How many solar panels do I need?

Investing in photovoltaic systems is undoubtedly a good idea. However, for non-specialist in this field, it is not easy to determine what kind of PV system and how many solar panels you need. Unfortunately, some solar businesses will use this knowledge gap and try to sell a larger solar systems than necessary.

In the solar installation business, the phrase “how many solar panels do I need” is referring to the sizing of a PV-System. In this blog post, we will give you a fact-based guide to understand the process of determining the question: how many solar panels do I need? – Here you find the factors which are influencing the amount of solar panels needed.

The Influencing Factors

The sizing of a PV-System is always an individual assessment based on location-specific, environmental and technical circumstances. Therefore, when determining the size of a PV system, the following factors must be considered.

Four factors influence the number of solar panels needed.

1. Location

If you would like to invest in a solar system, one of the first things to think of is were to install the system. Usually, the choice involves either the roof or installations on an open field.

At this stage, you must ask a very important question. Where is the location geographically positioned? Different geographical locations are subject to different degrees of solar irradiation. And different degrees of solar irradiation have an impact on the productivity of solar panels. Greater irradiation creates better energy flows inside the solar panel, enhancing its productivity. Countries closer to the equator experience stronger irradiation, which gradually decreases the more we approach the poles.

But also seasonal variations affect irradiation. For example, the Northern Hemisphere receives more sunlight during its summer months, which results in stronger solar radiation during that time. Regional variations also play a role. These include altitude, atmospheric conditions, cloud cover and temperature. Regions closer to the equator receive more direct sunlight throughout the year than areas further away the equatorial line.This results into stronger solar radiation for these regions. Additionally, temperature is negatively correlated to the efficiency of the PV system. An increase in temperature eliminates the produced current, reduces the power and decreases the overall PV system efficiency. However, this occurs at already high levels of solar irradiation.

A great indicator of how much radiation is present is the global radiation factor. Usually, this factor lies between 1.1 – 1.3 in northern European countries and from 1.3- 1.6 in the southern countries of Europe. You can check your radiation factor on Global Solar Atlas by inserting your location. This topic can be become quite complex. Fortunately, PV developers rely on software that takes all these factors and more in consideration, allowing for the best PV sizing outcome.

2. Space

Another factor is the amount of space available in the location. Is there enough space available? Am I limited due to small areas? The actually usable space can limit your PV-System capacity. It is very simple: if there is not enough room for more solar panels, then there is no way to increase the system size.

Moreover, it is important to consider which direction the space available is facing. This means: North, South, West or East? For a photovoltaic system, the optimal orientation is South. Nonetheless, also spaces facing East and West can be efficient. However, north-facing locations are not recommended as they receive a significantly lower amount of sunlight.

Additionally, the inclination of the roof or mounting system on an open space will influence the performance of the photovoltaic system. The inclination should be between 20-35%, depending on the exact location and height level. Both a lower and a steeper inclination will result in less output from the solar panels. Luckily, the inclination plays a less crucial role in terms of solar panel efficiency.

Lastly, it is important to take potential shading into account. Large trees or neighboring buildings can block precious sunlight to the solar panels. Therefore, we recommend finding a space with nearly no shade. Unfortunately, these are one of the major influences on the performance of a PV-System. Relatively little shading can already result in an overall decline in energy generation.

3. Consumption

Your electricity consumption has the biggest impact on how many solar panels you need. Electrical consumption refers to the electricity used in a given period. Usually, the period is one year and the value is kilowatt hours (kWh). For residential buildings the average consumption is around 5.500 kWh per year. For a small-medium enterprise, it is approximately 20.000 kWh – 30.000 kWh per year. Instead, the value varies extremely when considering large businesses. You can find your electricity consumption in your electricity bills. Otherwise, you can directly contact your electricity provider to retrieve such information.

Consumption is the essential factor when sizing a PV system. Consequently, the bigger the consumption, the bigger the PV system will have to be and, thus, the more solar panels you will need. The needed solar panels can be calculated backwards from the consumption. However, this calculation just gives you an approximate figure because other influences such as inclination and orientation are not taken into account.

