We have solar energy for free. So, by producing electricity from free solar energy, do we have the chance to enjoy free electricity? Is there enough sunlight for us to make sense of building such an installation? How much space do you need for a medium-sized solar plant? How to calculate and choose power plant components?
You will find the answer to these and many other questions in the article below. What’s more, if you decide to build your own solar farm and hire a contractor to carry out the task, you will be able to independently verify that it does not put a “putty” to you, so that after assembly your solar plant does not look like this 😀
IS POLAND A GOOD PLACE FOR SOLAR FARM?
Before we start counting and delving into the winding wind farms, we need some information about what we would intuitively call “the power of the sun” that shines above our heads.
Said “solar power” in technical language is nothing but the intensity of solar radiation . His unit is W / m2 (watt per square meter). This parameter (without delving into its physical definition) tells us about the strength of solar radiation falling on a specific surface at a given moment. We intuitively sense it ourselves when we go out for a walk and say: but today the strong sun, or: today the sun somehow heats poorly.
In the first case, when we feel a strong sun, it simply means a high intensity of solar radiation. However, when we feel that the sun is warming less, then of course the intensity of solar radiation is just lower.
The value of the intensity of solar radiation reaching the earth’s surface is highly variable and depends on many factors. Some of them are cyclical changes (seasons, time of day, angle of sunlight), and some are random phenomena (cloudiness, changes in air pollution, etc.).
For the curious:
From the Sun to the upper atmosphere reaches – approximately – a constant stream of solar radiation with an intensity of 1366 [W / m2]. Depending on the intensity of the phenomena (which I mentioned above) appearing on the path of the stream of sunlight, a stream of intensity varying in the range of 100 – 800 [W / m2] reaches the earth’s surface. In the most favorable conditions (cloudless sky, low level of pollution, sun at the zenith) the intensity of solar radiation reaches the level of 1000 [W / m2].
Operating solar radiation intensity while designing a solar installation would be very troublesome. In practice, a different size is used: Sunlight .
It takes into account all changes (cyclical and random) in the intensity of solar radiation during the full calendar year. The calendar year is the time interval in which the amount of energy reaching a specific point on the Earth’s surface is approximately constant (changes in the intensity of solar radiation at this time averaged so that deficiencies e.g. from cloudy days are compensated by days with excess sun).
Sunlight is the amount of energy that reaches the square meter of the Earth’s surface during the calendar year. The unit of sunshine is kWh / m2 (kilowatt-hour per square meter).
Sunlight is a characteristic and practically constant value for a given geographical location. So the maps of the sun have been created, from which one can read its value for the location of interest to us.
The map of Poland’s sun exposure looks like this:
As can be seen from the map above, the sunshine in Poland is in the range of 1000 – 1200 [kWh / m2]. In most calculations, the lower limit of this range is assumed, i.e.
Insolation in Poland is 1000 [kWh / m2]
As a curiosity, I also include a map of Europe’s sunshine:
The above sun exposure maps show that Poland is a decent location for the construction of a solar farm. Of course, in the south of Spain they are definitely better. This enables them to build power plants with the same power, using fewer panels, and thus smaller space.
WHAT AREA MUST A POWER PLANT HAVE IN POLAND?
The time has come for the first concrete calculations.
The surface panels must have results directly from their performance. If their efficiency was 100%, then from one m2 we would receive 1000 kWh per year, which is the amount of sunshine in Poland.
Unfortunately, their efficiency is much lower (in practice 15-20%), so to get 1000 kWh, their air must be proportionally higher. Fortunately, the power of the panels – called the peak power [Wp] – given by the manufacturers, is determined according to strictly defined criteria. The power of the panel is measured by illuminating it with a beam of light with a radiation intensity of 1000 W / m2, i.e. exactly as it occurs on a sunny day in favorable conditions. It perfectly simplifies all bills.
