How the calculator works

This page explains the calculator methodology in plain English: which data sources are used, where official rules are applied, which assumptions are made by the model, and where real-world results can still vary.

The calculator is intended to provide a practical, realistic estimate. It is not a performance guarantee, an electrical design tool, a product approval service or a replacement for manufacturer instructions.

What the calculator estimates

For a selected plug-in solar setup, the calculator estimates:

• Annual solar generation: the amount of electricity the panels are expected to produce over a year.
• Monthly generation: how production changes through the seasons.
• Solar used at home: the share of generation likely to be used by the household rather than left unused or exported.
• Annual bill savings: the value of solar electricity used in the home instead of bought from the grid.
• Payback period: the time it may take for bill savings to cover the selected kit cost.
• Longer-term net savings: estimated savings after the kit cost over 5, 10 and 15 years.
• Daily availability: an illustrative clear-day profile showing when power may be available during each season.

These outputs are connected. A kit can generate a lot of electricity, but only the electricity used in the home creates a bill saving in the current model.

Balancing simplicity and accuracy

The calculator is designed for people who may have no previous experience with solar panels, plug-in solar, baseload, inverter limits or electricity modelling. It keeps the questions deliberately short so users can get a useful estimate without needing to understand every technical detail first.

That does not mean the estimate is generic. Behind those simple inputs, the calculator combines postcode-area solar data, panel setup, kit size, household baseload, daytime presence, work-from-home patterns, electricity price and current UK plug-in solar rules.

This is a deliberate balance: enough user input to produce a grounded estimate, without turning the calculator into a long technical survey that many households would struggle to complete accurately.

How the UK plug-in solar rules are applied

The calculator is built around the current UK plug-in solar route, rather than around generic rooftop solar.

In line with the published UK framework, the calculator supports systems with a maximum inverter output of 800VA, up to four panels, and up to 2,000W of combined panel capacity. It also treats plug-in solar as a solar-only kit. Integrated plug-in battery storage is not included in the calculations.

The calculator models configurations within the UK framework. It does not certify individual products as compliant. A real product still needs to meet the applicable UK requirements and be installed according to the manufacturer's instructions.

Solar generation data and postcode modelling

Solar generation estimates are based on European Commission PVGIS solar data, a recognised public source for estimating photovoltaic generation across Europe, including the UK.

The calculator matches the user's postcode to one of 124 UK postcode areas, then uses solar data for that area. This is much more location-specific than using a single UK-wide average, because annual solar generation varies meaningfully between different parts of the country.

For a plug-in solar savings estimate, postcode-area modelling is a practical level of precision. It captures the main regional differences in solar generation while keeping the calculator fast and avoiding the need to send full postcode or property-level details to PVGIS for every calculation.

Exact-address results may differ slightly, especially in very large postcode areas or places with unusual local terrain or conditions. The calculator should still be treated as an estimate rather than a property-specific survey.

Panel direction and mounting angle

The calculator uses PVGIS generation data for each supported panel direction and mounting angle in each UK postcode area. These figures are not simple in-house percentage adjustments from a single south-facing result; they are based on solar modelling data from the European Commission PVGIS tool.

The calculator currently uses eight directions: north, north-east, east, south-east, south, south-west, west and north-west.

It also models three common mounting angles:

• 5 degrees: an almost-flat setup, such as a low-tilt frame.
• 35 degrees: a tilted setup, often closer to an efficient UK solar angle.
• 90 degrees: an upright setup, commonly associated with balcony-style mounting.

Direction and angle affect both annual generation and the time of day when power is most likely to be available. South-facing panels usually produce the strongest annual result. East-facing panels tend to produce more earlier in the day, while west-facing panels tend to produce more later. North-facing panels are included for comparison, but they are generally weaker.

System losses, inverter limits and clipping

A 15% system-loss deduction is already included in the solar generation estimates taken from PVGIS. In other words, the calculator is using PVGIS figures that have already been reduced for system losses, rather than taking raw generation figures and applying another deduction afterwards.

This helps keep the figures grounded and allows for normal real-world losses, including inverter conversion losses, cabling losses, temperature effects and less-than-perfect panel conditions.

The UK plug-in solar framework caps inverter output at 800VA. That means a larger panel array cannot simply feed 1,200W, 1,600W or 2,000W into the home. The inverter acts as the gatekeeper.

