Solar calculator methodology

Last updated: 2026-04-26. Tracking 26 SEG tariffs across 10+10+6 eligibility tiers.

The short version

We model your annual solar generation from system size, location, orientation, and tilt. We split it into self-consumption (what you use directly) and export (what goes to the grid). We then run that through every SEG tariff currently on the market, grouped by what you actually qualify for, and surface the best in each tier so you can see what's realistic versus what's marketing.

Every number on the results page comes from this calculation. Every assumption is documented below. Every supplier rate has a `verifiedAt` date and a clickable source URL.

Step 1: Annual generation

The core formula:

Annual generation (kWh) = System size (kWp) × Regional irradiance × Performance ratio × Orientation factor × Tilt factor

Regional irradiance

The amount of solar energy hitting a horizontal square metre in your region per year, measured in kWh/m²/year. We use 14 GSP regions matching the UK's electricity distribution map. Values range from about 770 (Northern Scotland) to 1,020 (South-East England) - so location really does matter.

Source: PVGIS, the EU's open photovoltaic geographical information system. This is the same dataset professional installers use.

Future improvement: V1.1 will integrate the PVGIS API for postcode-specific irradiance rather than regional averages, improving accuracy by 5-10%.

Performance ratio

Real-world losses including inverter efficiency, wiring losses, panel temperature, soiling, and module mismatch. We use 0.80 (80%) - the standard assumption for well-installed UK domestic systems. A new high-quality install might hit 0.85; an older or poorly-sited system might be 0.70.

Orientation factor

Multiplier vs perfect south-facing. Our values:

These are derived from PVGIS modelling for UK latitudes at optimal tilt.

Tilt factor

UK optimum is roughly 35° (which matches typical UK pitched roofs). Each 10° deviation from this costs about 2-3% annual generation. Flat (0°) loses about 9%, vertical (90°) loses about 30%. We use a linear approximation - more than accurate enough at this level of estimation.

Step 2: Self-consumption split

Solar generates during daylight hours, but most home electricity demand is morning and evening. Without storage, only some of your solar generation is used directly - the rest is exported to the grid.

We use these defaults:

These are realistic averages, but actual values vary widely. A household working from home might hit 50% without a battery. A family that's out all day might only hit 20%. If you know your actual figure, the "Advanced options" section lets you override it.

Self-consumption matters because every kWh you use directly saves you the import cost (currently around 25p/kWh on standard tariffs), while every kWh exported earns you the SEG rate (currently 3-25p/kWh depending on supplier and tier).

Step 3: Self-consumption savings

Self-consumption savings (£) = Self-consumed kWh × Import unit rate (p/kWh) ÷ 100

We use your current import unit rate as you've entered it. The default is the current Ofgem price cap rate, but you can adjust it if you're on a different tariff.

Notice this number doesn't change between tariffs - it's based on what you save by not buying grid electricity. Tariff choice only affects the export portion.

Step 4: SEG export comparison - the three tiers

Smart Export Guarantee (SEG) is the scheme that replaced Feed-in Tariffs. Suppliers with 150,000+ customers must offer one. Rates vary dramatically - from 1p/kWh (legal minimum) to 25p+ for top tariffs.

Almost every major supplier offers multipletariffs at different eligibility levels. Choosing "Octopus" doesn't mean a single rate - they have rates from 4.1p (open to anyone) to 32p (with battery, on closed Flux tariff). Most other UK calculators ignore this and pick one rate per supplier, often misleadingly.

We group the 26 tariffs we track into three tiers based on what you actually need to do:

Tier 1: Open (10 tariffs)

Anyone with an MCS-certified solar install and a smart meter qualifies. No need to switch electricity supplier. Top rate currently around 13p (Fuse Energy).

These are the only rates a user can definitely access without changing anything else about their setup. If you're happy with your current supplier, this is your realistic best.

Tier 2: Switch electricity supplier (10 tariffs)

You must take the supplier's electricity import tariff too. This is where the bulk of value sits - a willing user can typically jump from 4p to 12-16p. Top rate currently around 16p (Ecotricity).

The key caveat: check the import tariff isn't terrible before switching. A great export rate paired with an expensive import rate can be worse overall than a lower export rate with a cheaper import. We don't model that combined optimisation in V1, but we flag it loudly.

Tier 3: Installer-locked (6 tariffs)

You must use the supplier as your solar installer too. Highest headline rates (15-25p) but you give up choice of installer, which is a real trade-off - these aren't always the best installers in the market.

Many installer-locked tariffs are 12-month promos that revert to lower rates. We expose this with a separate "Effective/yr" column. See "Effective rates" below.

Effective rates: the honest comparison

Some installer-tier tariffs market themselves with a high headline number that only applies for the first 12 months, then drops sharply. Two examples in our current dataset:

Our calculator shows two annualised numbers per tariff:

On a typical 4kWp south-facing system without battery, So Energy So Bright shows £639/year in year 1 but only £333/year averaged across 25 years - which makes it worsethan several open-tier tariffs that don't require any commitment at all. The marketing rate is misleading. We surface this directly.

Step 5: Lifetime modelling

Solar systems lose efficiency slowly over time - typically 0.5% per year (the standard manufacturer warranty assumption). Over a 25-year lifetime that compounds to about a 7% reduction by year 25.

We use a simplified linear model:

Effective lifetime years = Lifetime - (Degradation × Lifetime² ÷ 2)

For 25 years at 0.5% degradation, this gives an effective ~23.4 years of full-output generation. The lifetime benefit calculation factors in both the rate schedule (including any promo period drop-off) AND degradation.

We use 25 years as the lifetime because that matches typical inverter warranty periods. Panels themselves often last 30+ years, but most installs need an inverter replacement once during their working life (typically £800-£1,500).

Things we deliberately do not model in V1:

Step 6: CO₂ savings

Every kWh of solar you generate displaces a kWh that would otherwise have come from the grid. The grid emits about 124 gCO₂/kWh on average (NESO 2025 figure).

CO₂ saved per year (kg) = Annual generation × 124 ÷ 1000

This is a conservative figure - the UK grid is decarbonising rapidly, but solar offsets at the time of use, which means it offsets daytime grid mix. As the grid gets cleaner, the relative CO₂ benefit of solar shrinks slightly. But solar is always net-positive for emissions.

What the calculator doesn't do (honest list)

V1 does not handle:

How we keep this data fresh

SEG rates change. We caught Octopus dropping Outgoing Octopus from 15p to 12p (March 2026) only because we manually verified before publishing. Our commitment:

Sources

The data underlying this calculator comes from:

Found a mistake or have a methodology improvement to suggest? Email hello@energycosting.co.uk. We genuinely want to know - calculator accuracy is the entire point.

See also: solar calculator, heat pump methodology.