Five things, of which two decisions genuinely matter and the rest are detail.
The parts
Panels Convert daylight into DC electricity. Rated in watts. Most panels sold in the UK today are good, and the difference between a mid-range and a premium panel is smaller than the price gap suggests. See choosing panels.
Inverter Converts DC from the panels into the AC your house uses. The component most likely to need replacing during the system’s life. See choosing an inverter.
Mounting Rails, brackets and flashings that hold the panels and keep the roof watertight. Unglamorous and the source of most of the problems that turn up years later.
Metering and monitoring Shows what the system is producing. Worth having something you will actually look at, because it is how you notice a fault.
Battery, if you have one Stores electricity for later. See solar batteries.
The two decisions that matter
1. How the system handles shading.
On a conventional string inverter, panels are wired in series and a shaded panel affects the others on that string. Two ways round it:
- Optimisers, small devices on each panel, working with a string inverter
- Microinverters, a small inverter on each panel
Both add cost. Both are worth it if you have real shading and largely wasted money if you do not. See microinverters and shading.
2. Whether the inverter is battery-ready.
If there is any chance you will add a battery later, a hybrid inverter now saves you replacing the inverter then. The extra cost up front is much less than the cost of doing it twice. See hybrid systems.
What matters less than you will be told
Panel brand, within reason. See which brands you will be quoted.
Panel efficiency Higher efficiency means more watts per square metre. That only matters if your roof space is the constraint. If you have room, a slightly less efficient panel that costs less per watt is often the better buy.
Small differences in warranty length A 25-year warranty from a manufacturer that may not exist in 25 years is not obviously better than a 20-year one from a firm that will. See warranties.
Aesthetics, unless you care. All-black panels cost a little more and produce slightly less. That is a perfectly reasonable thing to pay for if the look matters to you; just know you are paying for the look.
What to check in a quote
- Panel model and wattage, and how many
- Inverter model, its rated output, and whether it is hybrid
- Whether optimisers or microinverters are included, and why
- Mounting system manufacturer
- Whether monitoring is included and whether there is an ongoing fee
- Total system size in kW and estimated generation in kWh