Short answer
The inverter gets its own circuit in the meter cupboard with its own breaker, protected by an RCD. The cable runs from the meter cupboard to where the inverter is, usually the loft or utility room. An inverter above 3.7 kW should be connected three-phase, because a grid fed back on only one phase becomes unbalanced.
Who this article is for Anyone having solar panels installed or who already has them. Connecting the inverter to the meter cupboard is work for an installer; the part between panels and inverter belongs to the solar panel fitter.
The inverter belongs on its own circuit
An inverter feeds back its maximum current for hours on end. If you put it on an existing circuit, that adds to what is already on it. On sunny days that circuit trips, and at the worst possible moment: when you are not there.
So its own circuit is no luxury:
- its own breaker, chosen for the inverter's power
- protected by an RCD
- clearly marked on the circuit chart, so the next engineer knows what it is
Which cable
The cable runs from the meter cupboard to the inverter, which is usually in the loft or a utility room. The distance is often greater than people think: from the meter cupboard in the hall, through the crawl space, up the stairs to the loft.
A cable that is too thin over a long route is the most common cause of switching off.
| Inverter power | Connection | Cable rule of thumb |
|---|---|---|
| up to 3.7 kW | single-phase | 3 x 2.5 mm² |
| 3.7 to 10 kW | three-phase | 5 x 2.5 to 4 mm² |
| above 10 kW | three-phase | calculate on length |
Just as with an EV charger, length counts. A thin cable over a long distance causes voltage drop, and when feeding back that works the wrong way: the inverter sees too high a voltage at its end and switches off. That is one of the most common causes of "the inverter keeps cutting out". More on that in inverter cuts out.
Feeding back single-phase or three-phase
If you feed back on one phase with a larger inverter, the voltage on that one phase rises while the other two do nothing. That causes complaints in the house and at the neighbours', and it is the reason grid operators require three-phase above 3.7 kW.
If you currently have single-phase, upgrading means an application to the grid operator plus work in the meter cupboard. What that means is explained in upgrading the connection.
The DC side
Between the panels and the inverter runs DC voltage, and it can be quite high: several hundred volts. That needs a DC isolator at the inverter, and the cabling must be made with the right connectors.
This is where it goes wrong most often in practice, resulting in burnt connectors. What to look out for is explained in solar panels and fire safety.
Registering with the grid operator
An installation that can feed back must be registered. That is done through the national register and takes minutes. If the solar panel company does not do it, I do it at handover.
From my own work
At a home in Schimmert the inverter was in the loft and the solar panel fitters had connected it to the existing lighting circuit. On sunny days that circuit tripped. The inverter now has its own circuit with its own breaker and a cable that can handle the full power. No more cut-outs since.
When to call an electrician
- The panels have been installed but connected to an existing circuit
- The inverter is larger than 3.7 kW and you only have single-phase
- The circuit the inverter is on trips on sunny days
I connect the inverter and test the installation. See solar panels and the consumer unit and request a quote.
Frequently asked questions
Can the inverter go on an existing circuit?
No, it should get its own circuit. On a shared circuit it trips on sunny days, exactly when you are not at home.
Does my installation need to be registered?
Yes, an installation that can feed back must be registered. If the solar panel company does not do it, I do it at handover.
Why is three-phase required above 3.7 kW?
Because feeding back on one phase pushes the voltage up there while the other two do nothing. That causes complaints in the house and at the neighbours'.
Further reading
A dynamic contract and a home battery: is it worth it
With a dynamic contract the battery can charge when electricity is cheap. That works, provided you let it be controlled automatically.
The net metering scheme is ending: what changes for my panels
Your panels keep generating just as much. What changes is the value of the electricity you feed back.
Feed-in charges: why you pay for your own electricity
More and more suppliers charge for feeding your solar power back into the grid. Your bill shows what that costs you.
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