Solar Panel Calculator

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

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 to reduce your heating load before sizing solar, as lower consumption means a smaller system.

System Sizes by Home Type

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 before installing panels, as replacing a roof after solar installation is expensive.

Cost Breakdown

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.

How Much Do Solar Panels Cost — and the 25-Year ROI

Solar costs have dropped roughly 60% since 2015, making residential solar one of the best home investments available. Below is the cost by system size and the return over the full 25-year panel lifetime.

Cost and Savings by System Size

*Central European conditions (3.5 peak sun hours), EUR 0.30/kWh electricity price.

25-Year ROI Analysis

Assumes 0.5% annual panel degradation, 2% annual electricity price increase, and one inverter replacement at year 12 (EUR 600-1,200). Actual returns vary with self-consumption rate and local tariffs.

Hidden Costs, Grants, and VAT

• Hidden costs: budget 10-15% extra for scaffolding (EUR 300-600), consumer-unit upgrades (EUR 200-500), planning permission in conservation areas (EUR 100-300), and bird mesh (EUR 150-300).
• Grants/subsidies: Germany — low-interest KfW loans and 0% VAT up to 30 kWp; France — self-consumption bonus and 10% reduced VAT; Italy — eco-bonus schemes; UK — 0% VAT on residential solar. Always check your national energy agency for current schemes.
• Group buying: neighbours installing together can cut per-home costs 15-20% through bulk purchasing and shared scaffolding.

How Much Can You Save?

The financial return depends less on the panels and more on how effectively you use the energy they generate. Self-consumption is the single biggest lever.

Self-Consumption vs Export

EUR 0.30/kWh grid price. Effective saving = (self-consumption% x grid price) + (export% x export rate).

Annual Savings by System Size and Location

Assumes 50% self-consumption, EUR 0.30/kWh grid price, EUR 0.08/kWh export rate.

Strategies to Boost Savings

• Time-shift appliances (free): run the washing machine, dishwasher, and tumble dryer between 10am-3pm with delay timers — can lift self-consumption from 25% to 45%.
• Smart diverters: devices like an Eddi (EUR 400-500) route surplus solar to heat water before it's exported.
• Pair with a heat pump: heat pumps run during the day when solar peaks, absorbing 2-4 kWh of surplus daily and cutting both heating costs and grid exports. Reducing your heating load first with better insulation accelerates payback.