Where panel capacity is higher than 800W, the calculator models the inverter ceiling and associated clipping. If the panels could produce more power than the inverter is allowed to output, the excess potential output is not counted as usable AC output.

For these larger panel arrays, clipping is estimated using hourly PVGIS generation profile shapes, then converted into a monthly adjustment. It is not applied as a simple annual cap.

Larger panel arrays can still help in weaker conditions such as cloudier weather, winter, mornings and evenings. The model therefore does not scale oversized systems in a straight line, and it does not treat all extra panel capacity as fully useful.

Household baseload

Plug-in solar savings depend heavily on how much generated electricity is used in the home while it is being produced. The model starts with household baseload, meaning the background electricity used by devices such as a fridge, freezer, broadband router, standby equipment, heating controls or security equipment.

If the user enters a measured baseload, the calculator uses that value. If not, it estimates a modest household baseload from household size. The automatic estimate ranges from 90W to 120W, depending on the number of adults and children selected.

That is intended to be a safe representative estimate, not a claim that every household sits neatly in that range. Baseload can vary a lot. A small home with extra refrigeration, networking equipment, security systems or devices left running could have a higher baseload than a larger home with very efficient appliances. We intentionally avoid applying large automatic baseload guesses, because that could make savings estimates look stronger than they really are.

The model then estimates baseload solar use month by month. It looks at the selected baseload, the number of days in the month and the average solar-generation window for that month, then caps the result against the solar generation available. In simple terms, baseload gets first claim on the solar electricity generated during daylight hours, but it cannot use more solar than the kit is expected to produce.

Daytime electricity use

After baseload has been accounted for, the calculator estimates how much spare solar may be used by normal daytime activity. It uses the household presence and working-from-home answers to judge how likely it is that someone will be using electricity while solar power is available.

The model applies a small allowance for occasional daytime use, then adds cautious allowances for weekday presence and work-from-home use. A household that is home more often is assumed to have more opportunity to use spare solar, but the estimate is based on the selected routine, not on the user carefully reorganising appliance use after installation. It does not assume perfect scheduling or that every spare kWh will be captured.

The work-from-home answer adds a modest daytime load, intended to represent typical computer and screen use during daylight hours.

Spare solar use is also capped by the amount of solar left after baseload. In other words, the calculator can only allocate extra daytime use where the panels are expected to produce more than the household baseload during solar-generating hours.

This is a behavioural estimate, not a full smart-meter simulation. It does not know exactly when someone boils a kettle, runs a dishwasher or charges a laptop. Instead, it uses cautious rules to estimate likely spare solar use without overpromising savings.

Self-consumption and unused solar

Self-consumption is the share of generated solar electricity used in the home. It is central to the savings estimate, because a generated kWh only reduces the bill if it replaces electricity that would otherwise have been bought from the grid.

The calculator gives baseload first claim on generated solar, then estimates how much remaining spare solar is likely to be used by daytime activity. Any generation not expected to be used in the home is treated as unused for savings purposes.

This is why annual generation and annual savings are not the same thing. Two homes with the same kit can generate similar electricity, but save different amounts depending on how much of that electricity they use while it is being produced.

Electricity prices, savings and payback

Annual savings are based on avoided grid imports. In simpler terms, the calculator values the solar electricity estimated to be used in the home at the selected electricity unit rate.

Where no custom tariff is entered, the calculator defaults to the current Ofgem price-cap electricity unit rate. Users can enter their own rate if they pay a different price.

Standing charges are not affected by plug-in solar generation, so they are not included. Export income is also not assumed, because eligibility for export tariffs has not yet been confirmed for UK plug-in solar systems.

Payback compares the selected kit cost with the estimated annual saving. Longer-term net savings subtract the initial kit cost and then estimate cumulative savings after 5, 10 and 15 years.

Longer-term savings include a 0.4% annual panel degradation allowance after year one. This means the model slightly reduces expected generation each year to reflect the gradual ageing of solar panels. We chose 0.4% because recent NREL PV Lifetime Project data reports several modern module types degrading in roughly the 0.3% to 0.5% per year range, while also showing that degradation varies by panel technology and product. It is intended as a realistic modelling allowance, not a guarantee that every panel will degrade at exactly that rate.

The model does not assume electricity-price inflation, so future savings are valued using the selected unit rate rather than an assumed rising tariff.

How the recommendation is chosen

Within the calculator results, the Our recommendation section gives a plain-English assessment of whether the selected plug-in solar setup looks like a good return on investment for that household.

