Solar PV system sizing calculator
From your annual consumption, county, orientation and tilt, find the power (kWp) and number of panels you need, what the system produces in a year and how much of your consumption it covers.
How it is calculated
y = y_zone · f(orientation, tilt) [kWh/kWp/yr] kWp_req = consumption · target / y panels = ⌈ kWp_req · 1000 / P_panel ⌉ E_year = panels · P_panel / 1000 · y coverage = E_year / consumption
The specific yield y_zone is for south-facing panels at 30° tilt (table below). The factor f corrects for the real orientation and tilt. The location data is for Romania; use the “other location” option with your own yield for sites elsewhere.
Worked example
4,000 kWh/year consumption in Bucharest, south-facing roof at 30°, 450 Wp panels:
y = 1,299 kWh/kWp/yr kWp_req = 4000 / 1,299 = 3.08 kWp panels = ⌈ 3,079 / 450 ⌉ = 7 installed = 7 · 0.45 = 3.15 kWp E_year = 3.15 · 1,299 = 4,092 kWh coverage = 4,092 / 4000 = 102%
Specific yield by zone
South, 30° tilt, 14% system losses. Source: PVGIS 5.3, SARAH3 radiation database; mean of representative cities.
| Zone | kWh/kWp/yr | Counties |
|---|---|---|
| Dobruja and Black Sea coast | 1,307 | Constanța, Tulcea |
| South and South-East plains | 1,299 | Brăila, București, Călărași, Dolj, Galați, Giurgiu, Ialomița, Ilfov, Olt, Teleorman |
| West, South-West and southern Subcarpathians | 1,260 | Arad, Argeș, Bihor, Buzău, Caraș-Severin, Dâmbovița, Gorj, Mehedinți, Prahova, Timiș, Vâlcea |
| North-East and Moldavia | 1,198 | Bacău, Botoșani, Iași, Neamț, Suceava, Vaslui, Vrancea |
| Transylvania and North-West | 1,183 | Alba, Bistrița-Năsăud, Brașov, Cluj, Covasna, Harghita, Hunedoara, Maramureș, Mureș, Satu Mare, Sălaj, Sibiu |
Orientation and tilt factors
Ratio to south at 30° (= 1.00). Intermediate values are interpolated linearly.
| Orientation | 0° | 15° | 30° | 45° | 60° | 90° |
|---|---|---|---|---|---|---|
| South | 0.85 | 0.95 | 1.00 | 1.00 | 0.95 | 0.69 |
| South-East / South-West | 0.85 | 0.92 | 0.94 | 0.92 | 0.87 | 0.63 |
| East / West | 0.85 | 0.84 | 0.80 | 0.75 | 0.68 | 0.48 |
| North | 0.85 | 0.71 | 0.57 | 0.43 | 0.31 | 0.16 |
Frequently asked questions
What does kWp mean?
kWp (kilowatt-peak) is the rating of a PV system under standard test conditions. In Romania a 1 kWp system produces roughly 1,150–1,350 kWh a year, depending on location and orientation.
Where does the yield data come from?
From PVGIS, the European Commission’s free service (SARAH3 radiation database), with 14% system losses. For each zone I averaged 2–4 representative cities. For an exact site, check PVGIS directly.
Does 100% coverage mean no electricity bill?
Not necessarily. The calculator compares energy produced over a year with energy consumed. Output peaks in summer while consumption may peak in the evening or winter; how much you use directly (self-consumption) depends on your hourly profile and any battery. Local rules for exported surplus matter too.
What orientation and tilt are best?
In Romania, south at 30–35° gives the highest annual yield. East–west produces about 20% less at 30° but flattens output across the day, which can help self-consumption.
Does it account for shading?
No. Shade from chimneys, trees or neighbouring buildings can reduce output a lot and has to be assessed on site.