Thanks to this, to get energy from the power plant 1000 kWh per year, it is enough to use such a number of panels that their total power is equal to 1000 Wp. Proportionally, for power plants with a capacity of e.g. 3500 kWh, the total power of the panels used should be at least 3500 Wp.
HOW MUCH ELECTRICITY DO YOU USE?
You will find the answer in your electricity bill. Take bills from the last 12 months and count how much energy you used in kWh at that time. You will get the value you need by adding consumption from the next 12 months or by calculating the difference in meter reading between the beginning of the first and the end of the last month.
I counted my consumption. It amounted to 2875 kWh / year.
If you do not have bills and you do not have the ability to check your annual electricity consumption, then you can assume approximately 1000 kWh of annual electricity consumption per family member.
It follows that my power plant should have a minimum capacity of 2875 Wp .
We choose a panel
For the purposes of this article, we will choose a 250 Wp panel (made by SELFA), with the appearance and parameters as below:
The panel specifications are as follows:
The proposed panel was selected at the time the article was written, in 2007. Time is running and today panels are a bit more efficient. They allow you to achieve more power from a similar area, while their price is lower. It may turn out that due to better parameters you need fewer than you had before.
Below is the link where the current offer of solar panels will always be available . The list is generated based on the offers available at the time of clicking the link.
How many panels will we need?
The minimum number of panels (we divide the required power of the power plant by the power of one panel):
2875W / 250W = 11.4
In practice, I have to use 12 panels.
The dimensions of the 250 Wp polycrystalline panel used are 1640mm x 990mm, i.e. its area is 1.6236 m2.
The total area of 12 panels will be:
12 x 1.6236 = 19.48 m2.
To ensure 100% of the annual electricity demand (2875 kWh / year) I need to lay 20 m2 of solar panels. That’s the same as the area of one 4x5m room.
WELL, BUT PANELS ARE NOT EVERYTHING
It is true. Panels are just one element of the system. What else do we need then? Here again a few words of explanation.
From the beginning of 2016, new energy law came into force, which facilitated the construction and connection to the grid of small electricity-producing installations. Due to this act, the construction of this type of installation has been significantly simplified. The new law introduced the obligation on the energy network operator to connect such an installation to the network and collect excess energy. What does this mean for us?
The photovoltaic installation calculated above in theory provides me with 100% of my electricity demand, but for most of the day (e.g. when we are at work and children at school) it produces more energy than is consumed by devices installed at home. In turn, when we return home and start preparing dinner, watching TV, the children turn on the computers, and we set up some laundry, the energy consumption increases dramatically and my power plant is not able to cover all the momentary demand. Until now, the solution was batteries. At a time when electricity production was higher than necessary, excess energy was stored in batteries, so that at the time of increased demand use the accumulated reserves. Unfortunately, this solution significantly increased the cost of installation (purchase of batteries,
So what changed the new law? The provision on the obligation to collect surplus energy is important to us. Then, when our power plant produces energy with a surplus, the power grid operator is obliged to collect it from us. However, when we need more energy for a moment than our power plant produces, we can take the difference we need from the operator.
How does this happen in practice? The operator, after we report the readiness of the solar installation for acceptance, connects it to the power grid by installing at the same time instead of a traditional meter, a two-way meter. This means that when we draw energy from the network, the meter calculates the consumption normally. However, when we have surplus energy from our photovoltaic power plant, we give energy to the grid and a separate counter calculates the amount of energy returned. Ultimately, we have to pay (or refund) the amount arising during the billing period resulting from the difference in the counts counting consumption and refund. Currently, the reference period is one year.
If the meter at the end of the billing period has moved forward, we pay the amount resulting from the difference of indications. However, it may turn out that at the end of the billing period, the meter reading is lower than at the beginning (i.e. we have given more energy to the grid than we have used). In this situation, the production surplus is lost for free to the energy sector.
