People assume the inverter should match the panels exactly. It usually should not, and the reason is straightforward.
Why the inverter is often smaller than the array
Panels are rated at their output under standard test conditions: bright sunlight, a set temperature. In British conditions they reach that rating rarely.
So an inverter sized to the panels’ full rated output would spend almost its entire life running well below capacity, and you would have paid for capacity you never use.
Fitting a slightly smaller inverter, usually somewhere around 80% to 100% of the array rating, means the inverter runs closer to its efficient range for more of the time. It is standard practice and it is not a corner being cut.
Clipping
On the rare occasions when the panels do produce more than the inverter can handle, the inverter limits the output. That is clipping.
It sounds wasteful and it is, slightly. In practice the amount lost over a year in the UK is small, usually a fraction of a per cent, and it is outweighed by the saving on the inverter and by the better efficiency the rest of the time.
If an installer proposes significant oversizing, ask what they estimate the annual clipping loss to be. A reasonable answer is a small number. If they cannot say, that is a flag.
The interaction with grid connection
This is where inverter capacity becomes a decision rather than a detail.
The 3.68kW per phase threshold that determines whether you connect under G98 or G99 applies to what the system can export to the network, which is governed by the inverter’s output, not the panels’ rating.
So you can have an array larger than 3.68kW connecting under G98, if the inverter output is limited to 3.68kW or the system uses export limiting.
That is legitimate and common. What it means for you:
- Your panels can be bigger than your export limit
- You still use all of the generation you consume at home
- You cannot sell more than the limit at any moment
Whether that matters depends on how much you expect to export. See how much you will export and grid connection.
What to check in a quote
- Array size in kW and inverter AC output in kW, stated separately
- Whether export limiting is applied, and at what level
- The connection route, G98 or G99
- Number of MPPT inputs if panels are on more than one roof face
- Whether it is a hybrid inverter, if a battery is possible later
If you are adding a battery later
Inverter capacity is also about whether it can take a battery at all. A standard solar inverter cannot. See choosing an inverter and hybrid systems.