Inverter sizing depends on which kind of system you are building, and the two cases work in opposite directions.
Off-grid: size to the load
Continuous rating must exceed everything you might run at once. Add the running watts of your simultaneous loads and add margin — an inverter run continuously at its rating runs hot and dies early. 80% is a sensible ceiling.
Surge rating must cover the inrush of the largest motor. A well pump, a compressor or a fridge draws roughly three times its running watts for a second or two on start.
Count one motor's surge, not all of them. The chance of two large motors starting in the same half-second is small, and sizing for it buys an inverter twice the size you need.
| Load | Running | Surge |
|---|---|---|
| Fridge | 150 W | 600 W |
| Well pump ½ hp | 750 W | 2,300 W |
| Microwave | 1,000 W | 1,000 W |
| Table saw | 1,500 W | 4,500 W |
| Air conditioner 12k BTU | 1,200 W | 3,600 W |
Grid-tied: size below the array, deliberately
A DC-to-AC ratio of about 1.2 is standard — 6 kW of panels on a 5 kW inverter.
That looks like undersizing and it is not. An array reaches its nameplate rating only in ideal conditions for a few hours a year: cold, clear, sun perpendicular to the panel, panels clean. The rest of the time it produces substantially less, and an inverter sized to that rare peak spends its life running at a fraction of capacity, where it is both expensive and less efficient.
Clipping — losing the top of the curve on the brightest days — typically costs 1 to 3% of annual production at a 1.2 ratio. That is cheaper than the larger inverter.
Ratios up to 1.3 or 1.4 make sense for east-west arrays and shallow tilts, where the production curve is flatter and the peak lower.
String inverter, microinverters or optimisers
String inverter — cheapest per watt, one point of failure, and a shaded panel drags its whole string down.
Microinverters — one per panel. Each panel produces independently, so partial shade costs only that panel. Per-panel monitoring included. Higher cost, and the electronics live on the roof.
Optimisers with a string inverter — a middle path: per-panel MPPT, one central inverter.
The decision is almost entirely about shading. On an unshaded south roof a string inverter is fine and cheaper. With any partial shading, per-panel electronics recover more than they cost.
Pure sine, always
Modified sine inverters run resistive loads and mistreat everything else — motors run hot, some electronics refuse to work, and anything with a switching supply is unhappy. The price gap has narrowed to the point where there is no case for modified sine in a permanent installation.