How many solar panels do I need? Use the calculator
The calculator below works out how many panels match your yearly electricity use, draws how many fit on one face of your roof, and estimates the cost, the yearly saving and the payback. It uses PVGIS figures for the nearest of five UK places and rounds up to whole panels. It is a starting point for talking to installers, not a survey.
Worked example below.
To match 3,500 kWh a year on a south-facing roof near Manchester
9 panels of 450 W: 4.05 kWp, making about 3,589 kWh a year
Cost, savings and payback
Size multiplied by the average price per kW for its size band: about £6,700 for 4.05 kWp. The averages are the government's, for MCS installations on existing homes in Great Britain in 2025-26; this is our calculation, not a quote. DESNZ's average price per kW steps down at 4 kW and again at 10 kW, so systems near those sizes compare unevenly. A real quote for your roof is the better basis for payback.
| Export payment | Saving a year | Payback |
|---|---|---|
| 3p a kWh (example untied rate) | £320 | about 21 years |
| 12p a kWh (example tied rate) | £560 | about 12 years |
About 911 kWh (25 per cent) of the solar electricity is used at home, each kWh saving your unit rate of 26.32p (this assumes a single-rate tariff); the other 2,678 kWh is exported. The example rates are what-ifs, not offers: a tied rate needs that supplier's import tariff, so your unit rate and standing charge could change too, and the 12p row is not a like-for-like household saving. Payback is the price divided by the first year's saving: it leaves out price changes, panels slowly making less, and an inverter replacement during the panels' life.
Want installers to price a system for your roof? Request a free solar quote.
How the calculator works
- Panels needed: yearly use divided by the yearly output of 1 kWp on your roof, then by one panel's rating, rounded up. This matches a year's total; in winter the panels make much less than in summer.
- Yearly output per kWp: PVGIS 5.3, the European Commission's solar tool, queried 13 October 2026 for Belfast, Cardiff, Edinburgh, London, Manchester: 1 kWp, a 35 degree pitch, 14 per cent system losses, the hills on the horizon but no shade from nearby buildings or trees. PVGIS assumes free-standing panels; for London, building-integrated panels gave about 3 per cent less, so panels on a roof may make a little less than shown.
- Roof layout: the most whole panels in a grid on one rectangle, upright or on their side, inside the clear strip. It is not a survey: it does not check the roof's strength, the fixings or planning rules.
- Cost: your quote's price at its own size or, without one, size in kWp times the DESNZ mean price per kW for MCS installations on existing homes: £2,108.65 up to 4 kW, £1,651.40 for 4 to 10 kW and £1,319.52 for 10 to 50 kW (DESNZ solar PV cost data 2025-26: mean installed price per kW of total installed capacity, MCS installations on existing homes in Great Britain, April 2025 to March 2026, VAT included where it applied; batteries, extended warranties and other material or works left out). No battery. The steps between sizes come from the bands, not from real systems, so it is a rough guide, not a quote. Not shown for Northern Ireland, which these averages do not cover.
- Solar used at home: our model of representative days in each month, fitted to the published tables in MCS 032, the industry's estimate method, for your occupancy (on average 2.5 percentage points from the published shares of solar used at home, at most 6.7). The same model runs our battery planner. MCS has not reviewed or endorsed it.
- Export payments: your own rate if you give one. Otherwise two examples for Great Britain: 3p is an untied Smart Export Guarantee rate (for example British Gas Export SEG), read 13 Oct 2026; 12p is a tied export rate needing that supplier for import (for example Outgoing Octopus), read 13 Oct 2026. A tied rate needs that supplier's import tariff, so the example is a what-if, not an offer. See export tariffs.
- When we hold back: outside 1,500 to 6,000 kWh of use or output, the range the model was checked for, and for a Northern Ireland home without its own export rate, we show the panel count and layout but no savings or payback.
The figures you type stay on this page; nothing is sent to us.
How do you calculate the right solar system size?
Divide annual electricity use by the annual generation expected from each kWp of panels. Then divide that capacity by one panel's rating in kWp and round up. That gives an annual-matching benchmark.
Required kWp = annual use (kWh) ÷ annual yield (kWh/kWp).
Panel count = required kWp ÷ panel watts × 1,000, rounded up.
In our hypothetical household, 3,500 ÷ 900 = 3.89 kWp before rounding. A 450 W panel is 0.45 kWp. So 3.89 ÷ 0.45 gives 8.64 panels: round up to nine, totalling 4.05 kWp. Their illustrative annual generation is 4.05 × 900 = 3,645 kWh.
