Solar Panel Calculator
Size a solar power plant for your consumption: panel power and quantity, inverter, batteries for standalone systems, generation by region and the payback period of your investment.
What the calculator computes
- Peak solar panel power by monthly consumption and regional insolation
- Number of panels (standard 450–550 W) and occupied roof area
- Inverter power with a 20–30% margin
- Battery capacity for standalone systems (by daily consumption and reserve)
- Estimated monthly and annual generation by season
- Payback period at the current electricity tariff
How it works
The key parameter is peak sun hours (PSH): the equivalent number of hours per day with nominal insolation of 1000 W/m². In Southern Europe in summer this is 6–7 hours, in winter 1–2; in Northern and Central Europe summer is 4–6, winter 0.3–1. Annual generation of a 1 kW panel in Spain — 1400–1600 kWh, in Germany — 950–1100 kWh, in the UK — 850–1000 kWh, in India — 1500–1800 kWh.
Required panel power: P_kW = E_monthly / (30 × PSH_annual_avg × K), where K is the loss factor: 0.78 for grid-tied systems (inverter 96%, heating, soiling, wiring), 0.65–0.70 for standalone with batteries (battery efficiency 90%, inverter 93%).
Battery capacity (off-grid): C_Ah = E_daily / (U_system × DoD × η), where U is the system voltage (24/48 V), DoD is the depth of discharge (0.5 for lead-acid, 0.8 for LiFePO4), η is the circuit efficiency 0.85.
Regulatory framework and commissioning
In the EU, small grid-connected PV up to ~10–30 kW follows simplified permitting (in Germany up to 30 kW peak exempt from feed-in permit under EEG 2023; in the UK, permitted development rules under GPDO Part 14 apply to panels not protruding more than 20 cm). Metering: net metering (e.g., Poland, net billing since 2022) or feed-in tariffs (Germany 8.2–13 c€/kWh in 2024).
Technical requirements: anti-islanding protection, automatic disconnection when the grid fails, RCD 30 mA on the AC side, surge protection (SPD type 2), earthing per IEC 60364-7-712 (PV installations). Panels are mounted with a ventilation gap of at least 5–10 cm. Inverters follow IEC 62109.
Peak sun hours (PSH) by region
| Region | Summer, h/day | Winter, h/day | Annual yield of 1 kW, kWh |
|---|---|---|---|
| London, UK | 5.0 | 0.5 | 950 |
| Berlin, Germany | 5.8 | 0.5 | 1050 |
| Madrid, Spain | 7.2 | 2.0 | 1600 |
| Rome, Italy | 7.0 | 1.8 | 1500 |
| Warsaw, Poland | 5.8 | 0.5 | 1050 |
| Paris, France | 5.8 | 0.7 | 1100 |
| Bucharest, Romania | 6.5 | 1.0 | 1250 |
| Kyiv, Ukraine | 5.9 | 0.5 | 1100 |
| Almaty, Kazakhstan | 6.8 | 1.4 | 1300 |
| Tashkent, Uzbekistan | 7.5 | 2.2 | 1450 |
| Delhi, India | 6.2 | 4.0 | 1600 |
| Dubai, UAE | 8.0 | 5.0 | 1800 |
How to use the calculator
- Enter the average monthly electricity consumption in kWh (from your meter).
- Select the region — the average insolation will be filled in.
- Choose the system type: grid-tied (economical), hybrid with backup, or fully standalone.
- Enter the electricity tariff to calculate payback.
- Press «Calculate»: plant power, number of panels, area, inverter, batteries, generation and payback period.
Common mistakes
- Calculating generation by summer insolation — in winter the plant produces 5–10 times less, while consumption is higher.
- Standalone system without reserve days — on the 3rd cloudy day the house is left without power.
- Inverter sized tight — starting currents of refrigerators and pumps trip the protection.
- Saving on the charge controller — LiFePO4 batteries need MPPT; PWM loses 20–30% of generation.
- Ignoring panel cleaning — soiling reduces generation by 5–15% in southern regions.
Solar panels FAQ
How many panels for a 150 m² house?
Average consumption of such a house is 500–800 kWh per month. For Central Europe a 6–9 kW plant is needed — 14–20 panels of 450 W, roof area 30–45 m². Payback at 0.30–0.40 €/kWh is 7–12 years depending on the country.
Does a solar plant work in winter?
Yes, but generation drops 5–10 times due to the short day and low sun. Snow on panels almost completely stops generation — a tilt angle of 45–60° or regular cleaning is needed. In winter the plant covers 5–15% of consumption.
What is more profitable — grid-tied or off-grid?
Grid-tied (no batteries) pays back 2–3 times faster: batteries are the most expensive and short-lived part (lead-acid 5–7 years, LiFePO4 10–15). Off-grid is needed where there is no grid or frequent outages. A hybrid with a small reserve (5–10 kWh) is the golden mean for unstable grids.
Can I sell excess electricity to the grid?
In most EU countries yes: net metering (Poland, up to 2024), net billing, or feed-in tariffs (Germany, Spain, Italy). A bidirectional meter and a contract with the grid operator are required. In the UK, Smart Export Guarantee (SEG) pays 3–15 p/kWh.
What is the service life of solar panels?
Modern monocrystalline panels last 25–30 years with degradation of 0.5–0.6% per year (85% of nominal after 25 years). The inverter is 10–12 years, batteries 5–15 years depending on type. Panel warranty — 12–25 years.