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How to choose the power of photovoltaic panels?

The choice of solar panel energy is as follows: install as many solar panels on the roof as you can, taking into account the available space and finances. And better not go below 5 kWp. Why? You will learn from the article!

How to choose the power of photovoltaic panels?

As of April 2022, the discount system will disappear, and the current methods of calculating the power of a photovoltaic installation for prosumers will no longer be valid. What changed?

Under the previous rules, lowering bills to a level where only flat rates are paid was as much the purpose of the facility as it was to reduce its capacity. The larger the installation, the more profitable it was, but when the bills were as low as possible, there was no point in buying additional modules. Now it’s different: with a big enough installation, you can bring your bills down to zero and even get cash back.

What power to choose from solar panels?

After the legal changes that eliminate the discount system, the  power of the photovoltaic panels must be selected so that it simply generates as much money as possible  . It is about installing as many photovoltaic panels as possible in that part of the roof where there is a lot of sun and no shade.

It is worth investing in a high power photovoltaic installation because:

  • You can make money  with it  , not just lower your bills.
  • Higher installation = lower price of each module.
  • Your next car will probably be electric.
  • Your next heat source will probably be a heat pump.
  • Expanding a photovoltaic installation is difficult.

Of course,  for solar panels to have adequate capacity  , they must be installed in  the proper way  and in a suitable place for it. Do not install them in the shade and leave space from the edge of the roof.

At the same time, we do not want to say that low power photovoltaic installations are not profitable. However, we would like to point out that they will not be as profitable as they have been so far. Compared to a larger installation, they offer a higher return on investment and lower profits over 25 years.

Once the fitters have packed everything into the van, they come to you and climb onto the roof, adding a few modules at no cost to them. Hence, in installations of 2-3 kWp, the installation of each module costs a lot  . Therefore, even if you do not have a large power consumption, it is worth installing at least 5-6 kWp. It is best to compare a few offers from good installers and choose the one that suits you best.

What does the power of solar panels mean?

The power in electrical devices is the product of the voltage and the intensity of the current consumed or generated by a given device. Power is expressed in watts [W], and 1000 watts is the kilowatt [kW]  . However, when we talk about the power of solar panels, we can refer to one of three things: nominal power, maximum power or real power.

  • The nominal or maximum power is that reached by the module under the STC  test conditions   . It is expressed in Wp (ie peak watts).

The STC test result is rounded up, so when a given module outputs 381 watts in tests, it gets a label indicating that it is rated at 380 Wp. An installation made up of 10 of these modules has a power of 3,800 Wp, or 3.8 kWp. There are no test conditions on your roof, so a 1 kWp installation will never really reach 1 kW of power.

Analogy  : Your car’s engine has 80 kW of power, because that’s what the tests showed. When it’s parked, it still has plenty of power, even though it’s not running at the moment.

How many solar panels for 1 kWp?

1 kWp of power is equivalent to about 2-3 photovoltaic modules, depending on their size and efficiency. The smallest module is 1.6 x 1 meter and the largest module is 2 x 1 meter.

  • Real power  , that is, the one that actually reaches the installation at that moment. You can get this information from the investor. There it is expressed in kilowatts [kW]. Some systems (eg Solaredge) allow you to monitor each module separately. The power of the separate modules will be given in watts.

The real power of a photovoltaic system depends on the conditions, in particular the intensity of the radiation and the temperature. In summer, photovoltaics generate 5 times more electricity than in winter. There is no sun at night and the power is 0 W. After a year of measuring the actual power of the installation, its:

  • Efficiency  , that is, how many kWh of energy will 1 kWp of the installation produce annually. A kWh, or kilowatt-hour, is a unit of electricity, just as a liter is a unit of water and a kilometer is a unit of distance.

The efficiency depends on the installation method and the quality of the modules themselves. Solar panels installed in the south will produce more kWh per year than those installed in the east. The most efficient modules withstand the temperature better and work better in low radiation conditions, that is, when the sun is behind the clouds. This translates into higher yields, that is, more kWh produced per year.

The relationship between the power and performance of a module and the power and performance of the entire installation is shown in the following graph.

The power of a photovoltaic installation is the sum of the power of all the photovoltaic panels included in the installation. How can you estimate how much solar panel power you’ll be able to put on your roof? Let’s see!

How to calculate the power of solar panels?

With current technological advances, about 2 kWp of installation can be placed on a 10 square meter roof. The formula for the power of the photovoltaic panels on your roof is as follows:

Power of the photovoltaic installation [kWp] = Solar area of ​​the roof [m  2  ] x 2

Of course, this is a method for general understanding of the topic. Your  installer  in your offer will present you with a detailed presentation of the appearance of the installation on your ceiling. It could look like this:

Part of the offer of a photovoltaic installation for the company. 

Note that in the above design, as many solar panels as possible are placed, but only on the part of the roof where there is no shade. Shade is the worst thing about photovoltaics and should be avoided. Also note that in the previous design, the PV panels were not placed all the way to the edge of the roof, but  some space was maintained  .

Since there is no discount system, there is no more “correct” or “perfectly matched” power from the photovoltaic system.

If you have a high energy consumption, this is:

  • You have electrical heating, for example a heat pump, storage heaters or infrared heaters.
  • Do you have an electric boiler or a hot water heat pump
  • You work from home during the day (or are thinking of setting up for a business).