Use a full year of electricity readings. If panels already supply part of your demand, your import bill understates total household use: account for solar used at home as well. The MCS pre-sale method distinguishes total consumption, generation and self-consumption. [2]
How many panels do different usage levels need?
| Annual electricity use | Capacity before rounding | Panels | Installed capacity / generation |
|---|---|---|---|
| 2,000 kWh | 2.22 kWp | 5 | 2.25 kWp / 2,025 kWh |
| 3,000 kWh | 3.33 kWp | 8 | 3.60 kWp / 3,240 kWh |
| 4,000 kWh | 4.44 kWp | 10 | 4.50 kWp / 4,050 kWh |
| 5,000 kWh | 5.56 kWp | 13 | 5.85 kWp / 5,265 kWh |
| 6,000 kWh | 6.67 kWp | 15 | 6.75 kWp / 6,075 kWh |
A three-bedroom house does not have one correct panel count. Occupants, electric heating, EV charging and daytime demand change the answer. Start from bills and the roof forecast, not the bedroom count. For context, Ofgem's typical medium electricity use in Great Britain is 2,500 kWh a year on a single-rate meter (from 1 July 2026); MCS uses 3,500 kWh, a British average, when a home's own figure is not available. Your bills are the better guide. [14][2]
What annual generation figure should you use?
Use a roof-specific annual kWh forecast divided by the proposed array's kWp. For example, a forecast of 3,600 kWh from 4 kWp gives 900 kWh/kWp. Those numbers are illustrative.
PVGIS, from the European Commission, estimates output using location, panel technology, slope, direction and losses. It also reports monthly generation and year-to-year variation. PVGIS models shadows from hills and mountains, not trees, chimneys or neighbouring buildings. MCS says an accurate estimate of shade will typically require a site visit. [1][2]
| Assumed annual yield | Panel count | Resulting capacity |
|---|---|---|
| 700 kWh/kWp | 12 | 5.40 kWp |
| 900 kWh/kWp | 9 | 4.05 kWp |
| 1,100 kWh/kWp | 8 | 3.60 kWp |
Direction made more difference than pitch in PVGIS estimates we ran for five UK cities (London, Cardiff, Manchester, Belfast and Edinburgh; 35 degree pitch, 14 per cent system loss, 13 October 2026): an east or west roof produced 77 to 81 per cent of a south roof's annual output, south-east or south-west 93 to 95 per cent, and north 52 to 56 per cent. A south roof at 20 degrees produced 95 to 96 per cent of one at 35 degrees. [13]
An MCS installer's estimate is kWp × Kk × shade factor, where Kk comes from MCS tables by postcode zone, pitch and direction, drawn from PVGIS data and multiplied by 0.8. So a figure from PVGIS itself and one from an MCS quote are on different bases: compare like with like. [2]
Check whether the estimate already includes shade and system losses before applying another deduction. PVGIS calculates one slope and one direction at a time, so a split roof needs a separate estimate for each pitch; add their annual outputs. [1]
For one illustrative 450 W panel at the assumed 900 kWh/kWp, annual generation is 0.45 × 900 = 405 kWh. Dividing by 365 gives about 1.1 kWh per day as a yearly average. It is not a forecast for every day or a winter guarantee.
Will that many panels power your house all year?
Matching yearly generation to yearly usage does not make the house self-sufficient. Solar output and household demand occur at different times. A home battery shifts solar by hours, not seasons: in a PVGIS estimate we ran for a south-facing London roof, December produces under a third of July's output, so winter evenings rely mostly on the grid. [13]
MCS's worked example makes the distinction clear: its household, at home all day, uses 3,879 kWh and generates 4,059 kWh a year. Without a battery, it uses 1,177 kWh of that solar output at home, about 30 per cent of its annual demand. These are MCS example values, not a measured Worthwatt household. [2]
A battery can move some energy into later hours, but its usable capacity, power and losses need a separate calculation. See battery sizing and solar alone versus solar with a battery. This calculator does not simulate storage or off-grid operation.
How many solar panels can you fit on your roof?
Panel dimensions are only the starting point. A surveyed layout must allow for edges, mounting, roof obstacles and access, as well as which parts receive useful sunlight.
For one exact example, AIKO's 480 to 490 W Neostar 3S+ module measures 1,762 × 1,134 mm. That is 1.998 m² per panel, about 2 m². Nine cover about 18 m² of module area. Mounting gaps and roof clearances are extra, so 18 m² of roof is not a promise that nine fit. [3]
Ask for a drawing showing panel orientation and obstacles. Compare a smaller array that fits with a larger layout across another roof pitch. For a flat roof, tilted rows can need spacing to avoid shading one another; see the flat-roof guide. Roof structure and planning requirements also need checking.
Is 3.68 kW a limit on the number of solar panels?
