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.
4 articles
Solar Inverter Cost: String, Micro and Hybrid Compared per WattThe inverter is around ten per cent of a solar quote and the only component you are near-certain to buy twice.
Microinverters vs String Inverters: the Roof Decides, Not the BrochureThe comparison is not about quality. It is about how many planes your roof has and what falls across it at four in the afternoon.
String Inverter or Microinverters? Shade and Roof Shape Decide, Not PriceOn a clean south-facing roof with no shade, a string inverter is cheaper and there is little to argue about. Almost no roof is that roof.
Inverter Sizing: Continuous Watts, Surge Watts, and the DC-to-AC RatioTwo different numbers size an inverter, and a third one decides whether it should be smaller than the array on purpose.