Energy calculator

Your setup

Base load is a rough always-on estimate (fridge-freezer, router, standby devices) pre-filled at a typical UK figure - edit it to match your own home if you know it (e.g. from a smart meter's overnight reading). Generation up to this level is assumed to be almost always used instantly; generation above it depends on whether someone's actually home and using extra power, which is what the occupancy setting above controls. Whatever's neither used up to base load nor absorbed by extra daytime demand does not just disappear - it flows out to the grid the same as any small grid-tied inverter's would, but earns you nothing, since there is no metering or export tariff set up for a kit like this. In other words: generate more than your household can use at that moment (up to the kit's 800W cap) and the excess is exported, not banked or paid for.

Mounting type examples

Mounting type is illustrative only for the four outdoor options - it does not change the generation estimate itself, since a ground frame, a balcony rail, a wall-hang and a wall mount can each be set up tilted or vertical (the facing and orientation you choose above are what actually affect output). See Mounting for planning/fixing rules for each type. The two indoor options work differently - see below.

Estimated results

How this is calculated: panel capacity is capped at the 800W/800VA inverter limit, with capacity above that derated to 45% (representing the extra low/medium-irradiance capture an oversized array gets - see the worked explanation on Considerations), then multiplied by solar irradiance (kWh/m²/day) for the chosen period and an industry-standard 0.80 performance ratio, then by your panel placement factor. The usable share splits generation into "up to your base load" (assumed ~95% self-consumed, since something is almost always drawing that much) and "above your base load" (only counted as used at the rate set by your daylight-occupancy answer). Savings are shown against both Ofgem's Great Britain-average price cap and Octopus Energy's Flexible tariff, since actual unit rates vary by supplier and region. This does not model shading, an export tariff, a battery, or your own home's actual tariff.

Sun hours at this location

Monthly average solar irradiance (kWh/m²/day, "peak sun hours") for the selected location.

Why a bigger panel can be worth it even though output is capped

The inverter never passes more than 800W into your home, no matter how many watts of panel you connect - but a bigger array reaches that 800W ceiling in weaker light, holding it for longer through the morning, evening, and cloudier days, rather than only briefly around solar noon on a clear day. That is why the calculator still shows higher generation as you move the panel-capacity slider up, even past 800W. See Considerations for the full worked-example table.

About the placement options

South-facing and tilted ~30-40° is the best-case baseline this calculator's location data already assumes - a factor of 1.00. Facing away from south (east, west or north) and standing the panel vertically instead of tilting it each reduce that baseline; the two factors are multiplied together, and the "How your placement factor is worked out" card above shows exactly how for your current choices. East and west are treated the same, since there is no sourced UK dataset splitting them apart. Choosing one of the two indoor mounting options (flat against a window, or tilted behind glass) works differently: it overrides the facing/orientation multiplication with its own fixed, illustrative estimate instead, since those figures already assume a south-facing window and bake in an estimated glass-transmission loss - see the callout that appears above when you pick one, and Considerations for the full sourcing.

Working out your base load

"Base load" here means your home's minimum, always-on electricity draw - the fridge-freezer, the router, standby electronics, a boiler's controls - the stuff that is pulling power 24 hours a day regardless of who is home or what they are doing. It matters for this calculator because generation up to that level is assumed to be almost always used instantly (something is nearly always drawing at least that much), while generation above it depends on active use, which the occupancy setting controls separately.

The accurate way: read it off a smart meter

If you have a smart meter, this is worth doing properly rather than guessing. Check your in-home display or supplier's app for half-hourly (or more frequent) usage, and look at the overnight window - roughly midnight to 5am, when nothing should be actively running. Find the lowest reading in that window on a normal night, not the average - that floor value is your base load. If your display shows kW directly, multiply by 1,000 to get watts for the field above; if it only shows kWh for a half-hour period, divide by 0.5 to get the equivalent kW, then multiply by 1,000.