No. In Great Britain, the G98 threshold concerns the aggregate registered AC capacity of all generation and battery storage at the premises: up to 16 A per phase, which corresponds to 3.68 kW on a single-phase supply and 11.04 kW on a three-phase supply. For inverter-connected solar, the registered capacity is the lesser of the inverter's rating and the panels' rating, so once the panels' rating exceeds the inverter's, adding panels behind the same inverter does not raise it. [9][4] Northern Ireland uses NIE Networks' G98/NI, with the same 16 A per phase threshold. [10]
| Quantity | What it means | What to ask |
|---|---|---|
| Panel DC capacity, kWp | Sum of panel rated output | How much panel capacity is on each roof? |
| Inverter AC output, kW | Rated output feeding the house and grid | What is the inverter's continuous AC rating? |
| G98 threshold, A per phase | Connection-route test for aggregate AC generation | Are other generation and battery inverters included? |
| Agreed export limit, kW | Power the DNO permits to leave the premises | Is a G100 export-limitation scheme required? |
G99 applies where the total is above 16 A per phase or the equipment does not meet G98. Above 16 A, ENA's G98 says the DNO must be consulted before installation. [9][5] Fast-track routes exist: National Grid Electricity Distribution, for example, accepts up to 32 A per phase of equipment after an application, with a G100 scheme holding export to 16 A. [6] In Northern Ireland, NIE Networks' G99/NI Fast Track has covered up to 60 A per phase since 1 May 2026, with export held at 16 A per phase and approval needed before installation begins. [11]
Do not divide an inverter rating by a panel rating to decide what is safe to connect. String voltage, current, MPPT limits and the manufacturer's permitted DC sizing also belong in the design. An export cap can change annual yield, so it should be reflected in the forecast.
How many extra panels do you need for an EV or heat pump?
Add the electricity you expect to use, without counting demand already in your bills twice. Then rerun the annual-matching calculation. When that demand happens is a separate test.
| New demand | Inputs | Calculated addition |
|---|---|---|
| EV | 10,000 km/year × 0.2171 kWh/km; assume all charging at home | 2,171 kWh/year |
| Heat pump | 12,000 kWh/year useful heat ÷ assumed SPF 3.0 | 4,000 kWh/year for the stated heat and efficiency assumption |
For the EV calculation, the factor is the Energy Saving Trust fleet toolkit's figure for an average electric car, used when only the fuel type is known. [7] The government conversion factors the toolkit is based on already count charging losses, so do not add them again. [12] Better still, use your home charger's readings and allow for the share you charge elsewhere.
Our heat example assumes an annual Seasonal Performance Factor (SPF) of 3.0; it is not a single laboratory CoP used as a year-long guarantee. Get the useful space-heating and hot-water demand, seasonal performance and any auxiliary electricity from the installer. Winter heat demand can coincide with low solar generation. [8]
At the assumed 900 kWh/kWp and 450 W, the EV addition alone matches six extra panels annually; the heat addition alone matches 10. Calculate the combined demand before rounding: individually rounded counts should not simply be added.
What should the installer show before you choose a size?
Ask for the surveyed layout, total kWp, inverter AC rating, annual and monthly generation forecast, shade assumptions and network connection or export limits. Compare the extra installed price against the extra electricity you could use or export.
Having a bigger roof does not oblige you to cover all of it. A smaller array may suit the budget, while a larger one may support planned electric demand. Use the system-size cost guide and quote checklist to compare proposals using the same assumptions.
Sources
- European Commission JRC: PVGIS 5 user manual. PVGIS 5 manual, live page; read 14 October 2026. Back to the relevant section.
- MCS 032: Solar PV and Battery Pre-Sale Information. Issue 1.0, 1 January 2025; original PDF supplied in handoff; read 14 October 2026. Back to the relevant section.
- AIKO, Neostar 3S+54 AIKO-A-MCE54Db datasheet. 202601, V3.1 EN; read 14 October 2026. Back to the relevant section.
- National Grid Electricity Distribution: G98 connection procedures. Live DNO guidance, GB distribution network; read 14 October 2026. Back to the relevant section.
- National Grid Electricity Distribution: G99 connection procedures. Live DNO guidance, GB distribution network; read 14 October 2026. Back to the relevant section.
- National Grid Electricity Distribution: G99 procedures guidance. Issued guidance copy supplied in handoff; no date inferred; read 14 October 2026. Back to the relevant section.
- Energy Saving Trust: Car and van kWh calculations. Live fleet toolkit; fallback factor, not a household forecast; read 14 October 2026. Back to the relevant section.
- Energy Saving Trust: In-depth guide to heat pumps. Updated 16 July 2026; read 14 October 2026. Back to the relevant section.
- ENA: Engineering Recommendation G98, Issue 1 Amendment 7. Issue 1 Amendment 7 (2022), the edition we read; later amendments exist; read 14 October 2026. Back to the relevant section.
- NIE Networks: What is G98/NI?. Live network operator page; read 14 October 2026. Back to the relevant section.
- NIE Networks: G99/NI Fast Track. Live network operator page; read 14 October 2026. Back to the relevant section.
- DESNZ: 2025 Government greenhouse gas conversion factors, methodology paper. 2025 methodology paper, paragraph 5.31; read 14 October 2026. Back to the relevant section.
- European Commission JRC: PVGIS 5.3 PVcalc, Worthwatt runs for five UK cities. 1 kWp, 14% loss, free-standing mounting setting, PVGIS-SARAH3 2005 to 2023; queried 13 October 2026, London spot re-query 14 October 2026; read 14 October 2026. Back to the relevant section.
- Ofgem: Summary of changes to energy price cap 1 July to 30 September 2026 (revised TDCV). June 2026; read 14 October 2026. Back to the relevant section.