# Solar Panel Calculator

> Calculate solar panel system size, annual generation, savings, and payback period for your home. Free instant results with cost estimates.

Canonical: https://renoquant.com/solar-panel-calculator
Prices updated: 2026-03

## At a glance

A typical 3-bedroom European home using ~4,000 kWh/year needs a 3.5–4 kWp system (9–10 panels of 400 W) and about 18 m² of roof. Payback is 8–12 years in central Europe and 5–7 years in southern Europe (Spain, Italy); panels are warranted to retain 80%+ output for 25 years. South-facing roofs are ideal but east/west still produces 80–85% of peak — and panels generate 10–25% of peak even on overcast days, so cloudy regions are not disqualifying.

## How to Calculate Your Solar Panel System

Sizing a solar panel system correctly is the single most important decision in your solar journey. An undersized system leaves money on the table; an oversized system wastes capital on panels that cannot pay for themselves. This guide explains the sizing formula and provides reference data for the most common scenarios across Europe.

### The Solar Sizing Formula

The core calculation is:

**System size (kWp) = Annual consumption (kWh) / (Peak sun hours x 365 x 0.80 x Orientation factor)**

The 0.80 factor accounts for real-world system losses including inverter efficiency, wiring losses, temperature derating, and soiling. The orientation factor adjusts for roofs that do not face due south.

### Solar Panel Output by Region

| Region | Peak Sun Hours | Annual kWh per kWp | System for 4,000 kWh |
|--------|---------------|-------------------|---------------------|
| UK (London) | 2.8 | 820 | 4.9 kWp (13 panels) |
| Germany (Berlin) | 2.9 | 850 | 4.7 kWp (12 panels) |
| France (Paris) | 3.2 | 935 | 4.3 kWp (11 panels) |
| Spain (Madrid) | 5.2 | 1,520 | 2.6 kWp (7 panels) |
| Italy (Rome) | 4.8 | 1,400 | 2.9 kWp (8 panels) |
| Netherlands | 2.7 | 790 | 5.1 kWp (13 panels) |

Southern Europe generates nearly double the output per kWp compared to northern Europe. This means a smaller, cheaper system can offset the same electricity bill. Use our [insulation calculator](/insulation-calculator) to reduce your heating load before sizing solar, as lower consumption means a smaller system.

### System Sizes by Home Type

| Home Type | Annual Usage (kWh) | Recommended System | Panels (400W) | Roof Area (sqm) |
|-----------|-------------------|-------------------|---------------|-----------------|
| 1-bed flat | 1,800 | 2.0 kWp | 5 | 9 |
| 2-bed terraced | 2,900 | 3.2 kWp | 8 | 14.4 |
| 3-bed semi-detached | 4,000 | 4.4 kWp | 11 | 19.8 |
| 4-bed detached | 5,500 | 6.0 kWp | 15 | 27 |
| 5-bed detached | 7,500 | 8.0 kWp | 20 | 36 |

These figures assume central European sunlight (3.5 peak hours/day) and a south-facing roof. If your roof faces east or west, add 20% more panels to achieve the same annual generation. Check your [roofing condition](/roofing-calculator) before installing panels, as replacing a roof after solar installation is expensive.

### Cost Breakdown

| Component | % of Total Cost | Cost Range |
|-----------|----------------|------------|
| Solar panels | 40-45% | EUR 200-350 per panel |
| Inverter | 10-15% | EUR 600-1,500 |
| Mounting & wiring | 15-20% | EUR 150-250 per kWp |
| Installation labour | 25-30% | EUR 300-500 per kWp |

The total installed cost in Europe ranges from EUR 1,000-1,500 per kWp for residential systems. A 4 kWp system typically costs EUR 5,000-7,000 fully installed. Prices have fallen 60% since 2015 and continue to decrease as manufacturing scales up.

### Maximising Your Return

Self-consumption is the key to maximising savings. Electricity you use directly from the panels avoids both the generation cost and the grid markup. Export tariffs in most European countries pay only EUR 0.05-0.10 per kWh, compared to the EUR 0.25-0.35 you pay to import. Run your washing machine, dishwasher, and EV charger during peak solar hours (10am-3pm) to boost self-consumption from the typical 30% to 50-70%.

Adding a battery storage system (EUR 4,000-8,000 for 5-10 kWh) can push self-consumption above 80%, but the extra cost extends the payback period by 3-5 years at current battery prices. For most homeowners, optimising usage patterns is more cost-effective than adding a battery.

## Frequently Asked Questions

### How many solar panels do I need for my home?

The number depends on your electricity consumption and roof space. A typical 3-bedroom home using 4,000 kWh/year needs a 3.5-4 kWp system, which is 9-10 panels of 400W each. Our calculator sizes the system based on your monthly bill and available roof area.

### How much roof space do solar panels need?

Each 400W panel needs approximately 1.8 sqm of roof space. A 4 kWp system (10 panels) needs around 18 sqm. South-facing roofs are ideal, but east and west orientations still generate 80-85% of maximum output.

### What is the payback period for solar panels?

In central Europe, solar panels typically pay back in 8-12 years depending on system size, electricity prices, and sunlight hours. In southern Europe (Spain, Italy), payback can be as short as 5-7 years. The panels continue generating for 25-30 years after payback.

### Do solar panels work on cloudy days?

Yes, solar panels generate electricity from daylight, not direct sunshine. On a cloudy day, panels produce 10-25% of their peak output. Annual generation averages account for all weather conditions, so the calculator's estimates include cloudy days.

### How long do solar panels last?

Modern solar panels come with 25-year performance warranties guaranteeing at least 80% of original output. Many panels continue producing useful energy for 30-35 years. Inverters typically last 10-15 years and may need replacement once during the panel lifetime.

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Interactive version with calculator: https://renoquant.com/solar-panel-calculator. Figures are estimates; final quotes vary by site conditions, materials, and region.