Free tool

How much solar will you actually use?

Solar only saves money when you use it as it’s made. Add your own half-hourly smart-meter data, or pick a typical home, and we’ll match it against the sun, half-hour by half-hour.

of an 800 W kit’s output used by a typical Birmingham home: 54%
a year off the bill: £118
a year sent to the grid: 394 kWh

Photo: Siân Wynn-Jones / Unsplash

1How do you use electricity?

Your file stays on your device. Your browser reads it here; nothing is uploaded to us or anyone else, and nothing is saved. Close the page and it’s gone.

Electricity you use a year 3,323 kWh

3,323 kWh is the GB average DESNZ uses. Your bill or supplier app shows yours.

Use your own half-hourly readings

Octopus: My account → Electricity usage → Download, half-hourly consumption. Other suppliers and apps (Bright, n3rgy, Glow) export a CSV with a time and a kWh column, which works too. Use import data from before you had panels.

2Where will the panels go?

3Which way will they face?

South

The best direction for solar.

4Kit, battery and prices

Panel size 890 Wp

Most 800 W kits have 800–1,000 Wp of panels. Output is capped at 800 W.

Battery for the evening

Usable capacity. Batteries aren’t covered by the plug-in solar rules, so one needs an electrician. Batteries and plug-in solar.

Your unit rate (p/kWh)

Export rate (p/kWh)

Most plug-in kits aren’t paid for export.

Of what the kit makes, you’d use

—

Working it out…

Worked examples

Who uses the most of their solar

An 890 Wp kit (800 W inverter) facing south on a 30° stand in Birmingham, for homes using 3,323 kWh a year, the GB average DESNZ uses. Savings at the 25.60p price cap rate.

46%

Out all day

Uses 390 of the 853 kWh the kit makes, saving about £100 a year. 463 kWh goes to the grid. Solar covers 12% of the home’s electricity.

54%

In half the day

Uses 459 of the 853 kWh the kit makes, saving about £118 a year. 394 kWh goes to the grid. Solar covers 14% of the home’s electricity.

57%

Home all day

Uses 487 of the 853 kWh the kit makes, saving about £125 a year. 366 kWh goes to the grid. Solar covers 15% of the home’s electricity.

Direction and timing

Share of the output used, by direction

Upright panels make less in total but spread it more evenly, so more of it gets used. West-facing panels catch the late afternoon, when more people are home.

Kit set-up in Birmingham	Makes a year	Out all day	In half the day	Home all day
South, garden stand (30°)	853 kWh	46%390 kWh, £100	54%459 kWh, £118	57%487 kWh, £125
South, balcony railing (90°)	630 kWh	50%314 kWh, £80	59%373 kWh, £95	63%394 kWh, £101
East, upright (90°)	440 kWh	60%264 kWh, £68	66%291 kWh, £74	67%296 kWh, £76
West, upright (90°)	427 kWh	66%281 kWh, £72	69%294 kWh, £75	70%298 kWh, £76

890 Wp kit, 3,323 kWh a year, no battery, no shading. Solar from PVGIS 5.3 seriescalc, PVGIS-SARAH3 2020 for Birmingham, scaled to the PVGIS long-term monthly average.

Bigger homes, batteries

How usage and a battery change it

South at 30° in Birmingham. A home that uses more electricity soaks up more of the same kit’s output.

Use a year	Out all day	In half the day	Home all day
2,000 kWh	33%	39%	42%
3,323 kWh	46%	54%	57%
4,500 kWh	54%	63%	66%

With a battery

Storing the surplus for the evening

3,323 kWh a year. Usable capacity, 90% round trip. Batteries aren’t covered by the plug-in rules and need an electrician: see batteries.

Battery	Out all day	In half the day	Home all day
None	46%£100 a year	54%£118 a year	57%£125 a year
1 kWh	80%£174 a year	92%£201 a year	95%£207 a year
2 kWh	90%£196 a year	95%£207 a year	96%£209 a year
5 kWh	95%£206 a year	95%£208 a year	96%£209 a year

How it works

Half-hour by half-hour

Solar. We take an hour-by-hour year of output from PVGIS (the European Commission’s solar data service) for your area, direction and tilt, scale it so each month matches the long-term average, and cap it at the 800 W the rules allow.
Your use. Either your own readings, matched to the same calendar day and clock time, or a typical home (below).
Matching. In each half-hour, solar first meets your always-on use (fridge, router, standby), which we take as the 10th percentile of your half-hours. Of your use above that, we count 40% as able to use solar, because appliances switch on and off within the half-hour. The rest of the solar is exported.
Battery. If you add one, surplus charges it (up to 800 W) and it covers your use when the panels can’t, losing 10% on the round trip.

Our assumptions

What the typical homes are

The three patterns follow the occupancy types in the industry self-consumption method (MCS MGD 003): out all day, in half the day, home all day. The shapes are ours, not measured data:

25% of the year’s use is always on, spread evenly.
The rest follows a daily pattern with peaks at breakfast and early evening; weekends follow the “home all day” pattern.
Winter use is about 20% above average and summer about 20% below.

We set the 40% so that a home-all-day household on 3,323 kWh with a 30° south kit lands near the 62% DESNZ uses in its impact assessment. Your own readings beat any typical shape, so use them if you can.

Using your solar: questions

How much of a plug-in solar kit’s output will I actually use?

Usually between a half and two-thirds. For an 890 Wp kit facing south at 30° in Birmingham, a home using the GB average of 3,323 kWh a year uses about 46% if it’s empty on weekdays, 54% if someone is in half the day, and 57% if someone is home all day. The rest goes to the grid. Each kWh you use saves the full unit rate (25.60p on the price cap).

Is my smart-meter file uploaded anywhere?

No. Your browser reads the file on your own device and does all the sums there. Nothing is sent to us or to anyone else, and nothing is saved: close the page and it’s gone. The only thing the page asks our server for is the solar data for your postcode district and direction.

Where do I get my half-hourly smart-meter data?

Octopus customers can download it from their online account (electricity usage, then download) as a CSV. Other suppliers show half-hourly data in their apps; free services such as Bright, n3rgy and Glow can export it as CSV once you link your meter. You need a smart meter set to record half-hourly readings. Use data from before you had panels: once a kit is running your meter only sees what you still import.

Why not just use my yearly kWh?

Because timing is everything. Panels make most of their power around midday, so two homes that use the same kWh a year can use very different shares of their solar. A home that’s empty on weekdays and busy in the evening exports much more than one where someone works from home.

Does a battery help with plug-in solar?

It lets you keep daytime surplus for the evening. In our example, a 2 kWh battery takes a home that’s out all day from 46% to 90% of the kit’s output used. But batteries aren’t covered by the GB plug-in solar rules, so one has to be installed by an electrician, and the extra saving has to pay for it.

How accurate is this?

It’s an estimate. Solar comes from a real weather year (2020) from PVGIS, scaled to the long-term monthly average. Half-hourly readings can’t show what happens within each half-hour, when a kettle or oven switches on and off, so we count only 40% of your use above your always-on level as able to use solar. Shade, a different tilt or a dull year will move the answer.

Sources

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