This broad value can be calculated by watt per solar panels * the radiation factor / by the total consumption that should be covered. Here is an example considering a small enterprise with a consumption equal to 20.000 kWh per year. If the solar panels used have an output of 400 Watts and the PV-System is located in Greece with a radiation factor of 1,45, the small business will need 35 solar panels. The result is given by:

(400 Watts * 1,45 radiation) / 20.000 kwh = 34,48 -> 35 Solar Panels

This formula can help you find an approximation. However, it does not take into account all factors. Whereas professional simulation and analysis rely on one or more softwares that consider all the factors influencing the amount of solar panels needed and yield an accurate result.

4. Needs

The last, less fact-based factor, involves the individual needs. It is about what you want to achieve by investing in a photovoltaic system. These motives could be several. You might want to reduce your electricity bill, decrease your carbon footprint, become more or completely independent from the public grid and related power outages and price volatility. Although a PV system can address all these issues, different designs can maximize each one of these goals.

As an investor you should be clear about your intentions otherwise a consultancy can take longer or a system with wrong functionalities is offered.

Three Approaches to Solar Photovoltaic Systems

In this section we present 3 different approaches to the investment in solar photovoltaic systems. These can give you the broad limitation of your system needs:

The less intense investment approach

In the first approach the investor would like to reduce the intensity of the investment as much possible while having an effective PV system. In other words, the investor would like to spend less money on the PV-System while still enjoying the benefits.

The solar system should cover around 20-40% of the consumption. This will reduce the electricity bill in the long run, while keeping the initial investment as low as possible. For this approach, the combination with a battery system is not recommended because, on average, the battery extension costs around 30% of the total system. Moreover, if net metering is possible, the investor can benefit from it greatly. If you would like to know the existance and conditions of net-metering in your country, click here.

This approach results in a small PV-System that produces a small amount of electricity. However, if the generation of energy is limited also small amounts of energy are available to cut costs. Therefore, the systems are mediocre in terms of payback time. Nonetheless, this will be around 7 to 12 years.

The efficiency approach

For this approach the solar system should be sized to the point where costs-benefits are balanced as much as possible. Hereby, the solar system should cover around 50-75% of the total consumption. A small battery is recommended. The battery allows the investor to store some of the generated energy, normally during peak generation times as well as in periods of low consumption. Later on, when the solar panels are under-performing due to weather conditions, seasonal causes or simply because it’s night time, the battery releases the electricity needed. This represents also a financial advantage, as the energy stored in the battery is normally used during periods of peak demand, where the electricity from the public grid is more expensive.

This approach will incur a larger investment in the beginning. However, in the long run, this system yields the most output. The system will generate a lot of energy during peak time. It will even over-produce, thus storing electricity in the battery. Due to the heavier investment up front, the payback period will not drastically change and lies between 6 to 12 years. But keep in mind that on average a PV-System runs for 20-25 years. After the payback period, the system will still yield large amounts of electricity for the next 8-19 years.

To put it all together, the efficiency approach balances the intensity of the investment and gives high amounts of output. The payback time is average. But after the payback time the system will yield a lot of energy for free, which makes this system most effective in the long run.

Fully independent approach (Off Grid)

The last approach aims to give the investor the highest level of independence. Therefore, it is definitely the most substantial in terms of investment and planning. The system aims to cover 100% of the consumption or 100% of the space available. Moreover, the costs of the system are higher due to the bigger battery. However, the battery system will enable you to have electricity even in times without sun, in the night, evening, or early morning.

For this system, you can expect the cost to be twice as much as a ”conventional” one. This results from the extra investment for the large battery storage. But with such a system, every power cut or electricity price increase will not affect you because the system is designed to give you as much independence as possible. The payback period is longer due to the large investment made into the battery.

This approach aligns with the principle of the Off-Grid system. However, it is still possible to have a connection to the grid even with full independence. You can find more about Off-Grid systems here.


To figure out how many solar panels you need, you have to think about several contingencies ranging from the location, the space available, the consumption, as well as your individual wants and needs. Understanding and knowing about the factors will help you find a suitable system design and the right amount of solar panels. Moreover, the different approaches to solar photovoltaic system design can give you a broad idea of how to choose between the solar system sizing options. 

In summary, determining how many solar panels you need is not an easy question. The individual circumstances and environmental factors play an essential role. When facing the decision to invest in a solar system, we always recommend consulting professionals who can run adequate simulations and analyses.

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