Inverter Sizing for Off-Grid Solar: Continuous Power, Surge, and Idle Draw

The inverter converts DC battery power into the AC power used by standard household appliances. Size it too small and it will shut down when the water pump starts. Size it far too large and it wastes energy every hour it is switched on. This guide explains the three numbers that matter: continuous rating, surge rating, and idle consumption.

Continuous power: what runs at the same time

The inverter's continuous rating must cover all AC loads that can run at the same time, not every appliance you own. List your realistic worst case. In a cabin, that might be the microwave running while the fridge compressor and well pump happen to be on, with lights and internet also running.

Add a safety margin of about 25% so the inverter is not running at its limit, where it gets hot and less efficient:

Continuous rating (W) ≥ Simultaneous AC load (W) × 1.25

The 6th Star calculator takes a deliberately conservative approach: it assumes all listed AC loads could run together, multiplies by 1.35, and rounds up to the next 500 W. If you know some loads never overlap, you can choose a smaller inverter than the calculator suggests.

Watch out for microwave ratings: the advertised power is the cooking output. A "700 W" microwave typically draws 1,000–1,200 W from the outlet.

Surge power: motors and compressors

Induction motors in fridges, freezers, pumps, and power tools draw 3 to 7 times their running power for a fraction of a second while starting. Inverters have a surge rating, often about twice the continuous rating for a few seconds, but this varies a lot between designs:

• Low-frequency (transformer-based) inverters are heavy but handle large motor surges very well, often 3× for several seconds.
• High-frequency inverters are lighter, cheaper, and more efficient at light load, but their surge capacity can be limited to a very short peak.

Check the manufacturer's surge specification and, for well pumps especially, the pump's locked-rotor or starting current.

Worked example

Load	Running (W)	Starting (W)
Microwave (input power)	1,100	1,100
Well pump, 0.75 kW	750	~2,250–3,000
Fridge	150	~600–900
Lights + internet	115	115
Simultaneous total	2,115

2,115 × 1.25 ≈ 2,650 W, so a 3,000 W continuous inverter is the right size. Worst case for surge: the pump starts while everything else is running, about 1,100 + 150 + 115 + 3,000 ≈ 4,400 W for a moment. That is within the roughly 6,000 W surge rating of a typical 3 kW unit, but check the datasheet.

Pure sine vs modified sine

Choose a pure sine wave inverter for an off-grid home, cabin, or van. Modified (stepped) sine inverters are cheaper, but they make motors run hotter and noisier, can damage some chargers and dimmers, cause interference in audio equipment, and some appliances will not work on them at all. The price difference is small compared to the cost of the rest of the system.

Idle consumption: the hidden load

An inverter uses power whenever it is on, even with nothing plugged in. Typical idle draw is 10–40 W for 2–5 kW models. Over 24 hours, a 25 W idle draw is 600 Wh, which is a big share of a small system's daily budget. To reduce it:

• Run constant small loads such as fridges, lights, and routers directly on DC where good 12/24 V versions exist.
• Use an inverter with a power-saving or "search" mode that sleeps until a load is detected.
• Avoid buying an inverter much larger than you need, because larger units generally have higher idle draw.
• Add the idle consumption as a line in your daily energy audit if the inverter will stay on.

DC current: what the battery has to supply

The inverter's AC output comes from the battery as DC current, increased by the inverter's losses:

DC current (A) = AC power (W) ÷ (Battery voltage (V) × Inverter efficiency)

3,000 W load at	DC current (90% efficiency)
12 V	~278 A
24 V	~139 A
48 V	~69 A

At 12 V, a 3 kW inverter needs very heavy cables, large fuses, and a battery able to supply nearly 300 A. This is the main reason larger systems move to 24 V or 48 V. Battery-to-inverter cables must be sized for this current and fused close to the battery. See the wire sizing guide.

Inverter-chargers

If you have a generator or occasional grid access, an inverter-charger combines the inverter, a battery charger, and an automatic transfer switch in one unit. When the generator runs, it passes generator power through to your loads and charges the batteries at the same time. Some models can also add battery power to a small generator during peaks (often called "power assist"), so a small, quiet generator can support large loads.

Finally, set the inverter's low-voltage cut-off to match your battery chemistry, so the inverter shuts down before the battery is damaged.