Inverters

Inverter sizing works in opposite directions depending on the system.

Off-grid: size to the load

Continuous rating covers everything running at once, with margin — an inverter run continuously at its rating runs hot and dies early, so 80% is a sensible ceiling.

Surge rating covers the inrush of the largest motor, which draws roughly three times its running watts for a second or two.

Count one motor's surge, not the sum of all of them. Two large motors starting in the same half-second is unlikely, and sizing for it buys an inverter twice the size you need.

Grid-tied: size below the array, on purpose

A DC-to-AC ratio of about 1.2 is standard — 6 kW of panels on a 5 kW inverter.

An array reaches nameplate only in ideal conditions for a few hours a year. An inverter sized to that rare peak spends its life underloaded, where it is both expensive and less efficient. Clipping the top of the curve on the brightest days costs 1 to 3% of annual production, which is cheaper than the larger inverter.

Ratios of 1.3 to 1.4 suit east-west arrays and shallow tilts, where the curve is flatter and the peak lower.

String, micro or optimisers

The decision is almost entirely about shading.

A string inverter is cheapest per watt and a shaded panel drags its whole string down. Microinverters make each panel independent and put the electronics on the roof. Optimisers are the middle path: per-panel tracking, one central inverter.

On an unshaded south roof, a string inverter is fine and cheaper. With any partial shading, per-panel electronics recover more than they cost — and shade should be surveyed, not guessed.

Pure sine, always

Modified sine runs resistive loads and mistreats everything else. The price gap no longer justifies it in a permanent installation.

Hybrid capability is a decision you make once

Retrofitting a battery to a plain grid-tied string inverter usually means replacing the inverter. The premium for a hybrid-capable unit at purchase is far smaller than that replacement.