Inverters that meet this spec
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Assumptions used
Continuous = simultaneous load × diversity × 1.25.
Surge = largest single load × its starting multiplier (3× typical for compressors, up to 4–5× for pumps and air conditioning).
DC amps = continuous W ÷ (battery V × 0.9). Fuse rated 25% above that, rounded up to a standard size.
How this calculator sizes an inverter
An inverter has two ratings and they are sized against different things. Continuous rating must cover everything running at once. Surge rating must cover the instant a motor or compressor starts.
Surge is what actually trips inverters
Induction motors draw several times their running current while they come up to speed. A fridge compressor rated 45 W can pull 135 W or more for a fraction of a second. A half- horsepower well pump running at 750 W can spike past 3,000 W. If the inverter cannot supply that instant, it shuts down — and it will do so on a unit whose continuous rating looks comfortably adequate.
| Load type | Typical starting multiplier |
|---|---|
| Resistive — kettle, toaster, heater | 1× to 1.2× |
| Electronics — laptop, TV, router | 1× to 1.5× |
| Fridge and freezer compressors | 3× |
| Water pumps, power tools | 3× |
| Air conditioning, well pumps | 4× to 5× |
Bigger is not safer
Every inverter consumes power simply being switched on. A large unit can idle at 20–30 W, which over a day is 480–720 Wh — a substantial share of a small bank, spent on nothing. If your heavy loads are occasional, a smaller inverter plus some discipline about when you boil the kettle will save you battery every single day of the year.
The DC side decides your cable and fuse
DC current = continuous watts ÷ (battery voltage × 0.9). A 2,400 W inverter on 12 V pulls around 220 A. The same inverter on 48 V pulls 55 A. That factor of four is the difference between heavy welding cable with expensive lugs and something you can route comfortably.
The fuse protects the cable, not the inverter. Rate it above the inverter's maximum draw but within the ampacity of the conductor it guards, and fit it as close to the battery positive terminal as you practically can.
Common questions
What size inverter do I need for off-grid?
Size the continuous rating to the loads that could run simultaneously, plus about 25% headroom, then check the surge rating against your largest motor load. A van running lights, a fridge, a laptop and occasionally an induction hob typically lands around 2,000–2,400 W continuous. Avoid buying far larger than you need — idle consumption is a daily cost.
What is inverter surge rating and why does it matter?
Surge is the power an inverter can supply for a brief period, usually a few seconds, above its continuous rating. It matters because motors and compressors draw three to five times their running wattage at startup. An inverter with adequate continuous capacity but weak surge will shut down every time the fridge or pump kicks in.
Do I need a pure sine wave inverter?
For anything with a motor, a compressor, mains-charged electronics, or medical equipment such as a CPAP, yes. Modified sine wave runs resistive loads acceptably but causes motors to run hot and inefficiently, produces audible hum in audio equipment, and some devices refuse it outright. The price gap no longer justifies the risk.
How much power does an inverter use when idle?
Typically 5–30 W depending on size, drawn continuously whenever it is switched on. Over 24 hours a 25 W idle draw is 600 Wh — for a small van bank that can be a third of daily consumption. Many inverters offer a search or standby mode that reduces this substantially when no load is detected.
What size cable and fuse does my inverter need?
Both follow from DC current, which is continuous watts divided by battery voltage times about 0.9 for efficiency. Size the cable for that current over its full run length including voltage drop, then fuse to protect that cable — not to match the inverter. Always verify against the wiring regulations that apply where you are before installing.
These figures are planning estimates. Electrical installation carries real risk of fire and injury — have your design checked against the regulations that apply where you are, by someone qualified.