The answer first
A south-facing 35° roof near Nantwich makes about 959 kWh per kWp a year in PVGIS, and 69% of it arrives between April and September. Power used in the house as it is generated replaces bought units; the rest is exported. Under the SEG scheme, large suppliers must pay above 0p for exports from eligible systems up to 50 kW.
A Nantwich roof through the year
The PVGIS 5.3 model from the European Commission’s Joint Research Centre estimates output from satellite-derived sunshine records for 2005-2023. For a south-facing roof pitched at 35° near Nantwich, with 14% allowed for system losses, it gives these monthly figures per kWp of panels, and for a 4 kWp system:
| Month | kWh per kWp | 4 kWp system, kWh |
|---|---|---|
| Jan | 32.7 | 131 |
| Feb | 51.0 | 204 |
| Mar | 83.5 | 334 |
| Apr | 111.0 | 444 |
| May | 120.7 | 483 |
| Jun | 118.4 | 474 |
| Jul | 116.9 | 468 |
| Aug | 104.9 | 420 |
| Sep | 88.0 | 352 |
| Oct | 61.2 | 245 |
| Nov | 40.3 | 161 |
| Dec | 30.8 | 123 |
Across the year that comes to about 959 kWh per kWp, or about 3,838 kWh for the 4 kWp example. The six months from April to September produce 69% of it. May is the strongest month at about 121 kWh per kWp, and Dec the weakest at about 31, so the best month yields roughly 3.9 times the worst. The figures for other directions and pitches are tabulated on output on a Nantwich roof.
What self-use means
At any moment, the inverter delivers whatever the panels are making to the house wiring. If the house is using more than that, it draws the difference from the grid as normal. If it is using less, the surplus flows out through the meter to the network. Nobody has to switch anything; the split happens second by second, and the meter records how many units went each way.
Electricity used in the house as it is made is called self-use. It displaces units that would otherwise have been bought, at the household’s full import price. Exported units earn whatever the export tariff pays, which is usually a good deal less per unit. That is why the timing of a household’s demand matters as much as the size of the array. A fridge, a boiler pump and devices on standby use a little all day; a kettle, an oven or a washing machine use a lot for a short time.
Moving demand into daylight
- Timers. Washing machines and dishwashers with delay timers can be set to run around midday.
- Hot water. A diverter sends surplus to the immersion element in a hot-water cylinder; for an oil or LPG house this is often the biggest single use of summer surplus, as explained on off-gas villages.
- Electric cars. A charger that can follow solar output tops up a car parked at home during the day.
- Heat pumps. Some controls can warm the house or the cylinder a little harder when the panels are producing.
- Storage. A battery holds surplus for the evening, with the limits set out on batteries.
None of these is compulsory, and each has a cost. The point is to know which ones fit the household before the system is sized, rather than adding them afterwards.
How exports are paid: the SEG scheme
Under the rules Ofgem administers for the SEG scheme, every large electricity supplier must offer at least one export tariff to eligible small generators. The tariff must pay more than 0p per unit at all times. Eligible systems are those up to 50 kW that meet the scheme’s certification requirement, which for household solar normally means MCS certification of the installation, and that have a meter able to record exports, usually a smart meter.
Beyond that floor the scheme sets no price. Suppliers choose their own rates and terms: some tariffs are fixed for a year, some track wholesale prices through the day, and some are reserved for households that also take their import supply from the same company. Smaller suppliers can offer export tariffs too. Compare several before registering, check how the rate can change, and keep the MCS certificate and meter details to hand, since the application asks for both.
East, west and the shape of the day
Near Nantwich, a slope pitched at 35° and facing east gives about 766 kWh per kWp a year; turned to the west, about 757. Both are around 79% of the south-facing figure. The total is lower, but the output arrives earlier or later in the day. A house with panels on both slopes produces a flatter curve, with less of a midday spike and more power in the morning and early evening. For a household that is away from home around midday, that spread can mean a larger share of the output is used in the house.
Different homes, different patterns
Demand in the villages around Nantwich, where many homes heat with oil or LPG, is mostly lights, appliances and pumps, so summer surplus is common and a diverter is often the first use for it. In Nantwich North and West, 11.7% of households heat with electricity alone; their demand peaks on winter evenings, when the panels are idle, so most of the summer output is still likely to be exported unless it is stored. The building questions for those flats are on flats and solar, and in a rented home the question of who keeps the export payment is covered on renters and landlords.
Export limits and the network
The local network operator has to be told about every system, and larger ones need its approval before they switch on. Where the local network is constrained, approval can come with a cap on how much may be exported at once, enforced by a setting in the inverter. That cap changes the balance between self-use and export, so ask about it at the design stage. The steps, and how to identify the operator through the Energy Networks Association’s postcode search, are on grid connection.
Sizing with self-use in mind
A bigger array makes more electricity in every month, but the extra mostly arrives in summer, when a household is already exporting. That is not a reason to fit fewer panels than the roof can take, since winter output rises with size too, but it does mean the system’s size, the household’s routine and any diverter, car charger or battery should be considered together. Size also has a grid consequence: a system whose inverter stays within 3.68 kW on a single-phase supply can usually be connected on a simple notification, while a larger one needs the operator’s approval first. Ask the firm to show the expected monthly split between use and export for the size proposed, and how it would change with one size up or down.
Reading the meter and the statements
Once the system is running, two sets of numbers appear. The inverter’s monitoring shows what the panels made; the meter shows what was imported and exported. They will not match, because the difference is what the house used directly. Check in the first weeks that the meter is recording exports at all, since a meter set up only for imports will show nothing going out, and the export tariff cannot pay for units that were never measured. Keep a note of the generation and export readings each quarter. Over a full year they show how much of the output the household really used and whether a change of routine, or of tariff, is worth considering.
When the house changes hands
An export tariff belongs to the person who registered it, not to the house. A buyer has to apply for their own, using the system’s MCS certificate and meter details, so those papers need to pass with the sale; the checklist is on buying a home with solar. To ask how these figures apply to a particular roof, send a question.