Solar calculator: Calculate your individual solar system / photovoltaic system yourself
A photovoltaic system on the roof is long gone beyond just climate protection and has become one of the most return-strong small investments for property owners. Those who plan properly can achieve equity returns of 6–10 % per year — tax-free. This guide shows you in eight clear steps how to calculate the individual size of your solar system yourself, what realistic costs are, how the system pays off, and what homeowners, landlords, and investors need to pay attention to. In addition, you will find cost tables, economic calculations, tax notes, and a checklist before purchasing the system. Those who also want to assess the property will also find our methods for this.
Calculate a photovoltaic system in 8 simple steps
On the internet, there are numerous solar calculators that can roughly estimate the size of a photovoltaic system. The problem: Many of these tools ignore crucial factors such as roof pitch, shading, regional global irradiance, or future consumption changes due to heat pumps or electric cars. With our eight steps, you can calculate the individually suitable system size — precisely and verifiably.
An overview of the eight steps:
- Step 1: Determine annual electricity consumption
- Step 2: Calculate optimal electricity production (+25 %)
- Step 3: Determine regional energy yield
- Step 4: Assess roof pitch & orientation
- Step 5: Calculate shading
- Step 6: Adjust personal energy yield
- Step 7: Calculate system size in kWp
- Step 8: Determine required roof area
Step 1 – Annual energy consumption
The primary goal of a photovoltaic system is to cover the energy required for your household without needing to access the public grid. Therefore, the size depends first on your personal energy consumption. How much electrical energy do you consume per year? Your last electricity bills will answer this question.
Determine the average electricity consumption in kilowatt-hours (kWh) over the last three to five years. Important: Take into account future purchases that can significantly increase your demand:
- Heat pump: +3,000 to 6,000 kWh per year
- Electric car (15,000 km/year): +2,500 to 3,500 kWh
- Air conditioner / pool: +500 to 2,500 kWh
- Sauna / whirlpool: +1,000 to 2,000 kWh
Estimated electricity consumption per household (without heat pump/electric car):
| Household Size | Apartment | Single-family Home |
|---|---|---|
| 1 Person | 1.500 kWh | 2.300 kWh |
| 2 People | 2.500 kWh | 3.000 kWh |
| 3 People | 3.500 kWh | 4.000 kWh |
| 4 People | 4.250 kWh | 5.000 kWh |
| 5+ People | 5.000 kWh | 6.500 kWh |
Example: Family A has an average energy consumption over the last five years of 5.000 kWh. An additional electric car is planned (+3.000 kWh). Realistic planning value: 8.000 kWh.
Formula: Your personal electricity consumption per year (SV) = ______ kWh/year
Step 2 – Optimal Energy Production of the Photovoltaic System
To ensure that the system reliably covers your self-consumption, it should produce at least 25 percent more energy than you consume over the course of a year. These 25 percent are an empirical value that account for fluctuations, losses, and non-simultaneous consumption. Excess energy can be stored in a battery or fed into the public grid in exchange for feed-in tariffs.
Example: Family A with a consumption of 5,000 kWh calculates: 125 % × 5,000 kWh = 6,250 kWh optimal production amount.
Formula: Optimal electricity production amount = 125 % × SV
Step 3 – Regional Energy Yield
Even though it does not shine for 365 days a year in Germany, a photovoltaic system is worthwhile. The misconception that it is only profitable in southern countries is disproven: The global radiation averages 1,100 kWh per square meter and year. Modern modules with better cooling and self-cleaning also deliver economic yields in northern Germany.
The regional energy yield (RE) is given in kilowatt-hours per kilowatt peak and year (kWh/(kWp × a)). Overview of the average global radiation in Germany:
- Baden-Württemberg: 1.050–1.175 kWh/m²
- Bavaria: 975–1.175 kWh/m²
- Berlin / Brandenburg: 975–1.050 kWh/m²
- Hesse: 975–1.100 kWh/m²
- Mecklenburg-Western Pomerania: 1.000–1.050 kWh/m²
- Hamburg, Bremen, Lower Saxony: 950–1.025 kWh/m²
- North Rhine-Westphalia: 950–1.025 kWh/m²
- Rhineland-Palatinate: 975–1.125 kWh/m²
- Saarland: 1.050–1.100 kWh/m²
- Saxony: 975–1.100 kWh/m²
- Saxony-Anhalt: 975–1.050 kWh/m²
- Schleswig-Holstein: 950–1.025 kWh/m²
- Thuringia: 975–1.050 kWh/m²
Example: Family A lives in North Rhine-Westphalia with RE = 950 kWh/(kWp × a).
Formula: Your regional energy yield RE = ______ kWh/(kWp × a)
Step 4 – The Conditions of the Roof
Roof orientation, roof pitch, and shading are the three most important factors when choosing your solar modules. The old assumption that only south-facing roofs are economically viable is outdated: east-west systems often even provide more even yields throughout the day and are ideal for self-consumption. Even north-facing roofs can achieve acceptable values with a shallow pitch.
