Solar Panel Estimate Calculator

How to use this calculator

The estimator turns a single number — your monthly kWh — into a complete project view: system size, annual generation, gross and net cost (0% VAT), year-one bill savings, payback, and a 25-year cumulative figure with tariff escalation built in. Use it to sanity-check the first MCS quote you receive.

Inputs:

1. Monthly electricity use (kWh) — average the last 12 bills. The Ofgem typical domestic consumption value (TDCV) for 2024-26 is 2,700 kWh/yr = 225 kWh/month for medium households; a 4-bed home with electric heating, two cars and tumble dryer can run 4,500-6,000 kWh/yr.
2. Target offset (%) — UK roofs see most generation in summer, so 100% offset usually means around 30-40% self-consumption without battery. Households with battery typically reach 60-75% self-consumption and earn the rest at 15p/kWh export.
3. Peak sun hours/day — annual averages from the Met Office and BRE PVGIS-SARAH3: Cornwall and Channel Islands 2.8, South East 2.7, Greater London 2.6, Wales 2.4, Yorkshire 2.4, North England 2.3, Edinburgh 2.2, Highlands 2.0.
4. Installed cost per watt — the all-in MCS-certified turnkey price including 0% VAT. The 2026 Solar Energy UK median residential price is £1.55/W on systems 4-6 kWp, falling to £1.35/W on 8-10 kWp installations. Below £1.20/W usually indicates non-MCS gear or under-scoped DNO paperwork.
5. Grant / 0% VAT (%) — leave at 0 since the zero-rate is already in the £/W. For ECO4-eligible households or local authority schemes (e.g., Greater Manchester Solar Together, Solar Together Yorkshire), enter the grant percentage.
6. Electricity rate (£/kWh) — the all-in rate from your bill. April 2026 Ofgem price-cap unit rate is around 27p/kWh in most regions. Octopus Tracker and Agile customers should use their own rolling average.
7. Annual rate escalation (%) — UK tariffs have averaged 4-6% nominal growth since 2010 (with notable 2022-23 spike). 4% is a defensible long-run default.

How the math works

annual_target_kWh   = monthly_kWh × 12 × (offset% / 100)
system_kW           = annual_target_kWh / (peak_sun_hours × 365 × 0.78)
annual_generation   = system_kW × peak_sun_hours × 365 × 0.78
gross_cost          = system_kW × 1000 × £/W
net_cost            = gross_cost (0% VAT already applied)
year_1_savings      = annual_generation × £/kWh
simple_payback      = net_cost / year_1_savings
lifetime_savings    = sum over 25 years of (generation × (1-0.005)^year × rate × (1+esc%)^year)

The 0.78 derate matches BRE PVGIS-SARAH3 modelling — inverter losses (3-4%), wiring (2-3%), soiling (2-4%) and temperature derating (5-7%) at UK ambient.

Worked example for a London 3-bed using 3,200 kWh/yr at 27p/kWh:

• Annual target: 3,200 kWh
• System size: 3,200 / (2.6 × 365 × 0.78) = 4.32 kWp
• Annual generation: 4.32 × 2.6 × 365 × 0.78 = 3,200 kWh
• Gross cost at £1.55/W: 4,320 × £1.55 = £6,696
• Year-1 savings: 3,200 × £0.27 = £864
• Simple payback: £6,696 / £864 = 7.7 years
• 25-year lifetime savings (4% esc): ~£26,800

UK payback ranges by region (2026)

Solar Energy UK market data + Ofgem regional unit rates April 2026:

Sources: Solar Energy UK Member Survey 2025, MCS Installations Database, Ofgem Default Tariff Cap April 2026, BRE PVGIS-SARAH3.

What lowers UK payback below the headline

Three levers move the needle most:

• Battery (5-10 kWh) — pushes self-consumption from 30-40% to 60-75%, lifting effective savings by 15-25%. A 5 kWh GivEnergy or Tesla Powerwall battery adds £4,500-£7,500.
• Time-of-use tariff — Octopus Go (charge battery cheap at night, discharge at peak), Octopus Cosy (heat-pump optimised), or Agile (half-hourly wholesale) can cut effective import cost 20-40%.
• Solar Together / community group buys — Local-authority-led group purchases through iChoosr or Solar Together typically beat retail MCS quotes by 8-12%.

Pair this with the cost calculator, payback calculator, and ROI calculator

The estimator gives you the headline numbers in one pass; the cost calculator drills into £/W and adders (DNO upgrade, scaffolding); the payback calculator separates simple from discounted payback; the ROI calculator presents the whole project as an IRR you can compare against an ISA or NS&I bond.

Sources

• Solar Energy UK Member Survey 2025 — installed pricing benchmarks
• MCS Installation Database — verified installation data
• Energy Saving Trust Solar Calculator — generation modelling
• Ofgem Default Tariff Cap — price cap unit rates by region
• HMRC Notice 708/6 Energy-saving materials and heating equipment — 0% VAT confirmation
• BRE PVGIS-SARAH3 dataset — regional irradiation
• Octopus Energy Outgoing Fixed SEG tariff — current export rates