The question is almost never "which inverter is best". It is "what must this system still do when the grid is down, and what happens to the surplus at noon". Answer those two and the type chooses itself.
What each type actually does
| On-grid | Hybrid | Off-grid | |
|---|---|---|---|
| Needs the grid to run | Yes | No | No |
| Runs loads during an outage | No | Yes | Yes |
| Exports surplus to the grid | Yes | Yes | No |
| Requires a battery | No | Optional | Yes |
| Cost per kW | Lowest | Highest | Middle |
Start with the load profile, not the bill
Sizing from the monthly electricity bill tells you annual energy. It tells you nothing about when that energy is used, and the timing is what decides the inverter type.
- List the loads that must survive an outage, with their running watts and starting watts.
- Note how long a typical outage lasts and how often they occur.
- Note when consumption actually happens — daytime, evening, or overnight.
- Check whether net metering is available and what the export tariff is.
A household with a stable grid and daytime consumption gets the best return from on-grid, full stop. A household with four hours of evening outage needs storage and a hybrid. A farm with no grid connection at all needs off-grid, and no amount of budget pressure changes that.
Sizing the inverter against the array
Inverters are usually specified with a DC:AC ratio above 1 — more array than inverter. A ratio of 1.1 to 1.3 is normal, because an array almost never produces its nameplate output and a slightly undersized inverter spends more of the day near its efficiency peak.
The limits that matter are on the datasheet and they are absolute:
- Max PV input voltage — string Voc at the lowest expected cell temperature, not at 25 °C. Cold mornings are what destroy inverters.
- MPPT voltage window — string Vmp at the highest expected cell temperature must still sit inside it, or the tracker drops out at midday.
- Max input current per MPPT — parallel strings add current.
- Max short-circuit current — the protection rating, not the operating one.
Hybrid: the configuration people get wrong
A hybrid inverter has to be told what to prioritise. The same hardware behaves completely differently depending on whether it charges the battery first, serves the load first, or exports first. If the customer wants outage cover, the battery must be held at a reserve state of charge rather than being cycled to empty every evening for a marginally better bill.
Frequently asked
Can I add a battery to an on-grid system later?
Not to the on-grid inverter itself. You either replace it with a hybrid or add a separate AC-coupled battery inverter. Deciding this at design time is much cheaper than discovering it in year three.
Is a bigger inverter safer?
No. An oversized inverter runs at a lower fraction of its rating for most of the day, where conversion efficiency is worse, and it costs more. Size it to the array with a sensible DC:AC ratio.
Does an off-grid inverter need anti-islanding certification?
It has no grid to island from, so the anti-islanding test does not apply. It still needs the general safety qualification. Grid-interactive units need both.
