Plug-in Solar Savings Calculator

How It Works

Generation

Annual output starts from the regional yield of an optimally pitched south-facing array, then gets knocked down for the way a plug-in kit is actually mounted:

Regional yield runs from 1,055 kWh per kWp in southern England down to 800 in Scotland, read from the PVGIS satellite record (the UK solar map shows every cell). The mounting factor is the one that catches people out: a panel hung vertically on a south-facing balcony rail collects about 72% of what the same panel would collect on a proper 35 degree frame, because it is pointing at the horizon rather than at the sun. Facing east or west vertically drops that to half.

Clipping, when the panels are bigger than the inverter

Most kits sold under the 800 VA rule carry more panel than that, often 880 W or 1,000 W, because the inverter only rarely sees full sun. The calculator charges about 5% of annual output at a 1.5 DC to AC ratio and proportionally less below that, which is the usual northern-European figure. Oversizing is normally worth it: you lose a sliver of the summer peak and gain across every dull morning of the year.

Why the share used in the home decides everything

A rooftop system with an export tariff gets paid for its surplus. A plug-in kit does not. It pushes power out through the socket, the house consumes what it can at that instant, and anything left over goes into the grid unmetered and unpaid. So a kWh you use is worth the full 25p or so you would otherwise have paid, and a kWh you spill is worth exactly nothing.

That is why this calculator shows the spilled units as their own line. It is the number a kit's marketing never quotes, and it is usually between a quarter and a half of everything the panels make.

Worked example

An 880 W kit on an 800 VA inverter, hung vertically on a south-facing balcony in the Midlands, unshaded, at 25.7p per kWh, £499, with someone in the house for a good part of the day (60%):

• DC to AC ratio = 880 ÷ 800 = 1.1, so clipping costs 1% → factor 0.99
• Annual generation = 0.88 × 1,010 × 0.72 × 1.00 × 0.99 = 634 kWh
• Used in the home = 634 × 60% = 380 kWh
• Spilled to the grid for nothing = 253 kWh, which would have been worth £65.13 if anyone paid for it
• Annual saving = 380 × £0.257 = £97.69
• Simple payback = £499 ÷ £97.69 = 5.11 years; year by year, with tariff inflation, 4.9 years
• Over 20 years: £2,491.73 saved, a net gain of £1,992.73
• Cost of the electricity produced = 4.1p per kWh, against a 25.7p tariff

Now change one input and nothing else. If the house is empty every weekday and the share drops to 25%, the same kit on the same balcony saves £40.71 a year and takes 12.3 years to pay for itself. Take it off the rail and put it on a tilted frame in the garden instead, back at 60%, and generation rises to 880 kWh and the saving to £135.68.

Those three numbers, from one kit, are the whole argument for running your own figures rather than reading an average.