Using the provision of the new law, we can treat the power supplier’s network as a battery in which we store surplus energy. The meter counts how much energy we have sent to the network and in this way, we can collect it virtually free at a later time. Thanks to this, we can freely abandon the construction of a large part of the installation responsible for energy storage (batteries, charging controller, additional wiring).
Our installation is therefore simplified to two main elements:
– photovoltaic panels
– direct voltage converter (obtained from panels) to variable (power grid voltage). Such a converter is also referred to as an inverter or inverter .
Of course, you will need some additional accessories to assemble the power plant, such as:
– panel mounting brackets (for mounting panels on the roof)
– connection cables (for connecting panels with the inverter and connecting the inverter to the power grid)
– installation switches and safety devices (e.g. temporary or emergency disconnection of the inverter from the grid or panels from the inverter)
The above elements can, however, be treated as haberdashery, the cost of which can easily be calculated as a percentage of the cost of purchasing panels and inverter (in practice, it is assumed that the cost of purchasing accessories is about 20% of the value of panels and inverter ).
WHAT DOES THE INSTALLATION LOOK LIKE?
Thanks to the above considerations, we can already sketch the diagram of our power plant:
INVERTER – THE HEART OF THE SYSTEM
It is time to choose the inverter. When choosing, you need to pay attention to several parameters.
In order they are:
– Power and type of inverter
– Maximum input operating
voltage – Start voltage and shutdown voltage
– Maximum current
– Number of MPP inputs (MPPT)
– Noise level generated
– Mechanical properties (tightness)
Power and type of inverter
The first and most important thing is the type of inverter. We are looking for an inverter designed to work in an on-grid installation . This term ( on-grid ) refers to installations connected to the power grid of the power supplier, which is what we care about. When searching for a device, make sure it is suitable for on-grid operation.
We select the inverter power so that it is as close as possible to the power of the installation constructed by us.
You can see from here that the right inverter for my needs is one with 3000W power.
Let’s choose a specific device, as in the photo below:
This is the Growatt 3000TL inverter.
As in the case of panels, from 2017, progress is also visible in inverters. Today, for the price of the above inverter, you can buy a much better and more modern one. The rules of selection, of course, remain the same, but it’s always worth seeing the latest offer of inverters. The latest and current offer can be found at the link below.You can find and compare inverters at different prices from different suppliers by clicking on the link: .
The parameters of the selected inverter are as follows:
According to the technical specifications of the inverter, the maximum power of the panels cannot exceed 3200 W (this parameter is indicated by the red arrow in the figure below. In my case it is 3000 W, which means everything is correct.
Maximum input operating voltage from panels
This is the voltage that we get from interconnected photovoltaic panels in the open state (marked on the panel as Voc). In our case it will be 12 panels. When used on panels, they generate a voltage of Voc = 37.5 V. We connect them in series. This means that the output voltage from such a combined array of panels will be the sum of all 12 panels’ voltages, namely:
12 x 37.5 V = 450 V.
450 V voltage will appear at the output of the panel system, so we must check whether it falls within the range of the inverter we have chosen, according to the principle that: the voltage at the output of the panel system cannot be higher than the maximum input voltage of the inverter.
In the case of the inverter of my choice, the maximum input operating voltage is 500V and is higher than the maximum voltage possible to obtain from panels (450V). Everything is OK!
Maximum current from panels
This is the current obtained from solar panels. When connecting panels in series it is equal to the current of a single panel. In parallel is the sum of currents from each of the parallel branches.
In our case, we have a serial connection, i.e. we accept the current of a single panel. We can read it from the technical data of the panel (it is marked as Isc – in the case of our panel Isc = 8.8 A), or approximately calculate the power (Wp) and nominal voltage of the panel (marked as Vmp or Vmpp – it is always lower than the voltage in an open state).
Our panel power is 250W, and the nominal voltage is 29.9 V.
The resulting current is:
250 / 29.9 = 8.37 A
The maximum current obtained from the panels should be less than the maximum input current from the panels given in the inverter data.
