kicks in.
The safety margin is the lever most people underrate: running an inverter near 100% of its rating all day shortens its life and trips it on small spikes. A 20–25% cushion keeps it cool, efficient, and ready for loads you forgot to add up.
Worked example: 1,500 W running, 2,200 W surge, 25% margin
Continuous needed = 1,500 × 1.25 = 1,875 W. The next common inverter size at or above that is 2,000 W. Your largest startup is 2,200 W, so confirm the chosen inverter's surge rating is ≥ 2,200 W (most 2,000 W units surge to 4,000 W, so it clears comfortably). The result is a 2,000 W continuous inverter.
Common appliance running vs starting watts
| Appliance | Running watts | Starting / surge watts |
|---|---|---|
| Refrigerator | 150 W | 1,200 W |
| Well pump | 1,000 W | 3,000 W |
| Microwave | 1,000 W | 1,000 W |
| Air conditioner | 1,500 W | 4,500 W |
Start with the load, then check the battery
Every sizing job starts from an honest running total — add up your appliances with the off-grid load calculator first. Once the inverter is sized, make sure your storage can feed it under load with the battery bank sizing calculator. Motors with high surge set the inverter's peak rating, so always size around your single worst startup.
Frequently asked questions
What size inverter do I need?
Total your running watts, add ~25%, and pick the next common size up. 1,500 W + 25% = 1,875 W, so a 2,000 W inverter — if its surge also covers your largest startup.
What is the difference between continuous and surge (peak) watts?
Continuous is the power it can deliver indefinitely; surge is a brief burst for motors that spike on startup. Size continuous to the steady load, surge to the worst startup.
Why should I add a safety margin?
A 20–25% cushion keeps the inverter off its limit, covers forgotten loads, and improves efficiency and lifespan. Running flat-out all day shortens its life.
Pure sine vs modified sine — which inverter?
Pure sine produces clean power that runs anything; modified sine is cheaper but can buzz, heat motors, and harm sensitive electronics. Pure sine is usually worth it.
Will my inverter run a well pump or air conditioner?
Only if its surge rating covers the startup spike — a pump or AC can pull 3–5× its running watts for a second or two. Check the surge number, not just continuous watts.
Does a bigger inverter draw more from the battery?
Draw is set by the load you run, not the inverter's size — a 3,000 W unit on a 200 W load still draws ~200 W, plus a few extra idle watts.
Continuous and surge rating guidance follows inverter manufacturer data — see Victron inverter guidance. The watt and margin relationships here are exact arithmetic.
Last reviewed June 2026