Solar System Key Figures: The Glossary
Introduction: No Planning Without Numbers
As with all technical systems, key figures are essential for selecting the right components and adapting the system to the desired requirements. With the right metrics, every solar system can be optimally sized.
This article summarises all important key figures – from power and efficiency to battery parameters.
Power and Energy
Electrical Power (kW)
Definition: Power is work per unit of time – the amount of energy converted per second.
For solar systems: Electrical power is the amount of solar energy that can be converted into electrical energy per unit of time.
Unit: Kilowatt (kW) = 1,000 Watt
Examples:
- Small inverter: 3 kW
- Medium-sized system: 5–10 kW
- Heat pump: 3–12 kW
- EV wallbox: 11–22 kW
Peak Power (kWp)
Definition: The maximum possible power of a solar system under Standard Test Conditions (STC):
- Irradiance: 1,000 W/m²
- Cell temperature: 25°C
- Air mass: AM 1.5
Meaning: Kilowatt peak (kWp) is the unit for comparing solar systems. A 10 kWp system can deliver a maximum of 10 kW under optimal sunshine.
In practice: In Germany, systems only reach peak power for a few hours per year (clear summer day, midday sun).
Electricity Yield (kWh)
Definition: The actual amount of energy generated over a period.
Unit: Kilowatt-hour (kWh) = 1 kW power for 1 hour
| Examples: | Device | Power | Operating time | Consumption |
|---|---|---|---|---|
| LED lamp | 10 W | 5 h | 0.05 kWh | |
| Washing machine | 2,000 W | 1 h | 2 kWh | |
| EV charging | 11,000 W | 3 h | 33 kWh |
Annual yield: A 10 kWp system in Germany generates approximately 900–1,100 kWh per kWp, i.e. 9,000–11,000 kWh per year.
Efficiency Ratings
What Is Efficiency?
Definition: The ratio between usable energy and energy input.
Formula: η = Usable energy / Input energy × 100%
Illustration: An incandescent bulb converts only 5% of energy into light – 95% is lost as heat. LEDs achieve 40–50%.
Efficiency of Solar Modules
| Technology | Efficiency | Characteristics |
|---|---|---|
| Monocrystalline | 18–24% | Highest efficiency, dark appearance |
| Polycrystalline | 15–20% | More affordable, bluish structure |
| Thin-film | 8–15% | Flexible, partial shading resistant |
| Perovskite (laboratory) | up to 30% | Future technology |
| Tandem (laboratory) | up to 47% | Multi-layer cells |
Inverter Efficiency
Modern inverters achieve 96–98% efficiency. Losses arise from:
- Switching losses in semiconductors
- Self-consumption of electronics
- Heat generation
European Efficiency: A weighted average value that considers real partial load behaviour (more important than maximum efficiency).
System Efficiency
The overall efficiency of a PV system is typically 80–90%. Losses arise from:
- Cable losses (1–2%)
- Inverter (2–4%)
- Soiling (2–5%)
- Temperature losses (5–10%)
- Partial shading (variable)
Battery Key Figures
Capacity (kWh)
Definition: The amount of energy a battery can store and deliver.
Distinction:
- Gross capacity: Physical total capacity
- Net capacity: Actually usable (90–95% of gross capacity)
Typical values for home storage: 5–15 kWh
Charging and Discharging Power (kW)
Definition: How quickly the battery can absorb or release energy.
Meaning: Determines whether the battery can handle load peaks (e.g. simultaneous oven, heat pump, tumble dryer).
Typical values: 3–10 kW for home storage systems
C-Rate
Definition: Ratio between charging/discharging power and battery capacity.
Formula: C = Power (kW) / Capacity (kWh)
Example:
- 10 kW power / 20 kWh capacity = 0.5C
- At 0.5C, the battery charges/discharges in 2 hours
| C-Rate | Charge/Discharge Time | Meaning |
|---|---|---|
| 0.2C | 5 hours | Gentle charging |
| 0.5C | 2 hours | Typical home storage |
| 1C | 1 hour | Fast charging |
| 2C | 30 minutes | High performance |
Important: Higher C-rates stress the battery more and can shorten its lifespan.
