Three-Phase Supply: What It Means for Solar and Batteries

Most UK homes have a single-phase electricity supply. Some larger homes, newer builds and homes that have been upgraded have a three-phase supply, which is effectively three supplies in one. For solar and batteries, three-phase mainly changes two things: how much generation can connect under the simpler G98 notification route, and which inverter and battery equipment suits the house.

Key points

• You can usually tell from the main fuse: one fuse means single-phase, three fuses means three-phase.

• Under G98, generation up to 16 amps per phase only needs notifying. That is 3.68 kW on a single-phase supply and 11.04 kW on a three-phase supply.

• A three-phase home can use a three-phase inverter, which spreads generation across all three phases, or single-phase equipment on one phase.

• Batteries and backup need more thought on three-phase, because loads are spread across the phases.

• Upgrading to three-phase is a job for your network operator and is rarely worth doing for solar alone.

How to tell which supply you have

Look at the main fuse where the supply cable enters your home, sometimes called the cut-out or service head. UK Power Networks says a single-phase supply typically has one fuse, and a three-phase supply typically has three. Don’t open or touch it: it belongs to the network operator. If you aren’t sure, a photo for your installer or an electrician to look at is enough.

UKPN also notes that most homes with gas central heating need single-phase, and that two or more electricity meters need a three-phase supply.

What three-phase changes for solar

A larger system under G98. The G98 connection rules apply to generation up to 16 amps per phase. ENA’s G98 standard puts that at 3.68 kW on a single-phase supply and 11.04 kW on a three-phase supply. Below those levels, your installer notifies the network operator after installation. Above them, the system normally needs approval under G99 before it is connected. Our guide to DNOs and export limits explains both routes. On a three-phase home, a larger system can often go in without the wait for approval, which matters if you have a big roof, an EV or a heat pump.

Three-phase inverters. A three-phase inverter connects to all three phases and shares its output between them. That keeps the phases balanced and makes full use of the larger G98 allowance.

Single-phase equipment on a three-phase supply. It is also possible to connect a single-phase inverter to one phase. That suits smaller systems, but the generation all goes onto one phase, and the G98 limit applies to that phase.

Batteries on a three-phase supply

Batteries need more planning on three phases, because your household loads are spread across all three.

• Three-phase battery systems can supply all three phases, so they can cover loads wherever they are.

• Single-phase batteries connect to one phase. How well they cover the house depends on how the system measures your usage and how loads are spread, so the metering and CT clamp set-up matters.

• Backup in a power cut may cover only some circuits or one phase, depending on the equipment. If backup matters to you, ask exactly which circuits stay on. Our guide to what keeps working in a power cut explains the options.

Should you upgrade to three-phase for solar?

Usually not for solar on its own. Export limitation can let a larger system connect on a single-phase supply, and a battery lets you use more of what you generate. Our guide to how many solar panels you need covers sizing.

A three-phase upgrade starts to make sense when several large loads are coming together: a heat pump, an EV charger and solar with a battery, or a large extension. The upgrade itself is done by your network operator. UKPN lists microgeneration among the reasons for a three-phase supply. Ask an electrician to look at your total load first, as UKPN advises. Separately, some homes can get a larger single-phase fuse, which may be all that is needed for an EV or heat pump.

Frequently asked questions

Is three-phase better for solar panels?

It allows a larger system under the simpler G98 route and keeps generation balanced. On a typical home with a modest system, single-phase works perfectly well.

Can I put a single-phase inverter on a three-phase supply?

Yes, on one phase, and it is common for smaller systems. A larger system is usually better on a three-phase inverter.

Will a three-phase battery work in a power cut?

Only if it is designed and installed for backup. What stays on depends on the equipment and the wiring, so ask before you buy.

How do I get a three-phase supply?

Apply to your network operator, usually after an electrician has assessed your needs. In most of London that is UK Power Networks.

Request a solar and battery quotation. We’ll check your supply at the survey and design the system to make the most of it, whether it’s single or three-phase.