How the calculation works
An inverter has two ratings that matter: continuous watts (what it can deliver all day) and surge watts (what it can deliver for a second or two while a motor starts). Sizing means satisfying both. The continuous requirement is your simultaneous load total with about 25% headroom; the surge requirement is your steady total plus the start-up spike of the largest motorized appliance.
Motors, compressors and pumps draw two to three times their running watts for the instant they start, a 600 W fridge compressor can spike to 1,800 W. Resistive loads like kettles, heaters and incandescent lights have no spike at all. This is why the calculator asks about your largest load specifically: one compressor changes the surge requirement of the whole system.
Worked example
A cabin might run, at the same moment: a 600 W fridge, a 300 W laptop-and-lighting circuit, and a 300 W fan, 1,200 W simultaneous. The fridge is the largest load and has a compressor (3× start factor):
The purchase decision: a 2,000 W-class inverter, which typically carries a 4,000 W surge rating, comfortably above both numbers. Buying exactly 1,500 W would work on paper but leaves no room for the day you add one more appliance.
Pure sine wave or modified sine wave
Pure sine wave inverters reproduce grid-quality power; modified sine wave inverters approximate it with a stepped wave. The price gap has narrowed enough that pure sine is now the default recommendation: compressor motors run hotter and less efficiently on modified sine, some electronics buzz or misbehave, and induction cooktops, CPAP machines and variable-speed tools often refuse to run at all. Choose modified sine only for simple resistive loads on a tight budget.
Common mistakes
- Sizing from the sum of everything you own instead of what actually runs simultaneously. You do not microwave dinner while running a table saw. Oversizing the inverter wastes money and quietly burns more idle power around the clock.
- Ignoring idle draw. Inverters consume power just being on: 10–25 W for small units, 50 W+ for large ones. A 3,000 W inverter idling all day can consume 1 kWh, a meaningful slice of a small system's budget. Size to your real need, and use the inverter's standby mode if it has one.
- Forgetting the DC cable run. A 2,000 W inverter on a 12 V bank pulls up to ~170 A. That current demands short, thick cables and a properly rated fuse. If the cable math surprises you, it is often a sign the system voltage should be 24 V or 48 V.
- Trusting surge ratings blindly. Some budget inverters advertise surge figures they can hold for milliseconds only. Compressor starts need the surge held for a second or more, a reason to favor established brands for motor-heavy systems.
Frequently asked questions
What size inverter do I need for a fridge?
A typical 12 V-compatible or compact AC fridge runs 100–800 W but surges to roughly three times that at compressor start. For a fridge alone, a quality 1,000–1,500 W pure sine inverter is the usual answer; add your other simultaneous loads on top using the calculator above.
Can an inverter be too big?
Functionally no, economically yes. The oversize costs you twice: purchase price and permanent idle draw. A 5,000 W inverter serving 800 W of real load wastes battery capacity every hour it is switched on. Match the inverter class to your realistic peak.
Does inverter size affect battery bank size?
Indirectly. The inverter sets your maximum instantaneous draw, which determines the discharge current the bank must supply, relevant for battery C-ratings and cabling, but the bank's amp-hour size comes from daily energy use, not peak power. Size energy with the battery bank calculator, power with this one.