Four mountings in thirteen places. Read down a column to see geography. Read across a row to see the thing you can actually change.
Raw kilowatt hours on the left of each pair, index against this region's ideal roof on the right.
| Point | Ideal roof | Garden frame | South wall | East wall |
|---|---|---|---|---|
| South East Brighton | 897 119 | 890 118 | 645 86 | 424 56 |
| South West Exeter | 822 109 | 816 109 | 594 79 | 417 56 |
| East of England Norwich | 802 107 | 795 106 | 594 79 | 412 55 |
| Wales Cardiff | 798 106 | 792 105 | 575 77 | 385 51 |
| London London | 795 106 | 789 105 | 582 77 | 409 54 |
| East Midlands Nottingham | 760 101 | 754 100 | 564 75 | 391 52 |
| West Midlands Birmingham | 752 100 | 746 99 | 555 74 | 389 52 |
| Yorkshire Leeds | 736 98 | 729 97 | 550 73 | 376 50 |
| North East Newcastle | 734 98 | 727 97 | 558 74 | 372 50 |
| Northern Ireland Belfast | 714 95 | 708 94 | 534 71 | 367 49 |
| North West Manchester | 689 92 | 684 91 | 505 67 | 359 48 |
| Scotland, south Glasgow | 661 88 | 656 87 | 491 65 | 337 45 |
| Scotland, north Inverness | 628 84 | 623 83 | 470 63 | 321 43 |
All values kWh a year. Index = value divided by 752, the ideal roof figure for this region, times one hundred.
A garden frame is not a compromise. In every one of the thirteen rows the frame figure sits within about one per cent of the roof figure. Ground mounting costs almost nothing in output and asks nothing of the building.
A south wall costs roughly a quarter to a third. That is a real price but a survivable one, and brick is not a surface the planning conditions object to.
An east wall costs about half. It is also the arrangement most households default into, because it is the wall they happen to have, and nobody selling a kit is going to raise it.
An index can be built from one open dataset and checked by anybody. A savings figure needs a retail price and a tariff, and we have not sourced UK retail prices from anywhere we would be willing to cite. We would rather publish something verifiable than something persuasive.
| Parameter | Value used |
|---|---|
| Service | PVGIS, European Commission Joint Research Centre, API v5_2 |
| Peak power | 0.8 kWp |
| System losses | 14 per cent |
| Technology | Crystalline silicon, building-integrated |
| Roof case | Slope 35 degrees, azimuth 0 |
| Ground case | Slope 30 degrees, azimuth 0 |
| Wall cases | Slope 90 degrees, azimuth 0 and azimuth minus 90 |
| Queried | 13 September 2026 |
Free, open and requiring no registration. Anyone repeating this should get the same numbers, which is the entire point of publishing the method.