How Much Electricity Do Solar Panels Produce in Wiltshire?

The short answer: about 4,100 kWh a year from a 4 kWp system

A south-facing 4 kWp solar system in Wiltshire produces about 4,100 kWh of electricity a year. That is the PVGIS figure (the European Commission's solar model, v5.3, SARAH3 data, 35° roof, 14% system losses) for a roof near Melksham: about 1,025 kWh for every kWp of panels. Across the towns we cover the figure moves by only a few per cent, from about 1,015 kWh per kWp in Chippenham to about 1,051 in Salisbury. Roof direction matters far more than which Wiltshire town you live in, and the season matters most of all.

This guide sets out those figures so you can size a system, or check that an existing one is doing its job. If yours is falling short, our solar panel maintenance in Wiltshire service starts with exactly this comparison.

How much electricity do solar panels produce in each Wiltshire town?

PVGIS yields for a south-facing roof at 35°, per kWp of panels and for a typical 4 kWp array:

The spread is small because Wiltshire sits in one band of southern English sunshine. Salisbury does best, on the open edge of Salisbury Plain; the Avon valley towns sit a little lower. For local detail, our town pages for solar panels in Chippenham, Devizes and Swindon carry the same PVGIS figures alongside the planning and grid rules for each town.

Month by month: what a 4 kWp system makes

Annual totals hide a big seasonal swing. These are the PVGIS monthly figures for a south-facing 35° roof near Melksham:

June and July each produce more than three times December. On an average June day a 4 kWp array makes about 16.5 kWh, enough to cover a typical home's daytime use with plenty to export; on an average December day it makes about 4.4 kWh. That gap is the main reason to size a battery for your evening use rather than for the array, and why winter savings come mostly from charging a battery on a cheap overnight tariff.

Does roof direction change how much solar panels produce?

Yes, and by more than the choice of town. PVGIS figures for Chippenham, per kWp a year:

East and west roofs lose about a fifth of the annual total, but they spread generation into the morning and early evening, when many homes use more. In winter the gap widens: in December a Chippenham east or west roof makes under half of what a south roof does. Our guide to solar panel orientation in the South West covers split east-west arrays in more detail.

How much of that electricity will you use?

Ofgem's typical domestic consumption values put a single-rate home at 1,600 kWh a year (low use), 2,500 kWh (medium) or 3,800 kWh (high). A 4 kWp array making about 4,100 kWh therefore produces more than a medium home uses over a year, but much of it arrives in summer middays when nobody is in. The difference matters because self-used electricity is worth the unit rate you would otherwise pay, 26.32p/kWh at the Ofgem price cap for October to December 2026 (GB average), while exported units earn a Smart Export Guarantee rate, and the better fixed rates are around 12–13p. Adding a battery, or running a car charger or immersion diverter at midday, raises the share you use.

How much do solar panels produce in winter?

About a quarter to a third of the summer figure. In Wiltshire a south-facing kWp makes about 46 kWh in November, 34 kWh in December and 39 kWh in January, against 124–126 kWh in May to July. Snow and frost briefly stop generation, but cold itself helps panels: output falls as cells heat up, which is why spring days can rival midsummer.

Do solar panels produce less as they age?

Slightly. Panel makers warrant how fast output may fall: two current datasheets we checked allow no more than 0.35% and 0.4% a year after the first year. On those terms a panel still makes around 90% of its original output after 25 years. Faster losses than that point to a fault, not ageing.

How to check whether your solar panels are underperforming

Most inverters and batteries come with an app that shows monthly generation. To judge it:

• Compare like with like: this month against the same month last year, and against the PVGIS figure for your array size, direction and town.
• Allow for weather. A dull month can come in well below the PVGIS average without anything being wrong; two or three poor months in a row, or a sunny month that is down, deserve a closer look.
• Check strings and panels separately where the system allows it. One string well below its twin, or a single panel lagging on an optimised system, is a fault you can see in the data.
• Look for zero days. A day with no generation in daylight usually means the inverter has tripped.

Our guide to how often solar panels should be serviced explains what an annual service checks once you have spotted a shortfall.

Why a solar system produces less than expected

The usual causes, roughly in the order we find them: new shading from trees or extensions; dirt, lichen and bird fouling along the bottom edge of panels; an inverter that has tripped or is limiting output in heat; a failed connector or string; and a monitoring fault that under-reports a healthy system. Nesting pigeons under the array often bring both fouling and chewed cabling. Each shows up differently in the data, which is why a proper check starts with the monitoring history before anyone goes on the roof.

Using these figures to size a new system

Work back from your use. A medium home on 2,500 kWh a year is well matched by a 3–4 kWp array; a high-use or all-electric home on 3,800 kWh or more can use 5–6 kWp, roof permitting. Single-phase systems up to 3.68 kW connect under the G98 notification rules, while larger systems, including many solar-plus-battery designs, need G99 approval from the network operator before connection.

When to book a check

If your figures stay well below PVGIS for your roof after allowing for the weather, book an inspection rather than waiting for a bill to tell you. Our solar panel maintenance visits start from your monitoring data, then check the inverter fault log, connectors, isolators and panels. We cover Melksham, Trowbridge, Chippenham, Devizes and the surrounding area.