Solar Panel Calculator
Find out how many solar panels your home needs, estimated annual savings, and payback period. Free calculator for US and UK homeowners.
A solar panel system is one of the largest purchases most homeowners make. Before you request a single quote, it helps to have a clear number in your head: how many panels do you actually need?
This calculator gives you that number in seconds. Enter your annual electricity usage, your roof's available space, and your general location — and you'll see a panel count, estimated annual savings, and how many years until the system pays for itself.
How Many Solar Panels Do I Need for a 3-Bedroom House?
A typical 3-bedroom home in the UK uses around 3,100 kWh of electricity per year and needs 10–14 panels (a 4kW system). A comparable US home averages 10,500 kWh and needs 20–25 panels (a 7–9kW system). The exact number depends on your actual usage, panel wattage, and how much direct sunlight your roof receives.
What the Calculator Needs From You
Annual electricity usage (kWh) Find this on your electricity bill — most bills show annual or monthly consumption. If you only have monthly figures, multiply by 12.
- UK average household: 3,100 kWh/year
- US average household: 10,500 kWh/year
- Well-insulated UK flat: 1,500–2,000 kWh/year
- Large US home with EV: 15,000+ kWh/year
Available roof area (m² or sq ft) Measure the section of roof with the best sun exposure — typically south-facing in the Northern Hemisphere. Remove space for chimneys, skylights, and the required edge clearance.
A standard residential solar panel is roughly 1.7m × 1.0m (about 5.6ft × 3.3ft). A standard 400W panel needs approximately 1.7 m² of roof space.
Your location (peak sun hours) This is the average number of hours per day when solar irradiance is at sufficient intensity to generate meaningful power. It varies significantly by region.
| Location | Peak Sun Hours (avg) |
|---|---|
| Southwest USA (Arizona, California) | 5.5–6.5 |
| Southeast USA (Texas, Florida) | 4.5–5.5 |
| Northwest USA / Pacific Coast | 3.5–4.5 |
| South England / Wales | 3.0–3.5 |
| Scotland / Northern England | 2.5–3.0 |
| Southern Spain / Portugal | 5.0–6.0 |
| Central Europe (Germany, France) | 3.5–4.5 |
System efficiency and panel wattage Modern residential panels typically produce 350W–450W. Higher wattage panels cost more but need less roof space. The calculator uses 400W as a standard assumption unless you specify otherwise.
How the Solar Panel Calculation Works
Panels needed = Annual usage (kWh) ÷ (Peak sun hours × 365 × Panel wattage in kW × System efficiency)
System efficiency accounts for inverter losses, wiring, shading, and panel degradation — typically 80–85% for a well-installed system.
Example — UK home:
- Annual usage: 3,100 kWh
- Peak sun hours: 3.2
- Panel wattage: 400W (0.4 kW)
- System efficiency: 80%
Annual output per panel = 3.2 × 365 × 0.4 × 0.80 = 374 kWh/panel
Panels needed = 3,100 ÷ 374 = 8.3 → round up to 9 panels
A 9-panel, 3.6kW system covers this household's electricity demand with some margin.
Example — US home (suburban, Southeast):
- Annual usage: 10,500 kWh
- Peak sun hours: 5.0
- Panel wattage: 400W (0.4 kW)
- System efficiency: 82%
Annual output per panel = 5.0 × 365 × 0.4 × 0.82 = 599 kWh/panel
Panels needed = 10,500 ÷ 599 = 17.5 → round up to 18 panels
An 18-panel, 7.2kW system covers the household's demand.
Estimated Savings and Payback Period
The savings from solar depend on how much of the electricity you generate that you actually use yourself (self-consumption), and whether you export surplus power back to the grid.
| Scenario | Self-consumption | Annual bill saving (UK) | Annual bill saving (US) |
|---|---|---|---|
| Work from home, EV charger | 70–80% | £900–£1,200 | $1,400–$1,900 |
| Both adults work outside home | 30–50% | £400–£700 | $700–$1,100 |
| With battery storage added | 80–90% | £1,100–£1,500 | $1,600–$2,200 |
Typical system costs (2025–2026):
- UK: £5,000–£8,000 for a 3–4kW system (after 0% VAT)
- US: $15,000–$25,000 for a 6–9kW system (before 30% federal ITC tax credit — after credit: $10,500–$17,500)
Payback period at median savings:
- UK: 7–10 years
- US: 8–12 years (6–9 years after federal tax credit)
Solar panels typically come with 25-year performance warranties. Even at a 10-year payback, that's 15 years of effectively free electricity.
Does Your Roof Qualify?
Not every roof is ideal for solar. Key factors:
Orientation: South-facing (UK/Europe) or south-facing (US) produces the most power. East/west-facing loses about 20–30% efficiency. North-facing is generally not viable.
Pitch: Between 20° and 50° is optimal. Flat roofs can use mounting brackets to achieve an optimal angle.
Shading: Even partial shading from trees or chimneys significantly reduces output. Modern micro-inverter systems and optimisers can minimise the impact, but zero-shading is always better.
Condition: Panels last 25–30 years. If your roof needs replacing in the next 5 years, do the roof first.
Getting an Accurate Quote
The calculator gives you a reliable planning estimate. For a firm installation quote:
- Get at least 3 quotes from MCS-certified installers (UK) or NABCEP-certified installers (US)
- Ask each installer to model your actual consumption data, not averages
- Check whether your area qualifies for SEG (UK Smart Export Guarantee) or net metering (US) — these affect your return on surplus power
Related Calculators
- Electricity Cost Calculator — see exactly what your appliances cost to run
- EV Charging Time Calculator — size your solar system to cover EV charging
- Battery Life Calculator — estimate storage capacity you'd need to go off-peak
- Amps to Watts Converter — convert between electrical units
FAQs
How many solar panels does a 4-bedroom house need in the UK?
A 4-bedroom UK home typically uses 4,000–5,000 kWh per year and needs 11–14 panels (a 4.4–5.6kW system). The exact number depends on your actual usage, which you'll find on your electricity bill.
How much does a solar panel installation cost in the UK in 2025?
A typical UK home solar installation costs £5,000–£8,000 for a 3–4kW system. Solar panels are zero-rated for VAT in the UK since 2022. Battery storage adds £2,000–£5,000 on top.
What is the 30% federal solar tax credit in the US?
The Inflation Reduction Act provides a 30% federal Investment Tax Credit (ITC) on solar installation costs through 2032. If your system costs $20,000, you receive a $6,000 tax credit — reducing your effective cost to $14,000. This applies to both rooftop and battery storage systems.
How long do solar panels last?
Most residential solar panels carry a 25-year performance warranty guaranteeing at least 80% of their original output. In practice, panels often continue generating well beyond 30 years at reduced efficiency.
Can I go completely off-grid with solar panels?
Going fully off-grid requires substantially more panels and battery storage than a grid-tied system, plus a generator backup for extended low-sun periods. For most urban and suburban homes, grid-tied with battery storage (hybrid system) is more practical and cost-effective.
What is the Smart Export Guarantee (SEG) in the UK?
The SEG requires licensed electricity suppliers to pay homeowners for surplus solar electricity exported to the grid. Rates vary by supplier — typically 3p–15p per kWh. The highest rates currently come from Octopus Energy and OVO Energy. Check the Ofgem website for current rates.