Solar PV example: single garage

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This is an example design of a UK solar PV system.

  • Location: Cambridge 52.2degN, 0.14degE
  • Detached single garage
  • Flat roof 3x5m
  • Orientation: east-west
  • Prevailing wind: westerly

This is quite a small roof area and the flat roof means we have to build our own structures, we cannot rely on the roof pitch to incline our panels. On the other hand we don't need scaffolding for installation.

According to Global Solar Atlas, 1 kilowatt-peak (kWp) pointed due south at 38 degrees of inclination returns 983kWh per year as this location. This model includes compensation for cloud cover and panel efficiency at site temperatures. It assumes a full view of the sky, which we'll also assume for this example.

System design

Due to the roof having the long edge going east-west, we could mount four panels on frames angled due south, with one short edge meeting the long edge of the roof, and the ideal inclination would have the other short edge of the panel about 1.2m in the air. Such a setup would generate 1572kWh per annum according to GSA. Mechanically this is not ideal as it will be subject to strong crosswinds and need substantial anchoring to avoid the panels acting as a sail. It is also aesthetically less pleasing and may be awkward from a planning permission perspective.

Instead we can trade off a small part of our generation for an easier time by compromising on mounting angles. it makes more sense to align the panels east-west rather than due south. This is because we can butt a pair of panels, one facing east and one facing west, and have two pairs of panels on the roof. Each panel is about 2m x 1m and about 400 watt-peak. Using an east-west alignment also gives a smoother generation profile across the day, and also a better wind profile.

A simple mounting solution for flat roofs is the Renusol Console bucket system: you just put the plastic tub on the roof, fill it with ballast, and screw the panel to it. No attachment of the bucket to the roof is required. These mount the panels at 15 degrees inclination.

Let's do the calculation for 15 degrees and two panels of 400Wp each, facing east. GSA link. For 1kWp (GSA won't go lower) we'll generate 808kWh, so scaling down to 800Wp it'll generate 646kWh.

For the west facing panels, GSA gives 795kWh for 1kWp, so that's 636kWh for 800Wp.

Therefore our system is predicted to generate 646+636=1282kWh per year. Not as good as the panels facing south, but more practicable.

The cost of that electricity will vary based on tariff, but if entirely replacing grid electricity at 35p/kWh it would save £448.70 per annum. If it was fed back to the grid at an example Smart Export Guarantee rate of 4p/kWh it would earn £51.28pa, and at an Octopus Agile Outgoing average rate of 34p/kWh (source MSE 20-09-2022) it would earn £435.88.

These figures give plots of generation for the east and west facing panels at different times of the day in different months. Obviously there is more generation in the summer months, not least due to longer days. However there is still meaningful generation in February and October (about 2kWh per day).

Generation profile for 1kWp panels facing due east (multiply by 0.8 for our setup):

Generation profile for

Generation profile for 1kWp panels facing due west:

Generation profile for 1kWp panels facing due west

System costs

Let's price up the components we'll need. This is a rough sketch, the exact details including more detailed choice of parts would need to be done as part of a full system design. Prices correct in November 2022.

Item Type Quantity Supplier Unit cost ex VAT Line cost ex VAT
Solar panel Longi 54c HiMo5 400W 4 Midsummer £178.10 £712.40
Mounting system Renusol Console+ 5200 75 K 4 Midsummer £78 £312
Ballast assume a free source of aggregate etc local £0
Inverter Growatt MIC 1500 TL-X 1 Midsummer £218. £218
DC isolator Projoy 25A 3 pole 1 Midsummer £21.84 £21.84
AC isolator IMO 20A 4 pole 1 Midsummer £14.56 £14.56
Cabling 4mm2 solar cable 50m 1 Midsummer £52 £52
Connectors MC4 4mm2 pair 5 Midsummer £2.60 £13.00
Roof access assume already own suitable ladders £0
Total cost ex VAT £1343.80
Total cost inc VAT £1612.56

Professional installs would be free of VAT, while a DIYer would pay VAT. A professional install would obviously incur further costs in terms of labour etc, which we can't calculate here.

Based on the VAT-inclusive figure above, this would result in payback times of:

Return mode Rate for comparison Payback time
Self-consumption 35p/kWh 3.6 years
SEG (requires pro install) 4p/kWh 31.4 years
SEG Agile Outgoing (pro install) 34p/kWh (average) 3.7 years

If you could not consume all the generation the self-consumption payback time would be longer. This is a comparatively small system so generation is more likely to cover base loads from your house, while in a larger system a battery may be advantageous to avoid missing out on any generation.