Solar Panels kWh Calculator

Free solar panels kWh calculator for the UK. Enter your monthly kWh use, target offset, and panel wattage to see system size, panel count, and bill savings — calibrated to PVGIS-SARAH3 and MCS standards.

Solar Panels kWh Calculator

Monthly electricity use (kWh) Target offset (%) Peak sun hours/day System efficiency (%) Panel wattage (W) Electricity rate (£/kWh)

System size needed

3.24 kW

Panels needed

Offset achieved

82.6%

Daily production

6.57 kWh

Monthly production

200 kWh

Annual production

2,398 kWh

Year-1 bill savings

£648

How to use this calculator

This tool sizes a solar system from the kWh number on your electricity bill — the opposite direction of a production calculator. Enter six values and it returns required system size in kWp, panel count, daily/monthly/annual production, offset achieved, and year-1 bill savings:

Monthly electricity use (kWh) — average from your last 12 months of bills. Ofgem’s 2026 medium household is 242 kWh/month (2,900 kWh/year).
Target offset (%) — how much of your bill you want to eliminate. 80% is standard without a battery; 100%+ if you have storage.
Peak sun hours per day — UK average is 2.6. PVGIS-SARAH3 shows the exact value for your postcode (Plymouth 2.8, London 2.7, Manchester 2.4, Edinburgh 2.3, Inverness 2.2).
System efficiency (%) — leave at 78%. PVGIS default for UK rooftop installations.
Panel wattage (W) — STC nameplate. 2026 UK residential standard is 405–440 W.
Electricity rate (£/kWh) — your blended residential retail rate. The April 2026 Ofgem price cap is 27p/kWh; fixed tariffs from Octopus, EDF, and OVO range 24–30p.

The formula

annual_need_kWh   = monthly_kWh × 12
target_kWh        = annual_need_kWh × (offset / 100)
required_array_W  = target_kWh × 1000 / (PSH × 365 × derate)
panel_count       = ceil(required_array_W / panel_W)
actual_array_W    = panel_count × panel_W
daily_production  = actual_array_W × PSH × derate / 1000
year1_savings     = min(annual_production, annual_need) × rate

The savings figure assumes self-consumption — every kWh you produce displaces a kWh from the grid at retail rates. Exported kWh earn the SEG rate, which is much lower; the savings calculator handles the dual-stream model.

A worked example for the UK Ofgem medium household at 80% offset:

Need: 242 × 12 = 2,904 kWh per year
Target: 2,904 × 0.80 = 2,323 kWh
Required array: 2,323 × 1000 / (2.6 × 365 × 0.78) = 3,142 W
Panel count: ceil(3142 / 405) = 8 panels
Actual array: 8 × 405 = 3,240 W (3.24 kWp)
Daily production: 3240 × 2.6 × 0.78 / 1000 = 6.57 kWh
Annual production: 6.57 × 365 = 2,400 kWh (83% offset)
Year-1 savings at 27p/kWh: £648

System size by household consumption

Using 2.6 peak sun hours, 78% derate, 405 W panels, 80% target offset:

Monthly kWh	Annual kWh	System kWp	Panels	Daily kWh	Year-1 savings*
200	2,400	2.43	6	4.93	£486
242	2,904	3.24	8	6.57	£628
300	3,600	4.05	10	8.21	£778
400	4,800	5.27	13	10.68	£1,037
500	6,000	6.48	16	13.14	£1,296
750	9,000	9.72	24	19.71	£1,944

*At 27p/kWh April 2026 Ofgem cap. SEG export rate not included.

What changes the result

Peak sun hours by region

UK PSH varies more than most homeowners realise. Same 242 kWh/month household needs:

3.0 kWp in Plymouth at 2.8 PSH (8 panels)
3.2 kWp in London at 2.7 PSH (8 panels)
3.5 kWp in Manchester at 2.4 PSH (9 panels)
3.7 kWp in Edinburgh at 2.3 PSH (10 panels)
3.9 kWp in Inverness at 2.2 PSH (10 panels)

Always pull your exact PSH from PVGIS-SARAH3 — the Met Office figures are similar. Typical MCS-certified installer quotes use these values.

Smart Export Guarantee tariffs

The savings figure assumes 100% self-consumption. In practice UK households self-consume 30–50% of generation without a battery and 70–90% with one. Exported kWh earn the SEG:

Octopus Outgoing Fixed: 15p/kWh (best widely available)
E.ON Next Export: 16.5p/kWh (Solar Reward customers)
British Gas Export & Earn Plus: 6.4p/kWh
EDF Export+ Variable: 4.5p/kWh
OVO SEG Tariff: 4.0p/kWh

Source: Ofgem SEG database, Q1 2026. Battery storage shifts the economics dramatically — a 10 kWh battery raises self-consumption from 35% to 75% on a typical 4 kWp system.

Panel orientation and tilt

The reference case is south-facing at 35° tilt. Off-axis penalties (Energy Saving Trust 2026 figures):

East or west: 15–20% loss versus south
North-east or north-west: 25–35% loss
Flat (0° tilt): 8–10% loss
Steep (60°+ tilt): 5–8% loss

UK terraced housing often has east-west roof aligned with the street — split-array installs put panels on both pitches and lose only 5–8% versus a hypothetical south-facing equivalent.

