In short
A south-facing roof in Frodsham makes about 70% of its year between April and September, so most export happens in summer and almost all winter output stays in the house. The hill’s December cut for homes at its foot therefore falls on self-use, not export. Exports are paid through the SEG scheme, at rates each supplier sets.
Two places for every unit to go
Each kilowatt-hour a roof makes goes one of two ways. The house uses it at the moment it is made, or it flows out through the meter to the grid. Electricity used on site replaces power that would otherwise be bought at the household’s import rate. Electricity exported earns whatever export rate the household has agreed with a supplier, which is usually a good deal lower per unit. So the way output divides between the two matters as much as the total, and a well-designed system is planned around both.
A Frodsham roof month by month
The PVGIS figures for the town give a south-facing roof at 35° about 941 kWh per kWp over a typical year. The table shows how that divides between the months, for 1 kWp and for a 4 kWp system, with the average per day.
| Month | kWh per kWp | 4 kWp system, month | 4 kWp system, per day |
|---|---|---|---|
| Jan | 30.4 | 122 | 3.9 |
| Feb | 49.6 | 198 | 7.1 |
| Mar | 81.8 | 327 | 10.6 |
| Apr | 110.2 | 441 | 14.7 |
| May | 122.5 | 490 | 15.8 |
| Jun | 116.6 | 466 | 15.5 |
| Jul | 115.3 | 461 | 14.9 |
| Aug | 104.5 | 418 | 13.5 |
| Sep | 85.8 | 343 | 11.4 |
| Oct | 58.9 | 236 | 7.6 |
| Nov | 38.1 | 152 | 5.1 |
| Dec | 27.5 | 110 | 3.5 |
The spread is wide. May is the strongest month at about 122.5 kWh per kWp, roughly 4.5 times December’s 27.5, and April to September together supply about 70% of the year. For a 4 kWp roof, that means an average of 15.5 kWh a day in June against 3.5 in December.
For most households that shape decides when export happens. On a long summer day, much of the output arrives in the middle of the day when the house may be empty or nearly so, and a large share flows out. In December, the daily output is small enough that a house running a fridge, lights and an evening meal will normally use all of it, with nothing reaching the grid.
The December cut under the hill
For homes at the foot of the hill south of the centre, the terrain model takes about 32% off a south roof’s December: 19.1 kWh per kWp instead of 28.3. Across the year the loss comes to about 34 kWh per kWp, or 3.6%, and December alone accounts for roughly 27% of it. The rest comes from the months either side, when the sun is still low enough to graze the ridge.
Because winter output is almost all used in the house anyway, the hill’s bite falls on self-use rather than on export. A household under the ridge will buy a little more electricity on winter afternoons than an identical household on Main Street, but its summer export will be much the same, since the high summer sun clears the ridge with ease. The hill and winter sun gives the full horizon figures, and east and west roofs shows why an east-facing roof at the foot of the hill suffers more than a west-facing one.
Raising the share used at home
Without changing the panels, a household can move some demand into daylight: running a washing machine, dishwasher or tumble dryer around midday, charging an electric car while the sun is out, or heating water with surplus power. A diverter sends spare electricity to an immersion heater in a hot-water cylinder rather than to the grid, which helps only in homes that have a cylinder. A battery stores surplus for the evening, as set out on batteries in Frodsham. None of these makes more electricity; they change where it ends up.
Direction plays a part as well. A west-facing array delivers more in the late afternoon and an east-facing one in the morning, and a split between the two flattens the midday spike that would otherwise go out to the grid. For a household that is out all day, that can shift a noticeable share of output from export to use.
Why a bigger array mostly adds export
Adding panels raises output in every month, but not evenly in terms of what the house can use. In winter, a larger array simply covers more of the daytime demand that a smaller one left uncovered, so nearly all of the extra is used. In summer, the smaller array may already have been producing more than the house needed at midday, so most of what the extra panels add goes straight out to the grid. That is not a reason to choose a small system, since export still earns something and summer evenings still need power, but it does mean the export rate counts for more on a large roof than on a modest one. A firm’s estimate should show how its suggested size divides between use and export, month by month.
Checking a working system against the figures
Once a system is running, the inverter or its app records how much it has generated. Comparing a month’s total with the table above, scaled to the system’s size, is a simple check. Real months vary with the weather, and shade, pitch and direction move the figures, so a single poor month proves nothing. A system that falls well short of the modelled pattern for several months in a row, though, especially in late spring and summer when the hill plays no part, deserves a question to the firm that fitted it. The smart meter, meanwhile, shows how much has been exported, and the difference between the two is the share the house used.
The SEG scheme in brief
Exports are paid through the SEG scheme, which Ofgem administers. Under it, larger electricity suppliers must offer a rate above zero for exported power to systems of up to 50 kW that were installed under MCS and have a meter able to record exports, normally a smart meter. Suppliers set their own rates and terms, which differ a good deal, and a household does not have to take its export tariff from the company that sells it electricity.
Registration usually asks for the MCS certificate and the meter details, so keep those papers together from the day the system is commissioned. The agreement belongs to the account holder rather than to the house, so someone who sells up leaves it behind and a buyer applies for their own, as buying a Frodsham home with solar explains.
Is the best export rate worth chasing?
The export rate matters most to households that export a lot: those with a large roof who are out during the day. For a household that uses most of its output at home, the import rate is the larger influence. Comparing export tariffs is still worthwhile, but read the terms carefully. Some rates are open only to customers who also buy their electricity from the same supplier, some vary by time of day, and some can change at short notice.
Connection comes first
No export tariff can start until the system is properly connected: notified under G98 or approved under G99 with the network operator for the address, which the Energy Networks Association’s postcode search identifies. Grid connection explains which applies and when.
Putting a figure on one house
A survey turns these modelled figures into an estimate for a particular roof, with its own shade, pitch and direction. Ask for that estimate month by month rather than as one yearly total, so the summer surplus and winter shortfall are both visible, and set it beside the figures on output on a Frodsham roof. Prices are on what solar might cost in Frodsham, and anything specific to one house can be sent in using the question form.