Rough guide if you do not have one

Without a smart meter, typical whole-home base load in UK properties tends to fall into a broad range by size. These are this site's own illustrative planning figures, based on typical UK household appliance ownership rather than a single published study - useful as a sanity check, not a substitute for measuring your own:

• Small flat (1 bed)Roughly 100-200W continuous
• Mid-terrace house (3 bed)Roughly 200-350W continuous
• Larger detached (4+ bed)Roughly 300-500W continuous

Or add up individual always-on devices if you would rather build the figure from the ground up. The Energy Saving Trust publishes a running-cost figure for an F-rated 424-litre fridge-freezer of about £80/year in Great Britain: at the Ofgem price cap's average unit rate used elsewhere on this calculator (26.32p/kWh, Oct-Dec 2026), that works out to roughly 305 kWh/year, or an average continuous draw of about 35W once spread evenly across the year (a fridge-freezer's compressor actually cycles on and off rather than drawing a constant amount, but averaged over a day, this is what it comes to). That is one example, not a universal figure - an older or larger model draws more, a newer or smaller one draws less, so treat the 25-60W range as either side of it, not a precise number. We could not find a UK government or professional-body source giving a wattage figure for a typical WiFi router specifically; 2-20W, averaging around 6W, is a commonly quoted range based on manufacturer specifications rather than any single published study - a planning estimate, not a sourced fact. Beyond that, most homes have a scattering of small standby loads (a set-top box, a games console left on standby, phone chargers left plugged in, a boiler's control electronics, an alarm system) that are each individually tiny - often well under a couple of watts apiece - but can add up to a noticeable chunk of the total.

Sources: GOV.UK: Home Energy Model (HEM) methodology - the government's own source for the "base load" terminology (not to be confused with similarly-named commercial sites, which we have stopped citing). Energy Saving Trust: Top five energy-consuming home appliances - the fridge-freezer running-cost figure converted to an estimated wattage above. Citizens Advice: Using your smart meter's in-home display - how to read your own display. The by-property-size ranges, the overnight-reading technique and the WiFi-router figure are flagged above as this site's own practical guidance, not a specific external authority's published figures - your own smart meter reading, where available, is always more accurate than any estimate on this page.

Sources

Rules last checked: 29 October 2026, against the Interim Product Specification v2 (16 July 2026), SI 2026/848 and Ofgem's G98 decision of 11 August 2026. The specification has not been updated or replaced since July 2026.

• Plug-in Solar Device Interim Product Specification, version 2 (DESNZ, 16 July 2026) - the current version, referenced by name in the law.
• legislation.gov.uk: SI 2026/848 - amends the Plugs and Sockets etc. (Safety) Regulations 1994 and the Electricity Safety, Quality and Continuity Regulations 2002; in force 27 August 2026.
• Ofgem: decision to approve DCRP/MP/26/02 (11 August 2026) - the G98 amendment for plug-in solar: one device per household and notification to your network operator.
• sunhours.app: UK solar hours by region - the monthly irradiance dataset behind this calculator (20-year, 2001-2020 satellite climatology). Under review: we are working to replace this with a more authoritative UK solar-industry dataset (MCS's own published irradiance data, or the EU Joint Research Centre's PVGIS tool) - this session's network access could not reach either to pull the numbers; the figures above have not changed yet.
• Ofgem: Energy price cap unit rates and standing charges - the Ofgem comparison price.
• Octopus Energy: Flexible Octopus tariff and Octopus Energy developer API - the Octopus comparison price. Octopus's product/rates API (api.octopus.energy) is genuinely public and keyless; this page currently uses a manually checked, dated snapshot of the Flexible Octopus unit rate rather than a live per-visit fetch - see the CHANGELOG for this release for why, and octopus.energy for your own region's actual current rate.

This is a modelling tool, not a quote or financial advice. Real generation depends on shading, weather variability year to year, and your exact panel/inverter model; real savings depend on your own tariff, which may not match either comparison price shown.