The recommendation is based on four main checks: estimated annual saving, simple payback period, annual generation and the share of solar electricity likely to be used in the home.

• Good fit: usually shown when the setup saves at least £80 a year, has a simple payback period of 6 years or less, generates at least 600 kWh a year and the home is expected to use at least 60% of the solar electricity generated.
• Worth considering: usually shown when the setup saves at least £60 a year and has a simple payback period of 9 years or less, but does not meet all of the stronger good-fit checks.
• Not a good investment: shown when the setup falls below the worth-considering checks, such as annual savings below £60, a payback period longer than 9 years or a setup that is unlikely to justify the selected kit cost.

The calculator also explains the main reasons behind the recommendation, such as low generation, north-facing panels, limited daytime electricity use, high unused solar or a kit price that makes payback harder to justify.

The recommendation is not financial advice or product approval. It is a simple interpretation of the estimate, designed to help users understand whether the selected setup looks strong, borderline or weak.

Daily generation chart

The daily chart helps users understand power availability through the day. It shows an illustrative clear-day profile for each season, scaled against the relevant postcode-area and monthly generation estimate.

It is not a weather forecast. A real day can be much better or worse depending on cloud, shade, temperature and panel conditions. The chart is most useful for understanding when output may rise, when it may peak, how much may be absorbed by baseload, and when spare solar could be available.

What the model excludes

The calculator intentionally leaves some things out, either because they sit outside the current UK plug-in solar framework or because they would make the estimate look more precise than it really is.

• Integrated plug-in battery storage: not modelled because the current UK plug-in solar route does not provide for battery-integrated products.
• Export tariff income: not assumed because eligibility for UK plug-in solar export payments has not yet been confirmed.
• Grants or subsidies: not included unless a specific support scheme is added in future.
• Finance costs: not included, because users may buy outright or use different finance products.
• Maintenance and replacement costs: not modelled in the headline estimate, because costs will vary by product, warranty, installation and use.
• Property-specific electrical work: not included, because some homes may need no extra work while others may need professional assessment or upgrades.

If batteries become part of the UK plug-in solar framework in future, or if export-tariff eligibility becomes clear for compliant plug-in solar kits, the calculator can be updated to reflect that. For now, the priority is to keep the calculations aligned with the current UK framework and the information available today, so the estimates remain realistic rather than speculative.

Limitations and uncertainty

The calculator aims to be realistic, but every home is different. The model cannot know every physical, electrical and behavioural detail that affects performance.

Important limitations include:

• local shading from buildings, trees, railings, chimneys or roof features;
• the exact panel, inverter, cable and connector specification;
• balcony, wall, garden, roof or shed obstructions;
• installation quality and whether panels are kept clean and secure;
• weather variation from year to year;
• how household routines change after installation;
• future electricity prices;
• whether a specific product is approved for compliant UK use.

For that reason, the calculator should be treated as a useful estimate, not a guaranteed outcome. Its purpose is to help users compare plug-in solar setups, understand the main savings drivers and avoid unrealistic claims.

Model version and update history

The current calculator model is v1.5. The list below records substantive modelling changes that could affect generation, savings, payback or longer-term estimates.

• Calculator model v1.5 released 17 July 2026. Added a cautious spare-solar use adjustment for larger panel arrays where extra generation is created behind the output-limited inverter.
• Calculator model v1.4 released 13 July 2026. Added the almost-flat 5 degree mounting option, with PVGIS postcode-area generation data for all supported directions.
• Calculator model v1.3 released 9 July 2026. Refined self-use so baseload is modelled first, then behavioural daytime use is capped against annual spare solar and distributed back across months.
• Calculator model v1.2 released 8 July 2026. Added support for larger panel arrays behind an 800W inverter, using hourly PVGIS profile shapes to estimate monthly clipping effects.
• Calculator model v1.1 released 1 July 2026. Updated the default electricity unit rate to the July 2026 Ofgem price-cap rate, regenerated PVGIS postcode-area generation data with 15% PVGIS system losses, removed the separate practical generation deduction, and added a 0.4% annual panel degradation allowance to longer-term savings.
• Calculator model v1.0 released 29 June 2026. Initial live model using postcode-area generation estimates, household self-use assumptions, default kit costs, electricity tariff input and savings/payback calculations.

Cosmetic updates, layout changes and article edits are not normally treated as calculator methodology changes.