Cycle Life
Definition: Number of complete charge/discharge cycles a battery can withstand until a defined capacity loss (usually 80% remaining capacity).
Typical values:
- Lead-acid: 500–1,500 cycles
- Lithium-ion: 5,000–10,000 cycles
Conversion: At one cycle per day = 13–27 years lifespan
Depth of Discharge (DoD)
Definition: How deeply the battery may be discharged without damage.
Values:
- Lead-acid: 50% DoD recommended
- Lithium-ion: 80–100% DoD possible
Meaning: Higher DoD = more usable capacity, but potentially faster wear.
Autarky and Self-Consumption
Autarky Rate
Definition: Proportion of electricity consumption covered by your own solar system.
Formula: Autarky = Self-consumption / Total consumption × 100%
| Typical values: | Configuration | Autarky rate |
|---|---|---|
| PV only | 25–35% | |
| PV + small storage | 50–65% | |
| PV + large storage | 70–85% | |
| PV + storage + optimised behaviour | 80–95% |
Self-Consumption Rate
Definition: Proportion of generated solar electricity that is consumed on-site (not fed into the grid).
Formula: Self-consumption = Own consumption / Total generation × 100%
Meaning: The higher the self-consumption rate, the more economical the system (self-consumption saves approx. 25 pence/kWh compared to feed-in).
Economic Key Figures
Specific Yield (kWh/kWp)
Definition: Annual yield divided by installed capacity.
Typical values in Germany: 900–1,100 kWh/kWp
Depends on:
- Location (southern Germany > northern Germany)
- Orientation (south optimal)
- Tilt (30–35° optimal)
- Shading
Performance Ratio (PR)
Definition: Ratio of actual to theoretically possible yield.
Typical values: 75–85%
Meaning: Shows the quality of the system and installation.
Levelised Cost of Electricity (LCOE)
Definition: Cost per kilowatt-hour generated over the entire lifespan.
Calculation: Total costs / Total yield (over 20+ years)
Current values (2025):
- Rooftop systems: 5–10 cents/kWh
- Large-scale systems: 3–6 cents/kWh
- Grid electricity: 30–40 cents/kWh
Overview: Units at a Glance
| Unit | Name | Meaning |
|---|---|---|
| kW | Kilowatt | Power (work per time) |
| kWh | Kilowatt-hour | Energy (1 kW for 1 hour) |
| kWp | Kilowatt peak | Maximum PV power (STC) |
| % (η) | Efficiency | Usable / input energy |
| C | C-Rate | Charge/discharge power / capacity |
| % DoD | Depth of Discharge | Maximum discharge depth |
Conclusion
In Brief: With these key figures, solar systems of different sizes can be compared, the appropriate storage system sized, profitability calculated and system quality assessed. The most important parameters for planning are kWp (system size), kWh storage (storage capacity), autarky rate (grid independence) and self-consumption rate (profitability).
The Complete Article Series "How Does a Solar System Work?"
- From Photon to Volt: How Does a Solar Cell Work? – Photovoltaic fundamentals
- Structure of a PV System: From Module to Grid Feed-In – Components and power path
- AC/DC in PV: Inverters and Power Conversion – Power electronics
- Battery Storage: The Helper in Bad Weather – Energy storage
- Solar System Key Figures: The Glossary – You are here
Related Article Series
Energy Storage for Solar Systems:
- From Frog Legs to Batteries: How Does an Energy Storage System Work?
- Lithium vs. Lead: Which Battery for Solar Systems?
- The All-Rounder: Hybrid Inverters
- AC or DC? System Topologies for Solar Systems
Heat Pump Series:
- The Anti-Refrigerator: How Does a Heat Pump Work?
- Heat Pump Types and the Dream Team with Solar Systems
Battery Storage and Powerstations:
- Battery Technologies Compared: Lithium, Lead and Solid-State
- Powerstations: The All-in-One Solution for Solar Systems
- Market Analysis 2025: Battery Storage and Powerstations