In the case of the inverter of my choice, the maximum input current is 15A and is greater than that obtained from the panels. We are calm in the parameters.
Starting voltage and tripping voltage
First a few words of explanation about what these parameters say.
The starting voltage is the voltage at which the inverter can work effectively and start converting the energy obtained from the panels into energy generated into the grid.
After sunrise, along with the increasing amount of light falling on solar panels, the value of the voltage generated by them increases. Until the start voltage is reached, the inverter remains dormant and does not convert electricity. Energy production only begins when the start voltage is exceeded.
In practice, a lower start voltage means an earlier start each morning and the production of more energy than with a device with a higher start voltage.
The situation is similar with disconnection voltage. This is the voltage at which the inverter stops working and stops producing electricity for the grid. Lower shutdown voltage for longer work before each dusk and more energy produced each day.
In the case of the inverter of my choice, these parameters are as follows:
– start voltage: 150V
– switch off voltage (read as the lower voltage range): 100V
Number of MPP (MPPT) inputs. The
inverter MUST be equipped with the MPP function, i.e. maximum power point tracking functions. Thanks to this, he can fully use the power possible to get from solar panels. Due to the fact that the working conditions of the panels during the day change (main due to their heating), so-called maximum power point.
An inverter with maximum power point tracking function is able to get up to 20% more energy compared to an inverter without this function. Currently, the purchase of an inverter without the MPP function is rather unlikely, but you should be careful not to let yourself push the device, with the so-called inventory or bargain sale.
The inverter of my choice has 1 MPPT input.
Noise level generated
This parameter is particularly important when the inverter will be installed close to living quarters. Voltage converters (and the inverter is such an inverter) sometimes have a tendency to make strange noises. These are usually quite high sounds, which, despite the fact that they seem quiet, are also very penetrating. Although at first glance, the ears seem harmless, in the long run they can irritate.
The inverter does not require constant supervision, so it is best to install it a little further from the residential part (e.g. garage, attic, utility room, etc.). Most inverters can also be used outdoors.
My inverter generates noise less than 25 dB. It’s quite quiet.
It is worth paying attention to the tightness of the inverter housing. It is worth buying a device that is adapted to work in the open air, regardless of the fact that it will work under a roof. The tightness guarantees us resistance not only to water and moisture, but also to the penetration of dust and dust into the housing. Assuming that the installation is to work constantly and without fail for at least 20 years, it is worth taking care of this parameter.
The tightness of the inverter of my choice is IP 65, which means that it is dustproof and resistant to water jet (12.5 l / min) poured onto the housing from any side.
I suggest buying devices with the longest possible warranty. An inverter for an expensive device and its possible repair is not only the cost of this repair, but also the time when the power plant does not produce energy for the needs of our home and we must additionally buy it from the power grid. Not to mention the need to buy a new device when the repair is unprofitable. A guarantee of 5 years is the absolute minimum. There are also devices with a 10-year warranty and an option to extend it up to 20 years.
The manufacturer of the inverter of my choice gives a 5-year warranty, with the option of extending it to 10 years.
SELECTION OF WIRES
Selecting the type of cables When
constructing a photovoltaic system, DO NOT use standard electrical cables under any circumstances .
Cables for working in such installations must meet much higher requirements. The reason is definitely more difficult working conditions, requiring much greater resistance to temperature changes, a wider range of long-term work voltages, greater resistance to mechanical damage, resistance to UV radiation, ozone and atmospheric conditions. Solar installation cables must have so-called halogen-free insulation, which means that in a fire situation they do not emit substances that are dangerous to human life. They must be both flexible and flexible (cables with 5th or 6th class wires should be sought), guaranteeing durability and resistance to movements and multiple bends.
There is no room for compromise.
Detailed requirements for cables for solar installations are included in the PN-HD 60364-7-712: 2016-05 standard.
