Inverter Sizing Calculator

Sizing an inverter looks like a single-number problem and is not. Continuous watts is the obvious constraint: the inverter has to sustain whatever runs at once, indefinitely, without overheating. Surge watts is the second: motors draw several times their running current for a fraction of a second at startup, and an inverter that cannot supply that will simply refuse to start the load.

The third constraint is DC input current, and it is the one that determines what the installation costs. Power is conserved across an inverter, minus its losses, so a 3,000 watt output at 120 volts AC becomes about 275 amps at 12 volts DC. That current has to travel from the battery to the inverter through copper, protected by a fuse, often through a busbar and a disconnect switch — and every one of those components is priced by the amp.

This is why two people can buy the same inverter and spend wildly different amounts installing it. A 3,000 watt inverter on a 12 volt bank needs 4/0 welding cable, a 400 amp Class T fuse, and lugs that require a hydraulic crimper. The same inverter on a 48 volt bank needs 4 AWG, a 100 amp fuse, and crimps you can do with hand tools.

An induction motor at rest draws far more current than one already spinning, because there is no back-EMF opposing the supply until it moves. The inrush lasts a fraction of a second and typically reaches two to three times the running current — more for a compressor starting against pressure.

Inverters are built to handle this: most advertise a surge rating around twice their continuous output for a few seconds, exploiting thermal mass in the transformer and switching devices. What matters is whether that surge rating covers your worst moment.

The worst moment is not every motor starting together, which does not happen. It is the largest motor starting while everything else is already running. Add the running total to the biggest single surge above running, and that is the peak the inverter must supply. Summing every appliance’s starting wattage — as many guides suggest — produces a number roughly twice as large and sells a great many oversized inverters.

Where the surge is genuinely marginal, a soft-start module on the offending motor is usually the cheaper fix. Fitted to an air conditioner compressor it typically cuts inrush by more than half, and frequently brings a system that needed a 5,000 watt inverter inside the reach of a 3,000.

Twelve volts is the default for vans and small RVs for a good reason: the entire recreational vehicle accessory market is built around it. Fridges, fans, pumps, lights, chargers and USB outlets are all available in 12 volt form, often only in 12 volt form.

But the current scales inversely with voltage, and copper is priced by cross-section. At 12 volts a 2,000 watt inverter draws about 185 amps and a 3,000 watt one about 275. Those currents need cable in the 2/0 to 4/0 range, fuses in the hundreds of amps, and terminals that need proper crimping tools. The cable for a short run can cost more than a small inverter.

At 24 volts every one of those currents halves; at 48 volts it quarters. A 3,000 watt inverter at 48 volts draws under 70 amps — ordinary 4 AWG, an ordinary fuse, ordinary lugs. The voltage drop problem largely disappears too, because there is four times as much voltage to lose the same percentage of.

The practical dividing line sits around 2,000 to 3,000 watts of inverter. Below it, 12 volts is usually simpler and the copper cost is tolerable. Above it, the savings on cable, fuses and disconnects generally exceed the cost of a DC-DC converter to feed a 12 volt accessory bus, and the installation is easier to do well.

An inverter switched on and supplying nothing still consumes power. It has to keep its output stage running, its control electronics alive, and in many designs a transformer energised. Typical figures run from around 5 watts for a small unit to 40 watts or more for a large one.

Per hour that is nothing. Per day it is 480 watt-hours for a 20 watt idle draw left on around the clock — 40 amp-hours from a 12 volt bank, which on a 200 amp-hour lithium system is a fifth of the usable capacity consumed by an appliance doing nothing.

This is the single most common cause of the mystery overnight drain that sends people looking for a fault. It is also trivially fixed. Most inverters offer a search or standby mode, which drops to a very low draw and wakes when it detects a load being plugged in — though it can fail to detect very small loads. A remote on/off switch mounted somewhere convenient is the more reliable answer, and the habit of switching the inverter off when the kitchen is not in use is worth forming.

The related design point: anything that can be run natively on DC should be. A 12 volt compressor fridge, DC LED lighting and USB-C outlets all avoid both the conversion loss and the reason to leave the inverter on at all.

Inverters produce alternating current in one of two ways. A pure sine wave inverter synthesises a smooth waveform essentially indistinguishable from grid power. A modified sine wave inverter produces a stepped approximation — a square wave with a pause at zero — which is far simpler and cheaper to build.

Resistive loads do not care. A kettle, a toaster or an incandescent bulb turns any waveform into heat equally well. Everything else does care, to varying degrees.

Motors run hotter and less efficiently on modified sine, because the harmonics in a stepped waveform produce current that does no useful work. Transformers hum audibly. Some battery chargers and switch-mode power supplies misbehave or fail. Audio equipment picks up buzzing. Variable-speed motors and anything with a microprocessor controlling a motor — a modern washing machine, many power tools — can refuse to run at all. Medical equipment such as CPAP machines frequently specifies pure sine.

The price gap that once justified modified sine has largely closed. For a new build there is little reason to accept the compromise, and the failure modes are the kind that show up months later on an expensive appliance.