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UK Plug-In Solar Savings Calculator

How much could a plug-in solar kit realistically generate and save at your address, and how long would it take to pay back? This calculator works it out from your panel size, your region, how the panel is mounted, and your own electricity rate, using irradiance data from the EU Joint Research Centre PVGIS tool rather than a rule of thumb. Every assumption and the exact formula are shown below the result, and this calculator does not sell, recommend or price any product.

Before you use this: plug-in solar is not yet lawful to use in the UK. SI 2026/848 changes that from 27 August 2026. This calculator models the scheme that will apply from that date, so you can plan ahead — it is not an invitation to buy and connect a system before then. See the current legal status for the full picture.

UK plug-in limits: 800 VA / 3.5 A at the inverter, 2,000 W panel cap.
Ofgem price cap, Direct Debit, Great Britain average, 1 Jul-30 Sep 2026, incl. VAT. Replace with your own tariff rate if you know it.
Roughly 9am-4pm. Lower if most people are out at work or school; higher if someone is usually home.
Left blank on purpose — this site does not price or recommend kits. Enter your own quote to see a payback estimate.
Estimated annual generation 769 kWh 742–795 kWh a year, most years
Estimated annual saving £60 £58–£62 a year, most years
Simple payback Enter a system cost above to see this

    How we got this number: 0.80 kWp effective (from 800 W of panel) x 960.64 kWh/kWp/yr (Midlands (Birmingham)) x 1.00 (south-facing) x 1.00 (good - near-optimal angle, unshaded) = 769 kWh/yr, plus or minus 27 kWh from year-to-year weather.

    The assumptions and the formula, in full

    Every number above comes from the steps below. Nothing is hidden in the script — if a figure looks wrong, you can check the arithmetic by hand, or re-run the underlying PVGIS query yourself.

    1. How much sunlight your region gets

    Generation figures come from the European Commission Joint Research Centre’s PVGIS tool, queried directly for each region below (not estimated, not taken from a third party): 1 kWp, free-standing mounting, 30° tilt, due south, 14% system loss, PVGIS-SARAH2 radiation data, 2005-2020. A flat 0° panel at the same London point returns a noticeably lower 851.69 kWh/kWp/yr, which is why we do not use 0° as the reference; PVGIS own calculated optimum for this latitude is closer to 40° at due south (1,028.71 kWh/kWp/yr for London). We use 30°, below that true optimum, because a plug-in kit is usually fence-, rail- or ground-mounted rather than roof-pitched at an installer-chosen ideal angle — a slightly shallower reference is more representative than the best case.

    RegionkWh per kWp per yearTypical year-to-year range
    South East England (London) 1,017 978–1,055
    South West England (Bristol) 1,014 978–1,049
    Midlands (Birmingham) 961 927–994
    Northern England (Manchester) 880 848–911
    Wales (Cardiff) 1,021 982–1,060
    Scotland (Edinburgh) 905 871–940
    Northern Ireland (Belfast) 910 878–942

    Figures at 1 kWp; this calculator multiplies by your effective system size. Example request behind the first row: re.jrc.ec.europa.eu … PVcalc (only lat/lon change between regions). The range column is PVGIS own interannual standard deviation — real weather variation from year to year, not a margin we added. Retrieved 2026-08-15.

    2. Orientation and mounting angle

    The orientation and mounting-angle bands are also PVGIS pulls, not guesses. Holding London’s location and the 30° tilt fixed and changing only the compass direction: south-facing is the 1,016.57 kWh/kWp/yr reference above; east- or west-facing returns 808.42 (80% of south); north-facing returns 588.97 (58% of south). The mounting-angle band changes both the angle and the assumed system loss together, as a combined proxy for a less-than-ideal real-world setup: the “Average” band uses a 10° tilt and 20% loss (862.43 kWh/kWp/yr, 85% of the Good band); the “Poor” band uses a flat 0° tilt and 30% loss (693.24 kWh/kWp/yr, 68% of the Good band). PVGIS needs a full horizon profile to model shading precisely, which this calculator does not collect, so the mounting-angle band is a stated simplification, not a measurement of your actual site.

    3. What counts as a saving

    A plug-in system in this size band has no export arrangement, so on current guidance any generation you do not use the moment it is produced is worth nothing in this estimate — there is no payment for it. Your saving is the electricity you generate AND use at the same time, valued at your import rate. This calculator uses your “share of usage in daylight” input as a direct stand-in for the share of generation you actually use, which is a simplification stated plainly rather than hidden: a small plug-in system often generates less than a home’s baseline daytime load (fridge, router, standby devices), which tends to push real self-consumption higher than this simple proxy suggests. Treat the saving estimate as cautious rather than generous, for that reason. For the fuller argument and the same method in prose, see Plug-In Solar Costs and Payback in the UK.

    4. The UK limits this calculator enforces

    Per the DESNZ Interim Product Specification version 2.0, named directly in SI 2026/848: a 800 VA / 3.5 A ceiling on rated AC output at the inverter, and a 2,000 W cap on total panel (DC) capacity. Enter more than 2,000 W and this calculator refuses to produce a number, because that would model a system the specification does not allow. Between 800 W and 2,000 W of panel, the inverter is still the ceiling on what reaches your home, so the estimate is capped at 800 W of effective generating capacity — the extra panel mostly helps maintain output on cloudy days and at low sun angles, rather than raising the best-case number shown here. Above 960 W of panel, the specification recommends a professional wiring check. Separately, current guidance limits most households to one device in total regardless of how many circuits you have — see the legal status page for the detail. This calculator does not model battery storage: the UK definition excludes it entirely.

    The exact formula

    1. Effective capacity = the lower of your panel watts and 800 W.
    2. Generation (central) = effective capacity (kW) × regional kWh/kWp/yr × orientation multiplier × mounting-angle multiplier.
    3. Generation range = the same, using the regional figure minus/plus its year-to-year standard deviation.
    4. Used (self-consumed) = generation × your daylight-usage share.
    5. Saving = used × your electricity rate (p/kWh ÷ 100). No export payment is added.
    6. Simple payback = your system cost ÷ saving (only shown once you enter a cost).

    This is an estimate, not a quote. Real output depends on the weather in any given year, how precisely your panel is angled and sited, and how your household actually uses power hour to hour — all of which can move your real result outside the range shown, especially on the downside if you are more shaded than the band you picked. Nothing here is a guarantee, and nothing here recommends a specific product.

    Sources

    For the reasoning behind this method in full prose, see Plug-In Solar Costs and Payback in the UK. For what plug-in solar actually is, see How does plug-in solar work. For the legal position and the exact date it changes, see the current legal status.