Examples of cables meeting the above conditions are:
– SOLARFLEX-X PV1-F
– OLFLEX SOLAR XLS-R
– BiT 1000® solar
Selection of conductor
cross-section We select the conductor cross-section according to the dependence presented below, but it should be assumed that the minimum conductor cross-section is 2.5mm2, even if the calculation results less.
Conductor cross-sections are arranged in a standardized series: 2.5mm2; 4mm2; 6mm2, 10mm2, 16mm2 … etc.
In practice, we assume the nearest larger than calculated wire cross-section.
If we obtained 2.7mm2 from the calculations – we take a cross-section of 4mm2
If we received 3.6mm2 from the calculations – we take a cross-section of 4mm2
If we received 5.1mm2 from the calculations – we take a cross-section of 6mm2
In our case:
– power plant power: 3000W
– voltage from the panel section: 450V
– we will assume the distance from the panels to the inverter: 20m
– allowable voltage drop: 1%
we calculate the minimum cross-section:
We choose a 2.5 mm2 wire .
Depending on the type of panels used (dimensions, construction details, type, etc.), and the surface on which they will be mounted (type of roof, roofing, tiles, etc.), different mounting systems are used. Discussing them here is rather pointless. However, it is worth considering this issue in detail and asking for details about the potential supplier of solar panels before making the purchase.
HOW MUCH DOES MY SOLAR PLANT COST?
The time has come to sum up the costs of building a power plant. Let’s gather everything together. Let me just remind you that we will assume the cost of cables, assembly equipment and other electrical equipment as 20% of the value of the panels and the inverter.
The calculation looks like this (all calculations below relate to the state of 2017. Currently, the cost of purchasing materials for the power plant is lower. The payback time will be slightly shorter. I encourage you to make your own calculation based on current prices ( solar panels – CLICK and solar inverters – CLICK
Calculation is very easy and prices change quickly so there is no point in constantly updating it.)
The remaining part is labor and payment for connection to the network. Connection can only be made by a certified installer.
Of course, it is possible to negotiate the price for panels and inverter (most panel bidders in their offers clearly indicate that with a larger amount they are willing to provide additional discounts).
The calculated amount, however, gives us a view on the overall cost of materials needed to implement our solar plant.
If you decide to hire an external company for the comprehensive construction of such a power plant with you, then you have a benchmark in terms of the price that the contractor may demand from you.
DOES THE CONSTRUCTION OF A SOLAR FARM PAY OFF?
And finally we came to this place and this question. To count it, we need to remember our assumption at the beginning of the article, which was: my power plant covers 100% of my electricity demand. This means that I will pay PLN 0 annually for energy, i.e. after a year, a two-way energy meter in my home will show the same value as a year earlier.
In one of the previous articles (How much electricity costs?) I calculated that the price of electricity resulting from bills is 0.6 PLN / kWh and the annual consumption is 2875 kWh.
During the year, therefore, I will save 2875 * 0.6 = 1725 PLN
I will invest 16 800 PLN in the construction of the power plant,
so we calculate the payback time:
16,800 / 1,725 = 9.73
The power plant will pay for itself after less than 10 years.
Of course, you can think about the possibility of reducing the return on investment time. There are several ways to do this:
– buying slightly cheaper components and negotiating purchase prices (in the above valuation I did not focus on choosing the cheapest offer possible). Below the article, I will place several links to help you search current price offers.
– construction of a power plant with a slightly higher power and sale of surplus electricity to the operator (the guaranteed energy buy back price is currently PLN 0.75 / kWh (for power plants up to 3000 kWh), which means I sell more than I buy)new regulations came into force according to which we no longer sell energy to the grid. For installations up to 10 kWp, we can collect 80% of the transferred energy from the public network, and for installations above 10 kWp to 40 kWp, we can collect 70% of the transferred energy from the public network, so this point falls 🙁
– taking advantage of preferential financing for the construction of the power plant, from funds from The European Union. A low-interest loan is currently available for this purpose with a 30% write-off option.