Deviation value (DV) depending on roof pitch and orientation — guideline values in %:
| Tilt ↓ / Orientation → | Süd | Southeast / Southwest | Ost / West | Nord |
|---|---|---|---|---|
| 0° (Flachdach) | 88 | 88 | 88 | 88 |
| 15° | 96 | 95 | 90 | 78 |
| 30° | 100 | 97 | 88 | 68 |
| 45° | 97 | 94 | 83 | 58 |
| 60° | 89 | 86 | 76 | 50 |
Example: Family A has a 30° roof with a 40° south deviation → AW ≈ 97.5%.
Formel: Abweichungswert Ihrer Immobilie: AW = ______ %
Schritt 5 – Modulverschattung vermeiden
Shading can drastically reduce the performance of your system. If a source casts a shadow on just one module, the performance of the entire string is reduced — however, modern performance optimizers and micro-inverters mitigate this effect.
Typical sources of shading and losses:
- Tall tree to the south of the system: 5–15 %
- Neighboring house or roof: 3–10 %
- Chimney: 2–5 %
- Antenna, satellite dish: 1–3 %
- Ridge vent on the same roof side: 5–10 %
As about 80 % of the annual yield falls between March and October, summer shading is significantly more severe than winter shading. Anyone who has shading in the summer should definitely use performance optimizers.
Step 6 – Adjust personal regional energy yield
If your roof is not optimally located (AW < 100 %), the regional energy yield is reduced accordingly.
Example: Family A: 97.5 % × 950 kWh/(kWp × a) = 926.2 kWh/(kWp × a).
Formula: Personal energy yield (PE) = AW × RE = ______ kWh/(kWp × a)
Step 7 – The Size of Your Individual Photovoltaic System
Using the determined values, you now calculate the required system size in kilowatt peak (kWp). With a generous roof area, building a larger system can be worthwhile — because every additional kWp reduces the unit costs and provides feed-in tariffs.
Example: Family A: 6,250 kWh/a ÷ 926 kWh/(kWp × a) = 6.74 kWp.
Formula: System size (PV) = Electricity production ÷ personal energy yield = ______ kWp
Step 8 – The Required Roof Area
A typical photovoltaic module measures 1.67 m × 1 m (≈ 1.67 m²) and delivers 380–440 Wp. Accounting for spacing, row offset, and maintenance space, a standard calculation uses 5–7 m² per kWp — with modern high-performance modules it is more like 5 m².
Example: Family A: 6.74 kWp × 7 m²/kWp = 47.2 m² roof area.
Formula: Required roof area (RA) = System size × 5–7 m²/kWp
How Much Does a Photovoltaic System Cost? Cost Table
The investment costs per kWp decrease with increasing system size. Including installation, inverter, wiring, and commissioning — without storage — the following guidelines apply:
| System Size | Total Investment | € per kWp | Typical Annual Yield |
|---|---|---|---|
| 4 kWp | 7,200–9,200 € | 1,800–2,300 € | 3,600–4,000 kWh |
| 6 kWp | 9,500–12,500 € | 1,580–2,080 € | 5,400–6,000 kWh |
| 8 kWp | 11,500–14,800 € | 1,440–1,850 € | 7,200–8,000 kWh |
| 10 kWp | 13,500–17,000 € | 1,350–1,700 € | 9,000–10,000 kWh |
| 15 kWp | 19,000–24,000 € | 1,270–1,600 € | 13,500–15,000 kWh |
| 20 kWp | 24,000–30,000 € | 1,200–1,500 € | 18,000–20,000 kWh |
For apartment buildings, systems starting at 15 kWp are often sensible — these pay off especially in apartment buildings through tenant electricity models. Anyone who is planning a new build anyway or wants to buy a house should integrate the PV system into the financing from the start.
Energy storage: Is the battery worth it?
An energy storage system increases the self-consumption rate from typically 25–35 % (without storage) to 60–80 %. This makes it especially worthwhile with high electricity prices and low feed-in tariffs.
Guideline storage costs (Lithium-Ion, including installation):
- 5 kWh storage: 5,000–7,500 €
- 8 kWh storage: 7,000–10,500 €
- 10 kWh storage: 8,500–12,500 €
- 15 kWh storage: 12,000–17,000 €
Rule of thumb for storage size: 1 kWh storage per 1,000 kWh annual consumption — more is better with an electric car. Lifespan of modern Lithium-Ion storage systems: 6,000–8,000 full cycles, which corresponds to 15–20 years.
Economic Viability: Calculating Payback and Return
The most exciting question for every investor: When does the investment pay off and what return does it offer? Economic viability depends on four factors:
- Self-consumption rate (typically 25–35 % without storage, 60–80 % with)
- Electricity price from the utility provider (avoided purchase)
- Feed-in tariff for surplus electricity
- Investment costs including maintenance and insurance
Example calculation for an 8-kWp system without storage (Family A):
