In brief
Near Crewe, a south-facing roof makes about 69% of its yearly output between April and September. Electricity used in the home as it is made replaces units you would otherwise buy; the rest is exported. Under the SEG scheme, larger suppliers must offer a rate above 0p to installations certified under MCS, with a meter that records exports.
A year on a Crewe roof, month by month
PVGIS 5.3, the European Commission’s solar model, estimates output from satellite irradiance data covering 2005-2023. For a 1 kWp array facing due south at 35° near the centre of Crewe it gives about 951 kWh a year. The table shows how that total is spread through the year, with east- and west-facing slopes at the same pitch for comparison.
| Month | South (kWh) | Share of year | East (kWh) | West (kWh) |
|---|---|---|---|---|
| Jan | 31.9 | 3.4% | 15.3 | 14.5 |
| Feb | 50.1 | 5.3% | 29.6 | 28.7 |
| Mar | 81.9 | 8.6% | 60.3 | 58.7 |
| Apr | 110.3 | 11.6% | 92.6 | 90.3 |
| May | 120.0 | 12.6% | 110.6 | 109.1 |
| Jun | 117.0 | 12.3% | 111.1 | 109.8 |
| Jul | 116.5 | 12.3% | 108.3 | 108.7 |
| Aug | 104.7 | 11.0% | 93.3 | 92.0 |
| Sep | 88.1 | 9.3% | 68.0 | 68.3 |
| Oct | 60.7 | 6.4% | 39.7 | 39.2 |
| Nov | 39.5 | 4.2% | 19.8 | 19.3 |
| Dec | 30.3 | 3.2% | 12.6 | 12.1 |
Six months, April to September, produce about 69% of the year’s total. May is the strongest month on a south roof and Dec the weakest, with 4.0 times as much in the best month as in the worst. East and west roofs follow the same shape but lose more in winter, when the sun stays low in the southern sky. Other pitches and directions are on output on a Crewe roof.
Across a single day
A south-facing array produces a curve that rises from early morning, peaks around the middle of the day and falls away towards evening. Many households use the most electricity at the two ends of that curve: breakfast, and then the hours after work and school. The middle of the day, when output is highest, is often when the house is quietest.
That mismatch is why self-use and export matter. On a bright summer day a typical array can make far more around midday than the house is using at that moment, and the surplus leaves through the meter. On a dull winter day the same array may never make enough to cover even the background load of fridges and standby devices.
What self-use and export mean
Self-use is solar electricity used in the home at the moment it is generated. Each unit used this way is a unit that does not have to be bought from the supplier, so its value to the household is the import price it replaces. Export is the surplus that flows out to the grid when the panels are making more than the home needs.
Import prices are usually higher than export rates, so, unit for unit, using power at home normally beats sending it out. That does not make export worthless; for many systems, especially larger ones on homes that are empty during the day, it is a sizeable part of the output. It does mean that an estimate of how much the household will use itself is one of the most useful numbers in any quote, and one of the hardest to get right.
How exported electricity is paid for
Export payments come from electricity suppliers through the SEG scheme, overseen by Ofgem. Under it, larger suppliers must offer at least one export tariff that pays more than 0p per unit. To qualify, the installation must be no bigger than 50 kW, certified under MCS, and fitted with a meter that can record exports, which in practice usually means a smart meter.
The rules set a floor, not a rate. Tariffs vary from supplier to supplier and change over time; some pay a fixed rate, others vary through the day, and some are open only to people who also buy their electricity from the same company. Compare a few before choosing. You will need the MCS certificate to sign up, so keep it safe, and check how payments are made and how often. A firm can be checked on the MCS register before any contract is signed.
Using more of your own output
- Timing. Running a washing machine, dishwasher or tumble dryer in the middle of the day draws on the panels rather than the grid.
- Hot water. A diverter sends surplus to the immersion heater in a hot water cylinder; it matters most in homes with electric heating, covered on electric heating in Crewe.
- Storage. A battery holds daytime surplus for the evening; whether one makes sense is discussed on batteries in Crewe.
- Larger daytime loads. An electric car charged at home during the day, or a heat pump heating water, can soak up surplus that would otherwise be exported.
None of these is compulsory, and each has its own cost and practical limits. The point is that a system’s value depends partly on habits and equipment in the home, not only on the panels.
East–west roofs and the daily curve
Splitting an array between east and west slopes changes the daily shape. The east panels produce early, the west ones late, and the midday peak is flatter. In Jun an east slope makes 111.1 kWh per kWp against 117.0 for south, so the summer gap is small. In Dec the gap widens: 12.6 against 30.3. For a household that is out at midday but home in the morning and evening, the broader curve can mean a higher share of self-use, even if the yearly total is lower. Semi-detached roofs with east and west slopes are covered on semi-detached roofs in Crewe.
Export limits and the grid
The network operator decides how much a home may export. A small system goes in under the G98 notification route; a larger one, or one with a battery that takes the total over the limit, needs a G99 application first, and the operator may approve it only with an export limit set in the inverter. In that case the inverter caps what leaves the house and any extra is either used, stored or not generated. Find the operator for your postcode through the ENA search; the process is explained on grid connection in Crewe.
What a quote should say about self-use
Every quote should include an estimate of yearly generation, and a good one also says how much of it the household is expected to use. Ask what that figure assumes: whether anyone is at home during the day, what the household’s yearly use is, and whether a battery or diverter is included. Two quotes with different self-use assumptions are not comparable, even if the panels are identical. If a figure looks generous, ask for the working.
Reading your own pattern once the panels are on
After installation, the inverter’s display or app shows how much the panels have made, and a smart meter records how much went out to the grid. Note both at the start of each month for the first year. Set against the monthly table above, those readings show whether the system is performing as modelled and how much of its output the household is really using. A summer month that falls well short of the PVGIS pattern, with no obvious shade or weather reason, is worth raising with the firm while the workmanship warranty is fresh.
Rented homes and changes of owner
Tenants normally hold the electricity account in a rented home and so gain from self-use, while the export tariff can be in either name; renting in Crewe explains how to settle that. When a home with panels is sold, the new owner sets up their own export arrangement using the system’s paperwork, covered on buying a Crewe home with solar. To ask about the pattern for a particular home, use the question form.