Just  mount as many modules on the roof as you can  reasonably fit  or as much as you can afford  , with a reasonable minimum of 5-6 kWp. If you don’t want to fill up your entire roof because you’re on a tight budget, just invest as much as you can in a solar farm. To get the most for your money, compare at least 3 offers from good installers in your area. We can offer you several options. Free!

Lower or higher power of the photovoltaic installation – which is better?

The larger the facility, the higher the production and each panel costs less. Smaller installations, in turn, have higher  self -consumption  , but this does not change the fact that  a larger installation equals higher profits  .

  • A low power photovoltaic installation (2-4 kWp)  will pay off and its cost will be amortized over time, but the electricity bill can still be quite high, and the installation of each module will cost a lot.
  • A 5-6 kWp photovoltaic installation  costs much less per panel, so this is the minimum recommended power.
  • A higher power PV installation (>6kWp)  will significantly reduce your bills, and maybe even bring you a monetary return on the surplus value of the energy delivered to the grid!

Let’s try to demonstrate this by comparing small and large installations!

Photovoltaic installation power gross price Price per kWp Self-consumption
_
Annual production ~ Annual Benefits return period
3 kWp 17.000 zlotys 5.1 PLN miles 25% 3000 kWh 1.200 zlotys ~ 14 years
10 kWp 42.000 zlotys 4.2 PLN miles 10% 10.000 kWh 3500 zlotys ~ 12 years

And the real difference in earnings can be seen after 25 years. This can be seen in the following graph:

The formula for the power of a photovoltaic installation based on energy consumption

This is the formula for the maximum sensible size of a photovoltaic plant based on its annual energy consumption.

Photovoltaic installation power [kWp] = {(Z * A) + [120% Z * N / (Cs / Ck)]} / P

Z  – annual energy consumption [kWh]
A  – self-consumption [%]
P  – annual production from 1 kWp at the installation site [kWh]
N  – surplus [%], i.e. the reciprocal of self-consumption
Cs  – average price electricity sales [PLN]
Ck  – average electricity purchase price [PLN]
120%  – we give you this factor because it is the most you can recover in cash

If we assume that 1 kWp will produce 1000 kWh per year, will have 20% self-consumption and the sale price of electricity is 0.45 of the purchase price, then:

Power of solar panels [kWp] = annual energy consumption [kWh] / 450

At the same time, if you want to mount a photovoltaic installation on the ground, you will probably use double-sided panels, then this pattern will be more appropriate:

Power of photovoltaic panels [kWp] = annual energy consumption [kWh] / 550

But how do you know that your energy consumption will not increase in a few years because, for example, you will buy a heat pump or an electric car? How do you know how your self-consumption will evolve? Maybe in a few years energy storage will be really cheap?

Finalmente, ¿cómo será el precio de la electricidad a lo largo de los años? Es importante porque a partir de 2022 no se liquida la cantidad de energía devuelta, sino su valor . Como una central eléctrica, vendes energía al precio de venta de la energía de la bolsa (y no pagas ningún impuesto por ello). A su vez, compras energía al precio de compra (el que sabes en la factura). Después del año contable, la empresa de servicios públicos compara el valor de la energía que ha comprado y vendido, y luego prepara la factura o retira dinero sobre esta base.

Good to know:  the selling price of electricity on the energy exchange  is constantly changing and completely out of your control. Therefore, we recommend that you  make the most of the ceiling space  .

This is what a sensible photovoltaic installation looks like:

Optimal use of the roof surface. 

Admit that a photovoltaic installation made in this way looks very nice. Now think that someone didn’t install a module because by some calculations it would be better. Harmony would be disturbed and the ceiling would simply look bad.

Selection of energy storage for a photovoltaic installation

As a general rule, energy storage is not worth thinking about as long as there is room on the roof for more solar panels. Photovoltaic is more profitable and more ecological. We also believe that if you are already shopping for energy storage, an emergency power system is worth buying   .

The US institute NREL divided the photovoltaic and energy storage installations installed in the US into two groups, which it called “less resistant” and “more resistant”.

less resistant system more resistant system
Photovoltaic installation [kWp] superior to energy storage [kWh]. Energy storage [kWh] greater than the photovoltaic installation [kWp]
Example:
Photovoltaic power – 7 kWp
Storage capacity – 5 kWh
Example:
Photovoltaic power – 7 kWp
Storage capacity – 20 kWh
Characteristic:

  • More economical
  • Faster installation recovery
  • Turn on the selected circuit
Characteristic:

  • More expensive
  • Longer payback period
  • Longer magazine life
  • feeding the whole house

For this reason, installers usually propose the option of 1 kWh for each kWp of a photovoltaic plant. Increasing energy storage by another kWh has less and less impact on average self-consumption, so smaller storage facilities are initially more profitable. However, larger chargers run slower and wear less and therefore have a longer life expectancy.

The most resistant variant will power the whole house and all the devices for a few days, even in very bad weather, when the photovoltaic will only charge the battery with a fraction of its power. The key detail: during a breakdown, you don’t change your habits at all and continue to use as much electricity as you like.

However, a smaller energy storage can also power your home for several days, even in bad weather. All you need to do is  limit your energy consumption to critical appliances  , lighting, refrigerators, and mobile phones only.

The most accurate selection of emergency energy storage for your home requires determining which devices you want to power and for how long, assuming a worst case scenario. Our installers will help you.