Photovoltaic installation power calculator
Enter the annual energy consumption, available roof area, panel parameters and installation conditions. The calculator will automatically calculate PR from partial system losses and check whether the roof allows for the required number of panels to be installed.
Explanation of PV calculator parameters
The following parameters determine the calculator's result. Some of them describe the needs of the house, some are roof limitations and some are real installation losses. Thanks to this, PR is not entered manually, only calculated from specific assumptions.
| Parameter | What does it mean? | What value should I enter? | Impact on the result |
|---|---|---|---|
| Annual energy consumption | The amount of electricity your home uses per year. | Preferably from the energy invoice: kWh/year. | The greater the consumption, the greater the PV power required. |
| Coverage of consumption by PV | What percentage of annual consumption should the photovoltaic installation cover? | Most often 80-100%. With a heat pump or EV car you can test 100-120%. | Increases or decreases the target installation power. |
| Available roof area | Real surface on which panels can be mounted. | Not the entire roof surface, only the part without chimneys, windows, antennas and service zones. | Limits the maximum number of panels and the maximum kWp. |
| Assembly reserve | Space subtracted for spacing, roof edges, obstacle workarounds and measurement errors. | Typically 5-15%. With a complicated roof, even 20%. | Reduces the area actually available for panels. |
| The power of one panel | PV module power under STC test conditions. | From the panel's catalog card, e.g. 450 Wp, 500 Wp, 550 Wp. | It determines how many kWp can be obtained from one panel. |
| Area of one panel | The physical size of the PV module. | From the catalog card. Typically around 2.0–2.6 m². | It decides how many panels will fit on the roof. |
| Reference yield | Approximate annual production from 1 kWp before corrections. | For Poland, a starting range of 950–1050 kWh/kWp/year can be assumed, check it in more detail in PVGIS. | The higher the yield, the less PV power needed for the same consumption. |
| Roof direction | Correction of the installation geometry in relation to the sun. | South = best option; east-west usually lower annual yield, but better distribution of production in the morning and afternoon. | Reduces or maintains the reference yield. |
| Shading | Loss from the shade of trees, chimney, antenna, dormer, adjacent building or other obstacles. | None, light, medium or large. When choosing shades on a string, choose more carefully because the loss may be high. | It enters directly into PR and reduces annual production. |
| Air temperature on a hot day | Outdoor temperature used to calculate the instantaneous power loss on a hot day. | Use 30-35°C for the test. This is not an annual average, just a heat scenario. | Shows why a 500W panel doesn't always give you 500W. |
| Panel ventilation | Estimate how much hotter a PV cell is than air. | Good: +20°C, medium: +30°C, weak: +40°C. Roof hot and not very airy = poor. | It affects the temporary loss of power in hot weather. |
| Temperature coefficient Pmax | The panel power decreases for every 1°C above 25°C of cell temperature. | From the panel's data sheet. Typically -0.29%/°C to -0.40%/°C. | The higher the absolute value, the more the panel loses in heat. |
| Inverter efficiency | How much DC energy from the panels is converted into AC energy. | Typically 96-98.5%. You enter the percentage value in the calculator, e.g. 97.5. | Lower inverter efficiency lowers PR. |
| Cable and connection losses | Losses on DC/AC cables, connectors and connections. | Typically 1–3%. Long cable runs can mean more. | Lowers PR and annual production. |
| Dirt, snow, pollen | Loss from dirt, pollen, leaves, snow and sediment. | Typically 2-5%. In difficult conditions or at a flat angle, more. | Lowers PR, especially in winter and at low slopes. |
| Panel mismatch / MPPT | Loss from differences between panels, uneven working conditions and mismatching of strings. | Typically 1–3%. With different panel directions or partial shade more. | It lowers PR and may impair the performance of the string. |
| Downtime/restrictions | Losses from outages, service, network restrictions or voltage problems. | Typically 0.5–2%. If inverters frequently shut down in your area, enter more. | It lowers real annual production. |
| Annual temperature loss | Simplified annual production loss due to panel heating. | Good ventilation: approx. 4%, average: approx. 6%, poor/hot roof: approx. 9%. | It enters PR as an annual loss, separate from the momentary heat test. |
| Inverter clipping | Loss when the DC power of the panels temporarily exceeds the power that the inverter can output to the AC side. | 0-2% for normal oversizing. More with aggressive DC/AC. | Lowers PR, but moderate clipping is not always a design error. |
| Planned energy storage | Whether the user wants to add a battery in the future. | Select "yes" if you are considering self-consumption, backup or net-billing with storage. | It does not change the PR itself, but affects the inverter suggestion. |
How does the calculator calculate PR?
