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The inverters we offer

The inverter transforms the energy coming from the solar panels (direct current) into the power we use in our energy network (alternating current). There are various technologies available for this, which can be divided into three categories:

  • The String inverter
  • Power optimisers
  • Micro-inverters

If you are somewhat familiar with these technologies, you may hear different stories that are rather confusing. This is because representatives try to explain why their technology is the best. As always when there are multiple alternatives, each solution has its advantages and disadvantages.

Robisol carries all three technologies so that we can give you an objective and appropriate advice. We would like to give you a brief explanation of solar energy and the three categories that you can choose between. When explaining the categories we will also try to filter out the sense and nonsense.

 

Basic concepts in electrical engineering

The solar panel supplies a simple form of energy that runs from the plus to the minus. This is called direct current. A battery or a car battery works in the same way; you have a plus pole and a minus pole. Every device you connect to it is powered by the positive terminal and the negative terminal closes the circuit so that the electrons are also discharged.

Volt is the voltage. It is the amount of energy per unit. Perhaps it is best to compare this with the amount of air pressure you can put in a gas bottle. With 200 bar you have 200 times more air per unit than with 1 bar. With 200 volts you have 200 times more energy per unit than with 1 volt.

Ampere is the current. So this is actually the movement of the electron through the cable. Perhaps the easiest way to compare this is with water flowing through a river. With 100 Amps, 100 times as many electrons move through the cable as with 1 Amp.

Watt is power. This is the amount of energy that actually arrives. If you move 200 gas cylinders of 1 bar or 1 gas cylinder of 200 bar; in both cases you have moved the same amount of energy.

However, the amount of effort you need to expend to move 200 gas cylinders is much greater. If we translate this into electrical engineering, 200 volts is no problem at all, but 200 amps is an awful lot. You need thick cables for that and you will always have loss of current due to resistance in the cables. You can compare this with the flowing river where the water is slowed down by quays and bends. The resistance is also called an impedance and is expressed in Ohms (Ω). Resistance is often referred to when checking or measuring systems.

For the distribution of energy, we therefore have high-voltage pylons. But even that is not enough. In our energy network, we want to be able to bring in and take out energy at any point in the circuit. The trick is to move as little energy as possible. Alternating current can best be compared with a long track that is full of balls. If I add a ball at the beginning of the lane, it can be taken off again at the end without me actually moving it to the end of the lane. In our energy network we create this by continuously changing the direction of the current. All the balls go from left to right and you can arbitrarily take away and add power without actually moving all the balls.

The current we use in the house has a fixed voltage of 230V. Based on this, you know exactly how much current (Ampere) must flow through a cable to move a certain amount of energy (Watts). A commonly used calculation is that you can connect a maximum capacity of (230V x 16A =) 3,680 W to a 16A fuse.

 

Mountaineering in the solar market

The power of solar panels is indicated in Watt peak (Wp). This is the maximum power that the solar panel delivers at a solar irradiation of 1,000 W/m² and a temperature of 25°C. This is an international convention and all solar panels are individually tested to check whether they actually deliver the power promised.

Sometimes power is confused with kilowatt hour (kWh). A kilowatt hour is the amount of energy. If you run a machine with a capacity of 1,000 W for one hour, you have used 1 kWh. If solar panels generate 1,000 W for one hour. You have produced 1kWh. For you as a customer, a PV installer often looks at the amount of energy you consume on an annual basis; for an average family this is around 3,500kWh. With a heat pump, the rule of thumb is that you use another 3,500kWh, and if you drive electric (+/- 15,000 km), your annual consumption will be another 3,500kWh higher.


Performance expectations

The power of the solar panels is matched to your consumption. This is because you are allowed to net out. This means that you can offset the amount of energy you produce on an annual basis against the amount of energy you consume on an annual basis. If this is exactly the same, you pay nothing and receive nothing back. Normally, you pay between € 0.20 and € 0.25 for the electricity you consume. A saving at this rate can be easily earned back through an investment in solar power. If you produce more than you use, your feed-in tariff is normally between € 0.04 and € 0.08. This is very low. Because the additional investment in extra power is also less expensive, this need not stop you. But the optimum payback time is normally at a power output that produces approximately as much as you consume.

When comparing different PV installers, it is wise to compare offers on the basis of Watt peak (Wp). When installers make a kilowatt-hour (kWh) calculation, they are talking about the expected amount of energy you will produce on an annual basis. This takes account of location and orientation, but effects such as shadow and cable losses are influenced by the installer and one calculation model also assumes more hours of sunshine than another. Calculations made using an inverter manufacturer's software, for example, are less objective than software which can calculate all types of inverters. We therefore advise you to always base a comparison on capacity (Wp).

At Robisol we can make a PV Sol calculation for you if required. In this, various inverters and connections can be objectively compared with each other. As standard we use the amount of power in Watt peak (Wp). With a somewhat south-oriented installation, you may assume that in the Netherlands the rule of thumb is that the conversion factor is around 0.85. So with 1,000 Wp of power, you produce around 850 kWh per year. You therefore need approximately 4,100 Wp of power to produce 3,500 kWh of energy on an annual basis.

It is true that with a thin film panel or an installation with power optimisers or micro-inverters (if correctly connected) you will often end up with a slightly more favourable result. This depends on your set-up and the shading that is present. It is quite realistic that you will pay, for example, 15% extra for 5% more energy and that, in case of a lot of shadow, you will generate about 10 to 15% more energy. That in itself is a fine proposition, but if you have the space, it is generally more favourable to invest the extra costs in extra power. It is, for example, quite realistic that for 15% more investment you will install about 25% more power.

 

The detail of a solar panel

To complete the picture, we will briefly discuss how a solar panel produces energy and how it can be connected.

A solar cell produces a low voltage of approximately 0.5V. That is very low. If you want to connect a device to it, you need a lot of current and therefore very thick cables to transfer the energy. In every solar panel, the cells are therefore connected in series. This means that the plus is connected to the minus and so a solar panel with 60 cells delivers a voltage of approximately 30 Volts. If you connect them in series, the current (Ampere) remains the same; for a solar panel this is about 10A.

If you connect several solar panels, you can choose to connect them in series or parallel. When connecting solar panels in series, the plus is connected to the minus cable. The voltage (Volts) increases while the current (Amps) remains the same. Because you move more energy with the same current, this is favourable for your cable losses. Of course, you cannot do this indefinitely, because electrical components can break down if the voltage becomes too high. But with 10 panels in series, you are only at 300V, so this is generally done quite often. Solar panels connected in series are also known as a string.

With parallel connection, a splitter is used on both the plus and the minus side. The current then flows simultaneously through panels 1 and 2. Not the voltage (Volt) but the current (Ampere) will increase. If you would make a splitter with 10 solar panels in parallel, the voltage would remain at 30V and the current would increase to 100A. However, this is a very high amperage and you need thick cables to move an amount of energy.

On the pages String inverter, Power optimizers and Micro-inverters we explain the choice you have for connecting your PV system.

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