Comparing Solar System Sizes

When an installer offers you two sizes, the useful comparison is not the total price of each. It is what the difference costs and what the difference buys.

The principle

Much of a solar installation is a fixed cost: scaffolding, certification, network paperwork, the travelling, the setting up, the making good. Those cost roughly the same whether you fit eight panels or sixteen.

So stepping up a size adds mostly the cost of the extra panels and their mounting. The marginal panels are the cheap ones.

This is why installers usually suggest more rather than less, and on this occasion the incentive and the sound advice point the same way.

Working out the marginal cost

Ask for two quotes at different sizes and do this:

Extra cost       = larger quote - smaller quote
Extra generation = larger estimate kWh - smaller estimate kWh
Cost per extra kWh per year = extra cost / extra generation

Then compare that against the smaller system’s overall cost per kWh per year. If the marginal cost is well below the average cost, the larger system is good value on the generation alone.

The catch

Extra generation is only worth full value if you use it. Beyond a certain point, the extra panels mostly export, at roughly half the value of a unit used at home.

So the honest version of the calculation values the extra generation at a blend of your import price and your export rate, weighted by how much of it you will actually use. The larger the system relative to your usage, the more of the extra goes out at the export rate.

Where the step up is clearly worth it

You have, or are getting, a battery
You have, or are getting, an electric car or a heat pump
Someone is home during the day
You are on, or moving to, a time-of-use export tariff
The roof space is there and the marginal cost per panel is low

Where it is not

Low daytime use, no battery, no plans to change either
The step up pushes you from a G98 to a G99 connection, adding delay and the risk of a restriction. See grid connection
The extra panels go on a poor roof face, where they generate much less for the same mounting cost
The step up requires a larger inverter, which can make the increment less cheap than it first appears

The sizes you will be offered

The UK average is around 4.8 to 5.2 kWp. Most quotes for a typical house land between 3kW and 5kW, often with 3.68kW appearing because of the grid connection threshold.

Systems below 3kW rarely make sense now, because the fixed costs are spread over too few watts.