PR is the multiplication of coefficients after losses. For example, a loss of 3% means a coefficient of 0.97, and the inverter efficiency of 97.5% means a factor of 0.975.
PR = (1 - temperature loss) × inverter efficiency × (1 - cable loss) × (1 - dirt) × (1 - mismatch) × (1 - shade) × (1 - clipping) × (1 - downtime)
How to choose starting values?
| Installation conditions | Startup settings | When to change? |
|---|---|---|
| Strong south roof, no shade | Shade: none, cables 1-2%, dirt 2-3%, mismatch 1-2%, temperature 4-6% | Increase losses if the roof is dark, hot or the panel has poor ventilation. |
| East-west roof | Direction: east-west, other losses moderate | Increase mismatch if slabs have very different angles or different numbers of panels. |
| A roof with a chimney or a tree | Shade: light or medium, mismatch 2-4% | With multiple panels shaded, select "large" and consider optimisers/microinverters. |
| Installation on a hot, poorly ventilated roof | Ventilation: poor, annual temperature loss 9% | Particularly important in the case of dark sheet metal, BIPV or small clearance behind the panel. |
| Installation oversized relative to the inverter | Clipping 1–3% | Raise the clipping if the DC/AC is very high or the inverter frequently reaches the limit. |
How to calculate the power of a photovoltaic installation?
The simplest formula involves comparing the annual energy consumption with the expected production from 1 kWp of the installation. In practice, it is better to take into account the roof direction, shading and system losses.
Simplified formula
PV capacity [kWp] = annual energy consumption [kWh] / 1000
More accurate formula
PR = (1 - temperature loss) × inverter efficiency × (1 - cable loss) × (1 - dirt/snow) × (1 - mismatch) × (1 - shading) × (1 - clipping) × (1 - downtime)
PV capacity [kWp] = consumption [kWh] × coverage [%] / (reference yield × direction × PR)
Maximum number of panels = available roof area after reserve / area of one panel
PR, or Performance Ratio, should not be guessed by the user. In the calculator it is calculated from partial losses: temperature, inverter, cables, dirt, shading, mismatch, clipping and downtime. The available roof area acts as a limitation: if the roof is too small, the calculator shows the maximum possible installation instead of pretending that the required power will fit.
How does panel efficiency affect the selection of a PV installation?
If you know the power of a panel in Wp, efficiency is not needed to simply count the number of panels. The Wp power already tells you how much the panel gives under STC test conditions. However, efficiency is very important when calculating how much power will fit on a specific roof surface.
Panel efficiency = panel power [W] / (panel area [m²] × 1000 W/m²)
| Panel | Surface | Approximate efficiency | What does this mean? |
|---|---|---|---|
| 450 W | 2.1 m² | approximately 21.4% | A good option, but requires more space than a 500-550 W panel. |
| 500 W | 2.2 m² | approximately 22.7% | A good compromise between power and surface area. |
| 550 W | 2.55 m² | approximately 21.6% | A larger panel does not always mean higher efficiency. |
Why doesn't a 500W panel produce 500W in the heat?
The panel power is given for STC conditions: radiation 1000 W/m² and cell temperature 25°C. On a roof, on a sunny day, the temperature of the cell can be 55-70°C, which is much higher than the air temperature.
The key parameter is temperature coefficient Pmax. The lower the loss per degree, the better the panel retains its power in hot weather.
Actual power = STC power × [1 + temperature coefficient × (cell temperature - 25°C)]
| Air temperature | Ventilation | Cell temperature | Loss at -0.35%/°C | 500 W panel temporarily |
|---|---|---|---|---|
| 25°C | good | 45°C | about 7% | approximately 465 W |
| 30°C | average | 60°C | approximately 12.25% | approximately 439 W |
| 35°C | weak | 75°C | about 17.5% | approximately 413 W |
How does the roof influence the selection of PV panels?
The same set of panels can produce different amounts of energy depending on the direction, angle, shading and ventilation. Therefore, it is not enough to count the number of panels alone.
An east-west roof may still be good, but spreads production more into the morning and afternoon.
