About this calculator
How big should a rooftop solar power plant be to cover your yearly electricity use? The calculator works out the peak power (kWp) whose yearly generation equals the share of your use you choose, rounds it up to whole panels, checks whether they fit the usable roof area, and sizes the inverter from the DC/AC ratio.
It sizes by yearly energy balance, the way households selling surplus to the grid usually size: generation over the year equals use over the year. It makes no financial promises and contains no prices.
Formula
E_target = yearly use × share / 100 kWh
P_needed = E_target / Y kWp
N = ⌈P_needed × 1000 / P_panel⌉ panels, rounded up
P_inst = N × P_panel / 1000 kWp
E_gen = P_inst × Y kWh a year
A_panels = N × L × W; N_max = ⌊A_roof / (L × W)⌋
P_AC = P_inst / (DC/AC ratio) kW
Y— specific yield, the electricity one kWp of panels on your roof generates in a year. It depends on location, tilt, orientation and shading. Get it for your own address from the European Commission's free PVGIS tool (re.jrc.ec.europa.eu). The default 1000 kWh/kWp is a rough Lithuanian rule of thumb — "1 kW of solar produces 1000 kWh a year", as the Lithuanian Solar Energy Association put it at an APVA/BETA seminar (2020), with about 600 for a wall-mounted or shaded array. It is not a PVGIS value for your roof.- DC/AC ratio — panel peak power divided by inverter AC power. Panels rarely reach their peak rating, so the inverter is usually somewhat smaller. The default 1.2 sits just below 1.25, at which a University of Michigan study found clipping losses minimal for fixed-tilt systems.
Worked example
A household uses 4000 kWh a year and wants to generate all of it, with 430 Wp panels (1722 × 1134 mm) and 30 m² of usable roof:
P_needed = 4000 / 1000 = 4.0 kWp
N = 4000 / 430 = 9.3 → 10 panels → 4.30 kWp
E_gen = 4.30 × 1000 = 4300 kWh a year (108 % of the use)
area = 10 × 1.953 m² = 19.5 m² (the roof fits 15 by area)
inverter = 4.30 / 1.2 = 3.6 kW
FAQ
Where do I get the yield for my roof?
Open PVGIS, mark your house, enter 1 kWp, your roof's tilt and orientation
(azimuth) and the default system losses, and read the yearly PV energy
production in kWh. That figure is Y.
Will 100 % generation make my bill zero? Not necessarily. The plant generates most in summer and around midday, while use is higher in winter and in the evening. What happens to the surplus sent to the grid and the electricity taken back depends on your contract with the network operator and supplier.
Why fewer panels fit than the area suggests? The check compares areas only. Panels come in rows that must leave clearances from roof edges, ridges, chimneys and roof windows, so a real layout fits fewer. Draw the layout before ordering.
Can the inverter be larger than the panels? It can, but that adds no output, so the ratio here goes no lower than 1.0.
Assumptions and limits
- Sizing by yearly energy balance; no hourly self-consumption, storage or battery modelling.
- The specific yield is your input; the default is a rough figure, not PVGIS. No tilt, orientation or shading correction is applied by the calculator.
- Roof fit by area only.
- Inverter size from the DC/AC ratio only; string voltages, MPPT inputs and grid-connection limits are not checked.
- No prices, payback or subsidy calculations.