System derate

The default 78% comes from PVGIS-SARAH3 and accounts for inverter losses (3%), wiring (2%), soiling (1.5%), shading (2%), mismatch (2%), and temperature derating. UK soiling is much lower than southern Europe because rain washes panels — 1–2% annual loss versus 4–5% in Spain.

Why size from kWh, not from roof space

Roof-area sizing is the planning-stage shortcut. kWh sizing is what eliminates a bill. Two adjacent UK semis can use 1,800 kWh/year (gas heating, no kids) and 7,000 kWh/year (electric heating, EV, family of five) — same roof, very different systems.

The right workflow:

Sum 12 months of bills. Add projected load if you are buying an EV (3,000 kWh/yr at 12,000 mi) or heat pump (3,500 kWh/yr replacing gas in a typical UK home, per Energy Saving Trust 2026).
Decide your offset target. Without battery: 70–80%. With battery: 100–110%.
Run this calculator with local PSH from PVGIS.
Sanity-check roof area: each 405 W panel needs ~1.9 sq m. A 4 kWp system needs ~19 sq m of unshaded roof.
Get MCS-certified quotes — required for the 0% VAT zero-rate (HMRC Notice 708/6) and for SEG eligibility.

Common mistakes

Using a single-occupant baseline for a family: Ofgem medium 2,900 kWh/year is misleading. A four-person household easily uses 4,500–6,000 kWh.
Sizing for cloudy December: UK December production is 15–20% of June output. Solar is annualised — you bank summer surplus, but only with a battery or net-metering-style billing. Most UK domestic SEG tariffs do not net.
Ignoring the 0% VAT timing: 0% VAT (HMRC Notice 708/6) on residential solar is in force through 31 March 2027 and converts to 5% reduced rate after — get quotes booked before March 2027 to lock in.
Forgetting MCS sign-off: SEG eligibility requires MCS certification. A non-MCS install saves on labour but cannot register for SEG.

Sources

PVGIS-SARAH3 — European Commission JRC irradiance data for any UK postcode
Ofgem Typical Domestic Consumption Values 2026 — UK household kWh benchmarks and SEG database
Energy Saving Trust — UK residential solar generation data and orientation tables
MCS MIS 3002 — UK solar PV installation standard
Solar Energy UK 2026 Annual Review — installer benchmarks and SEG rate tracker
Met Office Solar Resource Maps — alternative PSH source

Frequently asked questions

How many kWh does a solar panel produce per day in the UK?

A 405 W residential panel at 2.6 peak sun hours and a 78% derate produces about 0.82 kWh per day (405 × 2.6 × 0.78 / 1000). Annualised, that is 300 kWh per panel per year. The Energy Saving Trust's 2026 figures cite 850–950 kWh per kWp for England and Wales — our calculator's defaults match this when you size by panel count. PVGIS-SARAH3 from the European Commission JRC is the authoritative source for UK irradiance data.

What is the average UK household electricity consumption?

Ofgem's 2026 Typical Domestic Consumption Values set the medium household at 2,900 kWh/year (242 kWh/month) for electricity-only customers. Low-use households are ~1,800 kWh/year, high-use 4,300 kWh/year. Households with electric heating, EVs, or heat pumps typically use 5,000–9,000 kWh/year. Always size from your last 12 months of bills — not the Ofgem average — because UK consumption varies enormously by heating fuel.

Should I aim for 100% offset or less?

Most UK domestic solar is sized for 70–90% of consumption because the Smart Export Guarantee (SEG) export rates are typically 5–15p/kWh — well below the 27p/kWh April 2026 Ofgem price cap retail rate. Octopus Outgoing Fixed pays 15p/kWh, the highest standard SEG. Self-consumption is therefore worth 1.8–5× more than export. Add a battery (10–15 kWh) and aim for 90–110% offset; without one, 70–80% is more economical.

Why is my actual production lower than the calculator says?

Three common UK-specific causes. First, you are using a southern England PSH for a Scottish install — PVGIS shows 2.4 PSH for Inverness vs 2.8 for Plymouth. Second, your roof faces east or west — common in UK terraced housing — costing 12–20% versus due south. Third, the system is partly shaded by chimneys, neighbouring buildings, or trees. MCS MIS 3002 requires a shading analysis, but optimiser-equipped systems (SolarEdge, Tigo) limit shading losses to the affected modules only.

How does panel wattage affect the count?

Higher-wattage panels mean fewer panels for the same kWp. A 4 kW system needs 10 panels at 405 W (Q CELLS, JA Solar), 9 at 450 W (REC Alpha Pure-R), 8 at 500 W (Maxeon, Trina Vertex S+). Roof real estate is the constraint — a typical UK semi has 12–16 sq m of viable south-facing roof, fitting 6–8 standard panels. MCS MIS 3002 requires panels be installed within 90° of due south for a clean PSH calculation; otherwise the orientation factor must be applied.

Solar Panels kWh Calculator