The short version
A south-facing 35° roof near Northwich makes most of its electricity between April and September, often more than the house uses at midday. Spare units are exported and can earn an export tariff under the SEG, but a unit used at home usually counts for more because it replaces electricity bought at a higher import price.
Three places a solar unit can go
Solar panels produce direct current, which an inverter converts into the alternating current your house runs on. From the inverter, each unit of electricity, measured in kilowatt hours (kWh), has three possible destinations. It can be used straight away by whatever is switched on in the house; it can be stored, in a battery or as hot water in a cylinder; or it can flow out through the meter onto the local network. Nobody has to direct it. Electricity feeds the nearest load first, so the house always takes what it needs before anything leaves the property.
That simple physical fact shapes the value of every array. How much of the output you use yourself depends more on when you use electricity than on how much you use in total, and on a Northwich roof the timing is set by the sun.
A Northwich roof month by month
The figures below come from PVGIS 5.3, the European Commission’s solar model, for a roof near Northwich facing due south at a 35° pitch. The run allows 14% for system losses and uses the PVGIS-SARAH3 radiation record for 2005-2023, so each month is an average across many years rather than a forecast for any one of them. The middle column is output per kilowatt peak (kWp), the standard size rating of an array. The right-hand column multiplies it by four for a 4 kWp array, a mid-sized domestic system.
| Month | kWh per kWp | 4 kWp array |
|---|---|---|
| Jan | 30.0 | 120 |
| Feb | 48.9 | 196 |
| Mar | 82.5 | 330 |
| Apr | 110.2 | 441 |
| May | 120.2 | 481 |
| Jun | 116.6 | 466 |
| Jul | 115.0 | 460 |
| Aug | 104.2 | 417 |
| Sep | 86.6 | 346 |
| Oct | 59.3 | 237 |
| Nov | 38.6 | 154 |
| Dec | 26.9 | 108 |
| Year | 939.1 | 3,756 |
Over a year the model gives 939 kWh per kWp, or about 3,756 kWh for the 4 kWp array. The six months from April to September produce 69.5% of the total. May is the strongest month at 120.2 kWh per kWp and December the weakest at 26.9, so an average May day yields roughly 4.5 times as much as an average December day.
Two consequences follow. In summer a typical array makes more around midday than most households are using at that moment, so part of the output is exported unless something captures it. In winter the output is small, and almost all of it is absorbed by the fridge, the lighting and the heating controls. The export question is, for the most part, a summer question.
Why self-use usually counts for more
A unit of solar electricity used in the house replaces a unit you would otherwise have bought from your supplier at your import rate. A unit exported earns whatever your export tariff pays. At the tariffs typically on offer, the import price per unit is higher than the export rate, so a unit used at home is normally worth more to you than one sent to the grid. That is why advice about solar keeps coming back to self-use, and why the time of day you run appliances matters as much as the size of the array.
This guide does not print prices. Tariffs change often and any figure here would soon be out of date. Instead, compare your own import rate, which is on your bill, with the export rates suppliers are offering when you come to sign up. The gap between the two is what makes shifting your use worthwhile.
Ways households move use into daylight
- Setting the washing machine, dishwasher or tumble dryer on a delay timer so they run in the middle of the day.
- Charging an electric car at home while it is parked during daylight.
- Heating water with surplus solar through an immersion heater controlled by a diverter, a device that sends spare output to the cylinder instead of the grid.
- Adding a battery to hold daytime surplus for the evening, covered in batteries.
None of these is compulsory, and none suits every house. A household with someone at home all day may already use most of its output without trying. One that is out until the evening will export more unless it changes habits or adds storage.
Roof direction changes the shape of the day
A south-facing roof peaks around midday. An east roof peaks in the morning and a west roof in the afternoon, which can line up better with when a family is actually using power. PVGIS puts an east-facing 35° roof near Northwich at 752 kWh per kWp a year and a west-facing one at 743, against 939 for due south. The annual total is lower, but the output is spread across more of the day, and a house with both an east and a west slope can use them together. Every direction and pitch the model was run for is set out on output on a Northwich roof.
The SEG: being paid for exports
Selling exported electricity in Great Britain works under the Smart Export Guarantee, known as the SEG, which Ofgem oversees. The rules in brief:
- Large electricity suppliers must offer an export tariff, and it must pay more than 0p per unit.
- The offer has to be open to installations of up to 50 kW that are certified under MCS. A house roof falls comfortably inside that limit.
- You need a meter able to record exports; in practice that usually means a smart meter.
There is no single SEG rate. Each supplier sets its own tariff, some fixed and some variable, and the rates differ from one company to the next and change over time. The export tariff does not have to come from the company that sells you electricity, so it is worth looking beyond your current supplier. Check what is available when your system is commissioned, and look again every so often.
What a supplier will ask for
- The MCS certificate for the installation, which the installer issues after commissioning.
- Your meter details, including the export reference if you have a separate one.
- The size of the system in kWp and the date it was commissioned.
The installer also has to tell the network operator about the system, or get its approval first for larger set-ups. That step is separate from the export tariff and is explained in grid connection.
Working out how much you export
Nobody can give you a reliable export share in advance, because it depends on your household. The installer’s performance estimate should state the expected annual generation in kWh; how much of that you use depends on who is at home, which appliances run during the day and whether you add storage. Once the system is running, the export register on your meter and the inverter’s app between them show the split. A few months of readings across spring and summer tell you far more than any estimate made beforehand.
Compare those readings with the PVGIS table above. If the array is producing noticeably less than the model suggests for its direction and pitch, ask the installer to check for shading, a fault or a configuration problem. If generation matches but exports are higher than you expected, it is a sign that your use is concentrated in the evening, which is where a timer or storage can help.
Sizing with export in mind
Because self-use is usually worth more per unit, a bigger array is not automatically better value. Extra panels add most of their output in the summer months, when a household is already exporting. That does not make a larger array a poor choice: a roof only has room for so many panels, the scaffolding costs much the same whatever the size, and future loads such as an electric car or a heat pump may raise your daytime use. It does mean the decision should be made with your own pattern of use in view rather than on price per panel. The cost guide covers how size affects price.
Questions for the installer about export
- What annual generation in kWh do you estimate for my roof, and how was it worked out?
- How much of that do you expect me to use at home, and what assumptions sit behind the figure?
- Will I need a new or reconfigured meter to record exports?
- How soon after commissioning will I receive the MCS certificate, so I can apply for an export tariff?
- Would a diverter or a battery suit my pattern of use, and why?
A firm that replies with a headline savings figure and nothing else has not really answered. The other checks worth making are in choosing an installer.
Export tariffs, import prices and supplier offers change frequently. This page explains how the arrangement works and shows modelled output for a typical roof near Northwich; check current rates with suppliers before you decide anything.