Solar Panel Sizing and Self-Consumption Calculator
Calculate annual production of an FV plant, percentage of self-consumption and surplus to feed the grid. Free tool for companies, installers and energy managers.
Configure
Result
No results
Enter values and press the button to calculate.
Frequently Asked Questions
What is a kWp and how does it differ from a kW?
The kWp (kilowatt peak) indicates the maximum nominal power of a solar panel under standard test conditions (1,000 W/m² irradiance, 25°C cell temperature). It differs from the kW that measures actual instantaneous power, which varies with light and temperature. A 10 kWp system never produces exactly 10 kW in real conditions.
What are peak sun hours?
Peak Sun Hours (PSH) express the amount of solar radiation available in a location as equivalent hours to 1,000 W/m². In Italy: Torino ≈ 1,200 h/year, Milano ≈ 1,300 h/year, Roma ≈ 1,500 h/year, Palermo ≈ 1,700 h/year. More precise data is obtained from PVGIS (the European free instrument).
What is solar self-consumption?
Self-consumption is the percentage of electricity produced by the FV system and consumed directly on-site, without passing through the grid. High self-consumption (>70%) reduces dependence on the grid, maximizes bill savings, and improves investment return. Optimization is achieved by pairing batteries or flexible loads (heat pumps, EVs).
How is a commercial FV system properly sized?
Optimal sizing depends on annual consumption, daily load profile, roof space availability, and budget. A practical rule of thumb is that a 1 kWp system produces around 1,000-1,500 kWh/year in Italy. To maximize self-consumption, the system is sized to cover 70-90% of energy needs. An authorized technician (designer under FER ex DM 10/9/2010) can perform simulations with software like PVsyst.
Is excess energy paid by GSE?
Energy excess injected into the grid can benefit from on-site swapping (SSP) or dedicated withdrawal (RID) through GSE. With Renewable Energy Communities (CER, D.Lgs. 199/2021 and D.M. 7/12/2023), it's possible to share surplus with other parties and receive additional incentives. Tariffs vary; consult the GSE and your distributor.
Are the calculator results valid for a practice with the GSE?
No. This calculator provides an approximate estimate based on simplified physical formulas (without hourly load profile). For GSE practices, FER incentives or Terna/e-Distribution connection, a certified design by a qualified technician with validated software simulations (PVsyst, PVWatts NREL or equivalents) is required.
How is it used?
- Insert power of the system
Indicates the nominal power of the solar system in kWp (peak chilowatt). This value is shown on the provider's quote or the module technical sheet.
- Specify solar hours and efficiency
Enter the equivalent annual peak sun hours for your location - in Italy they vary from 1,100 h/year (North) to 1,700 h/year (South). System efficiency takes into account cable losses, inverters, and temperature (typically 0.75–0.85).
- Indicates annual consumption
Insert annual electricity consumption in kWh. You can find it on your electricity bill (voice "energy consumed" or "kWh billed").
- Interpret energy production and self-consumption
The calculator shows estimated annual production, energy self-consumption percentage (FV energy used on-site), and the amount of surplus fed into the grid. Higher self-consumption values indicate a well-sized system relative to consumption.
How to size a corporate solar plant
Solar panel sizing is the process of determining the optimal power output of an FV system based on a company's energy demand, location irradiance data and available budget. The basic formula is simple: annual production (kWh) = nominal power (kWp) × equivalent hours of sunlight (PSH/year) × system efficiency.
Self-consumption is the key parameter to evaluate the return on investment of an FV system: it represents the share of energy produced that is consumed directly on-site, reducing withdrawals from the grid and lowering bills. A high self-consumption rate (above 70%) can be achieved by aligning FV production with the company's load profile - ideally with daytime loads (machinery, air conditioning, EV charging).
In Italy, peak sun hours vary significantly by geographic area: the Northwest records around 1,100–1,250 PSH/year, the Center 1,300–1,450, and the South 1,500–1,700. Precise data for each municipality is available on PVGIS, the European irradiance tool (RE.JRC.EC.EUROPA.EU), or through ENEA-ISPRA maps.
The unused surplus can be sold to the grid through on-site swapping (SSP) managed by GSE or valued in a Renewable Energy Community (CER), where DLgs 199/2021 and DM 7/12/2023 provide additional tariff incentives. This calculator's estimate is indicative: for certified design, FER incentives and connection require a licensed technician.
All calculations occur in the browser: no data is sent to external servers. A simplified physical model (without hourly load profile) provides valid estimates for pre-factibility; detailed simulation software like PVsyst or PVWatts NREL are used for simulations.
Practical example: 10 kWp installation, Central Italy
- Power plant capacity: 10 kWp (e.g., 25 modules of 400 Wp)
- Annual sunshine hours: 1,300 PS (Rome/Italy area, source PVGIS)
- System Efficiency: 0.80 (cable losses 3%, inverter 4%, temperature 8%, shading 5%)
- Annual production = 10.4 kWh/year
- Annual consumption usage: 8,000 kWh → Self-consumption = 8,000 / 10,400 = 76.9%
- Surplus in network: 10,400 - 8,000 = 2,400 kWh/year
Vocabulary Dictionary
- Peak kilowatts
- Unit of nominal power measurement for a module or solar plant under standard test conditions (STC): irradiance 1,000 W/m², cell temperature 25°C, AM 1.5 spectrum. Does not match actual output power under real conditions.
- Peak Sun Hours (sunlight hours equivalent)
- Number of hours per day (or year) equivalent to 1,000 W/m² solar irradiance. A site with 1,300 PSH/year receives the same energy as 1,300 full days of sunshine. Data available from PVGIS or ENEA for every Italian municipality.
- Self-consumption (%)
- Electricity production and consumption on-site from the FV system. Formula: autoconsumption% = min(consumption / production, 1) × 100. Typical values: 60-80% without battery, 80-95% with energy storage system.
- System Efficiency (PR - Performance Ratio)
- Real production vs theoretical full irradiance ratio. Accounts for temperature losses, cabling, inverter, reflection and dirt. Typical values: 0.75 (outdated installation/heat) - 0.85 (modern installation/no shade).
- Renewable Energy Community
- Renewable energy producer-consumer association regulated by DLgs. 199/2021 and DM 7/12/2023 (CER decree). Allows sharing of FV surplus with connected primary cabin users to receive tariff incentives from the GSE.
- In-place exchange (SSP)
- GSE mechanism allowing surplus FV to be injected into the grid and withdrawn at a different time, "compensating" withdrawals with injections. Available for installations up to 500 kW. Regulated by Delibera ARERA ARG/elt 74/08 and subsequent modifications.
Do you need a custom analysis?
This tool is free and informative. For in-depth analysis with AI on-prem - private data, zero cloud - contact Federico.