The legal limit for plug-in solar in the UK is 800W, which typically means two 400W panels. Most marketing pushes the maximum configuration, but that is not always the right answer for every household. Here is how to think about size.
The 800W case: nearly always the better choice for savings
If you have the space and the budget, 800W generates twice as much electricity as 400W for roughly 1.5 times the cost. The economics almost always favour the larger system if you can accommodate it. An 800W south-facing system in central England generates around 600 to 700 kilowatt hours per year. A single 400W panel generates half of that, for a saving of around £55 to £85 per year rather than £110 to £180.
The payback period on 800W versus 400W is often very similar because the incremental cost of the second panel and second set of mounting hardware is relatively small compared to the doubling of generation.
When 400W makes sense
Space constraints are the main reason to choose a single panel. If your balcony railing only has room for one panel, or your ground frame position is limited, a single 400W panel is the right size for that space rather than a compromise two-panel installation that does not fit properly.
Budget can also be a reason to start with one panel. A single 400W setup with microinverter typically costs £150 to £400 and delivers meaningful savings while you assess whether you want to expand. Most microinverters support a second panel being added later, so starting at 400W does not lock you out of upgrading.
Matching system size to your electricity use
An 800W system generating 600 to 700 kWh per year covers roughly 20 to 25 percent of a typical UK household’s annual electricity consumption of around 2,700 to 3,100 kWh. That is a meaningful chunk but leaves plenty of room for the generation to be used — you will not be drowning in surplus electricity most of the time.
For a one-person flat using perhaps 1,200 kWh per year, an 800W system covers around half their annual consumption in theoretical terms. Self-consumption will be lower in practice due to timing, but the relative impact on the bill is larger for smaller households.
Think about base load first
The most reliable saving from plug-in solar comes from base load: the appliances that run continuously regardless of what else is happening. Fridge, freezer, broadband router, devices on standby. These typically draw 200 to 400 watts continuously. A well-positioned 800W system can cover this base load entirely during summer daylight hours, effectively stopping the meter on your most consistent energy costs.
Use the savings calculator to model your specific situation, and see the Best Kits page for current pricing on both 400W and 800W configurations from the main UK suppliers.