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 doesn't just disappear - it flows out to the grid the same as any small grid-tied inverter's would, but earns you nothing, since there's 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 doesn't 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's 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's 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's pulling power 24 hours a day regardless of who's home or what they're 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 don't have one

Without a smart meter, typical whole-home base load in UK properties tends to fall into a broad range by size - 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'd rather build the figure from the ground up. A fridge-freezer is usually rated around 400W on its nameplate, but that's the compressor's running power, not a constant draw - it cycles on and off, so averaged across a full day a typical fridge-freezer works out much lower, commonly somewhere in the 25-60W range depending on the model and how old it is. A WiFi router typically draws somewhere between 2W and 20W, averaging around 6W. 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: Home Energy Model: Base load energy explained - the overnight-reading method and the by-property-size base-load ranges. Power NI: Which appliances use the most electricity? - fridge-freezer and router wattage figures. Citizens Advice: Using your smart meter's in-home display - how to read your own display. These are general industry guides, not a measurement of your specific home - your own smart meter reading, where available, is always more accurate than any of the ranges above.

Sources

• sunhours.app: UK solar hours by region - the monthly irradiance dataset behind this calculator (20-year, 2001-2020 satellite climatology).
• 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.
• Sunsave: How much electricity do solar panels produce? - UK regional yield cross-reference.

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.