Chimneys, trees, antennas and dormers may require optimizers or a different string system.
Better ventilation means lower cell temperature and lower losses on hot days.
How many panels are needed for 5 kWp, 8 kWp and 10 kWp?
The table below shows a quick comparison for a 500 W panel with an area of 2.2 m². Actual production depends on installation conditions and system losses.
| Installation power | Number of panels 500 W | Panel area | Approximate annual production |
|---|---|---|---|
| 5 kWp | 10 pieces | approximately 22 m² | approximately 4200–5000 kWh |
| 8 kWp | 16 pieces | approximately 35 m² | approximately 6700–8000 kWh |
| 10 kWp | 20 pieces | approximately 44 m² | approximately 8,400–10,000 kWh |
How to choose PV panels for an inverter?
Selection of an inverter is not only about comparing the kWp power of DC panels and the kW power of the AC inverter. You need to check the string voltage, input current, MPPT number and allowable DC/AC oversizing.
- MPPT – system for tracking the maximum power point of the panels.
- String PV – a chain of panels connected in series.
- DC/AC oversizing – greater power of the panels than the power of the inverter.
- Clipping – temporary power limitation when there are more panels than the inverter can provide on the AC side.
See also: replacing the inverter with a hybrid one, hybrid inverter for LiFePO₄ 48V batteries, how to choose panels for the inverter.
The most common mistakes when selecting photovoltaic panels
1. Counting only after annual energy consumption
kWh consumption is a starting point, but you need to take into account the roof, direction, shading and real system losses.
2. Ignoring the temperature of the panels
On a hot day, the panel may have a cell temperature of 60°C or more. This lowers the instantaneous power relative to STC conditions.
3. Confusing panel power with efficiency
A 550 W panel is not always more efficient than a 500 W panel. It may simply be larger.
4. No plan for energy storage
If you want to add a battery in the future, it is worth considering the selection of the inverter and the method of expansion during the PV project.
Glossary of terms
| Concept | Explanation | Why is it important? |
|---|---|---|
| Wp | Watt-peak, panel power under STC test conditions. | Used to compare panels, but does not mean constant power in all conditions. |
| kWp | Sum of PV panel power in peak kilowatts. | The basic parameter of the size of a photovoltaic installation. |
| STC | Standard Test Conditions: 1000 W/m², cell temperature 25°C. | Laboratory conditions under which panel power is measured. |
| NOCT / NMOT | Conditions closer to the panel's operation in a real environment. | They help to understand why the panel in practice has lower power than in STC. |
| Temperature coefficient Pmax | It indicates by what percentage the panel power decreases for each degree above 25°C of cell temperature. | A key parameter when comparing panels on hot days. |
| Performance Ratio | The real efficiency factor of the entire PV system, calculated from partial losses. | It includes, among others: temperature, inverter, cables, dirt, shading, mismatch, clipping and downtime. |
| MPPT | A system in the inverter that optimizes the operation of panels. | Important for different roof directions and division of panels into strings. |
| Clipping | Power limitation by the inverter when DC production exceeds the AC capacity of the inverter. | It may occur when the panels are oversized relative to the inverter. |
FAQ
How to calculate the power of a photovoltaic installation?
The simplest way is to divide the annual energy consumption by the expected yield from 1 kWp. More precisely, you need to take into account the direction of the roof, available square footage, shading, automatically calculated PR and local sunlight conditions.
Does panel efficiency affect the number of panels?
If you know the panel power in Wp, the number of panels results from the installation power and the power of one panel. Efficiency mainly affects the roof area needed to obtain a given power.
Why doesn't a 500W panel always produce 500W?
500 W is the power under STC conditions. In real operation, the panel may be warmer, partially dirty, less ventilated or limited by the inverter, therefore the momentary power may be lower.
How many 500 W panels are needed for 10 kWp?
About 20 panels. Additionally, you need to check the roof surface, string arrangement, MPPT parameters of the inverter and permissible DC/AC oversizing.
Does energy storage change the selection of PV panels?
It may change the selection strategy. When it comes to energy storage, it becomes more important to increase auto consumption, select a hybrid inverter and properly control battery charging.
Sources and tools for further verification
For an accurate design, it is worth using local data, panel and inverter data sheets, and PV production simulation tools. When analyzing the location, you can check, among others: European Commission PVGIS: PVGIS.