Most UK homes have a single phase electricity supply. Some larger, newer or converted properties have three phase. It affects how large a system you can fit without a lengthy application.
How to tell which you have
Look at your main incoming fuse, usually near the meter. A single phase supply has one large fuse and one thick incoming cable. A three phase supply has three fuses, often in a row, and a thicker incoming arrangement.
If you are not sure, your installer will check. Do not guess, because it changes the design.
Why it matters for solar
The 3.68kW threshold that decides whether your system connects under G98 or G99 is per phase.
- Single phase: up to 3.68kW under G98, with the installer simply notifying the network operator afterwards. Above that, a G99 application first.
- Three phase: roughly 11kW under G98, because the limit applies to each of the three phases.
So a three phase property can fit a much larger system without applying for permission first. That is a real advantage and it is worth knowing before you start comparing quotes.
See grid connection.
Single phase and three phase inverters
If you have a three phase supply you can use either:
A single phase inverter connects to one of the three phases. Cheaper, and fine for a smaller system. The drawback is that generation goes onto one phase while your house may draw from all three, which can mean exporting on one phase while importing on another. Whether that costs you depends on how your meter handles it, so ask.
A three phase inverter spreads generation across all three phases. More expensive, but it balances properly and it is the sensible choice for a larger system on a three phase supply.
If you have a single phase supply, the question does not arise. You use a single phase inverter.
Should you upgrade to three phase?
Rarely worth it for solar alone. An upgrade means work by your network operator and can be expensive, sometimes very.
It becomes worth investigating if you are also adding a heat pump and a fast EV charger, where total demand may exceed what a single phase supply comfortably handles. In that case ask the network operator for a quote before committing to anything, because the figure varies enormously by location.
Practical points
- Tell every installer which supply you have, so quotes are comparable
- If you have three phase and one installer proposes a single phase inverter while another proposes three phase, that is a real difference, not a pricing quirk
- If you are planning a heat pump or EV charger, have the whole electrical capacity assessed once rather than three times. See EV charging