How We Make Our Solar Calculator Accurate in the UK

Why most solar calculators get it wrong

The typical online solar calculator does something simple: it multiplies your system size by a generic "sun hours" figure, then multiplies that by your electricity rate. The result looks precise, but it misses the most important variable in the real world.

Your panels do not care whether you are home. They generate electricity whether your kettle is on or not, whether anyone is in the flat or whether you are at work until six. The electricity they produce has to go somewhere. If your home is not consuming it at that exact moment, it flows back into the grid through your socket. Most plug-in solar users on a standard tariff do not get paid for that export. It is simply lost.

We built our plug-in solar savings calculator to address this directly. It does not assume you use 100% of what you generate. Instead, it estimates how much of your solar output actually displaces grid electricity you would have bought anyway.

How we model sunlight using real sun position data

Generic calculators use a single annual sun hours figure per country. We use something more precise: the actual position of the sun throughout the day, specific to your UK city, on today's date.

Our calculator uses the open-source SunCalc library to compute the sun's altitude at every 15-minute interval between sunrise and sunset. We then apply the standard solar energy formula:

This matters in the UK because our solar window changes dramatically by season. London gets around 3.5 peak sun hours in January and over 5.5 in July. A calculator using a single annual average ignores that seasonal variation and the way it affects when your panels actually produce power during the day.

You can see this in action on the sun panel that appears above the calculator inputs. It shows today's actual sunrise, sunset, solar noon, and peak sun hours for your chosen city, including a visual arc showing where the sun sits right now. Read more about how we track the sun if you want the technical detail.

Real solar irradiance data from PVGIS

Sun position tells us when the sun is up. It does not tell us how much energy actually reaches your panels. For that, we use PVGIS: the Photovoltaic Geographical Information System run by the European Commission's Joint Research Centre.

PVGIS holds decades of satellite-measured solar irradiance data for every location in Europe. When you click Calculate, our calculator sends your city's coordinates, your system size, your panel orientation, and the optimal tilt angle for your latitude to the PVGIS API. It returns the expected annual generation in kilowatt-hours and a month-by-month breakdown.

You will see this data appear in the results section after you calculate, labelled "Live PVGIS data," with a monthly bar chart showing how generation varies across the year. Summer months in the south of England will show two to three times the generation of winter months. That is the reality of UK solar, and your savings estimate reflects it.

New: postcode lookup for a location-aware estimate

City averages do the job, but a postcode does better. The calculator now has an optional UK postcode field in the main flow. Enter one and we validate it against postcodes.io, the free open-data UK postcode service, and feed the returned latitude and longitude straight into the PVGIS request. Your postcode also auto-selects the nearest supported city so the monthly profile and sun-position model stay anchored to a sensible regional baseline.

The field is entirely optional. Leave it blank and the calculator falls back to the city you choose in the dropdown. If postcodes.io is unreachable or the postcode is mistyped, we show a note under the input and revert to the city-level estimate. Nothing is saved or stored, your postcode is used only to look up coordinates for that session.

The biggest mistake: assuming all solar is used

Even with accurate generation figures, a calculator can still overstate savings by assuming you use every kilowatt-hour your panels produce. We call this the self-consumption assumption, and it is where most tools fail.

Self-consumption is the proportion of your solar output that your home actually uses as it is generated. For a typical UK renter who is out at work from 8am to 6pm, self-consumption might be 40 to 55%. For someone working from home with appliances running through the day, it could reach 80%. These are very different numbers, and they produce very different savings estimates.

A calculator that assumes 100% self-consumption would overstate savings by as much as double for someone who is out all day. That is the difference between a three-year payback and a six-year payback. Our goal is to give you the honest figure.

How we estimate real usage: the hybrid self-consumption model

We do not ask you to guess a percentage. Instead, we ask two questions that most people can answer accurately:

What is your daytime baseload in watts? This is the total wattage of appliances that run continuously while you are home or even while you are out: a fridge (typically 80W), a router (10W), a set-top box on standby, smart home devices, a fridge-freezer. Most UK flats have a baseload of 150W to 300W.

How many hours per day is your solar generating usefully? This is the solar window, not the full daylight hours. UK panels generate at meaningful output for roughly 3.5 hours in winter and 5.5 hours in summer, with a year-round average around 4.5 hours. We pre-fill this based on today's data for your city.

From those two numbers, we calculate your estimated monthly daytime demand and compare it to your monthly solar generation:

If your baseload demand exceeds your solar generation during that window, you use all of it (rate = 100%). If your generation far exceeds your baseload, you will export the surplus (rate < 100%) and those exported kilowatt-hours do not count toward your savings.

The auto-calculated rate appears in real time as you adjust your inputs. If you know your own usage pattern well, you can switch to the manual override and enter your own percentage instead. Either way, the figure feeding into your savings calculation is transparent and based on your actual situation, not a generic assumption.

Want to see your own estimate right now? Try the solar calculator and see how the self-consumption figure changes as you adjust your baseload and solar window.

How adding a battery changes the picture

A battery fundamentally changes the self-consumption calculation. Instead of exporting surplus solar, the battery stores it and makes it available in the evening when you are home and consumption is high.

In our model, adding a battery increases the self-consumption rate by 25 percentage points, capped at 90%. So if your auto-calculated rate is 55%, a battery brings that to 80%. That additional 25% of your annual generation becomes usable solar saving rather than exported surplus.

There is also a second source of savings with a battery: smart off-peak charging. If you are on a tariff like Octopus Go or Octopus Agile, you can charge the battery from the grid at around 7p to 9p per kWh overnight, then use that stored electricity during the day instead of paying 27p or more at peak rates. Our calculator models this separately, so you can see exactly what each source of saving contributes.

Curious whether an 800W system or the full EcoFlow STREAM kit with battery makes sense for your home? Run the numbers in the calculator with and without battery storage to compare payback periods directly.

Continuous improvement

We update the calculator regularly based on feedback from real users and changes in UK energy pricing. The Ofgem price cap figure is reviewed each quarter. PVGIS irradiance data is updated as new satellite measurements are processed. Our self-consumption model has been refined based on community feedback from renters and flat-dwellers who shared their actual usage patterns.

We have also expanded city coverage to 29 UK locations, added support for four system sizes from 400W to 1600W, and introduced the hybrid self-consumption model described above as a direct improvement over the fixed percentage dropdown used in earlier versions.

If you spot something that does not look right or if your real-world savings differ significantly from what the calculator predicted, we want to hear about it. Use the contact page to share your data.

A quick summary

The result is a savings estimate that reflects your actual home, your city, and your usage pattern rather than a marketing figure designed to make the numbers look as good as possible.

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