🔌 Inverter Sizing

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 1 independent reference calculation · how PhDino checks its numbers

Sizing an inverter for both its steady running load and the momentary surge a motor draws on startup.

An inverter converts the DC power stored in a battery bank into the AC power household appliances actually use, and it needs to be rated for two different numbers, not one. Its continuous rating has to cover everything running at once, steadily, for as long as it's on. Its surge (or peak) rating has to cover the much larger, momentary current spike that motor-driven appliances draw the instant they start.

That startup spike is a real physical phenomenon, not a safety margin — an electric motor at standstill has far less internal resistance to its own current than one already spinning, so it briefly draws several times its running current before it gets up to speed. An inverter that's only sized for steady-state running loads can trip or shut down the moment a compressor or pump tries to start, even though it would have handled that same load just fine once running.

Key formula

Continuous rating needed = total continuous load × (1 + safety margin)
Surge rating needed = (continuous load − largest motor's running watts) + largest motor's running watts × surge multiplier

How to use the Inverter Sizing calculator

Use this to find the two ratings an inverter must meet: the continuous power it can deliver all day, and the surge power it can deliver for the few seconds when a motor starts. Both matter, and an inverter that satisfies one and not the other trips or shuts down the first time the well pump starts.

Enter the total power of the loads that can run at the same time, not the sum of everything you own, plus the biggest motor and how strongly it surges on starting. The margin is a share added on top for safety and future growth.

Total Continuous Load (W)

The combined running watts of everything that can be on at once. Loads that never overlap, such as a kettle and an oven you use at different times, do not add.

Largest Motor — Running Watts (W)

The running watts of the single biggest motor or compressor: a well pump, a refrigerator compressor, a power tool. Its starting surge is what stresses the inverter.

Motor Starting Surge Multiplier

How many times its running watts the motor draws while starting. Refrigerator compressors are often around 3, pumps and air-conditioner compressors from 3 to 7, and induction motors in general several times; the nameplate or data sheet has the actual value.

Safety Margin (%)

A percentage added to the continuous rating for safety, inverter aging and load growth. Twenty to twenty-five percent is common.

Worked example: the cabin's inverter

At its busiest the cabin runs 2,400 W of loads together: a refrigerator, lights, a well pump, a laptop and a small microwave. The well pump is the largest motor at 750 W running and surges to about 3 times that on starting. A 25% margin is wanted. Which inverter rating is needed?

Worked by hand:

1. Continuous rating. Add the margin to the running load: 2,400 W × (1 + 25%) = 3,000 W.
2. Surge on starting. While the pump starts, the other loads keep running, and only the pump's own watts are replaced by its starting surge: 1,650 W + 750 W × 3 = 3,900 W.
3. Battery current. At 92% inverter efficiency, 3,000 W on a 48 V bank draws about 67.9 A from the batteries.

A 3,000 W inverter with a surge rating of at least 3,900 W covers the cabin; in practice a 3 kW model with about 6 kW of surge is a common size. The battery side matters too: the same inverter on a 24 V bank would pull about 136 A, and on 12 V about 272 A, which needs cable and fusing on the scale of welding leads. That is why a 3 kW inverter almost always goes with a 48 V battery bank.

Reading the result: continuous, surge and the battery side

The continuous rating is a thermal limit: the inverter can supply that power indefinitely. The surge rating is an electrical limit that holds for only a moment, usually a few seconds, so it covers motor starting but not a load that stays high. A load that needs the surge rating for longer than that will trip the inverter.

• Sizing the continuous rating to the average draw instead of the peak leaves no headroom, and running an inverter near its limit all day shortens its life.
• Pure sine wave inverters run motors, compressors and electronics without trouble; cheaper modified-sine units can make motors run hot and some equipment misbehave.
• Converting from DC to AC costs a few percent, and an idle inverter draws power just by being on. Both come out of the battery, and both are worth checking for a system that must last through a dark week.
• The battery bank must be able to supply the inverter's peak current. A small bank on a large inverter voltage-sags under load and shuts the inverter down.

Notes & limitations

• The surge calculation only needs to account for the single largest motor starting at a time — in a well-designed system, appliances aren't all switched on simultaneously, so it's the worst one-motor-starting-while-everything-else-runs scenario that actually matters, not every motor surging at once.
• Motor starting surge multipliers vary by motor type — a refrigerator compressor or well pump can spike several times its running wattage, while some newer soft-start or variable-frequency-drive equipment barely surges at all. Checking a specific appliance's actual starting characteristics beats guessing a generic multiplier.

Common mistakes

• Adding up the running watts of every appliance. Only loads that can operate simultaneously count, and over-adding produces an inverter twice the size needed.
• Ignoring surge. An inverter can cope with the average load all day and still trip every time the pump or compressor starts.
• Confusing VA and watts. Some appliances are rated in volt-amperes, which can be higher than watts when the load has a poor power factor.
• Choosing a large inverter for a small battery bank. The battery must be able to deliver the inverter's current, and a bank that cannot will sag and shut down.
• Using 12 V for a large inverter. The current at a few kilowatts is several hundred amperes, which is hard to cable safely.

Frequently asked questions

Papers worth reading

Performance model for grid-connected photovoltaic inverters Boyson, W. E., Galbraith, G. M., King, D. L. & Gonzalez, S. (2007), Sandia National Laboratories (report). Describes the Sandia model of how a grid-connected inverter’s efficiency varies with loading, so a real inverter is not simply rated power in, rated power out.