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How many panels would make sense on my building?

How many kWh can I generate on my roof?

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FAQ

Solar energy concepts

Air mass factor
Impact of the distance which the sun covers through the atmosphere. In winter when the sun is low, sunlight has to travel a relatively long distance through the atmosphere before hitting the solar panel. The sun's strength is therefore a bit less in winter than in summer when the sun shines practically straight down.

Autonomous system
An autonomous solar panel installation is one in which the output generated is stored in accumulators or is used directly. In this case, there is no connection to the public electricity grid, and any inverters present work independently.

CO2
Carbon dioxide, or greenhouse gas, increases the greenhouse effect which increases global warming. A 250 Wp PV panel saves about 125 kg CO2 a year which enables eight trees to grow.

Thin-film solar cells
These cells are made using vapour deposition techniques. They have no crystal structure. This type is considerably cheaper but generates much less electricity.

Efficiency
Figure indicating how much solar radiation energy is converted to electricity by a solar panel. Current solar panels have approximately 15% efficiency. The rest of the solar energy is unfortunately converted to heat. This means that solar panels can get quite hot.

Island protection
Built-in protection inside an inverter against external power failure. The inverter switches off automatically. In the event of a power failure, the solar panels will therefore not work either.

Electricity meter
A device metering your electricity consumption.

Energy cost recovery time
In the Netherlands a solar panel produces in a period of three years about as much energy as was required to make the panel (from raw material to finished product). In Southern Europe that time is of course shorter because of the more abundant sunshine there, namely two years.

Feed-in tariff
This is paid for surplus energy fed back into the grid. It is a fixed tariff which energy companies (have to) pay for electricity returned to the grid. Nowadays many countries have a feed-in tariff under which the costs are evenly apportioned to energy consumers.

Photovoltaic
Refers to the use of (sun)light to generate electric energy. Photovoltaic solar energy concerns the part of solar radiation that people perceive as light, in contrast with solar thermal energy in which the heat of solar radiation is used e.g. to heat up water.

Direct current
A type of current in which one pole is always positively or neutrally charged in relation to the other pole. The difference in charge is not necessarily constant ; the voltage generated by a solar panel varies e.g. depending on the incident light. However, as electrons always move in one direction, there is still direct current. The inverter therefore serves not only to absorb these fluctuations but also to convert the direct current to alternating current and the mains.

Grid
The grid is the electricity network. On-grid means that the installation is connected to the electricity network. Off-grid installations usually supply their energy to an accumulator-based system.

Grid parity
Situation in which sustainable electricity (e.g. from solar panels) competes with the "grey" (non-renewable) electricity of the grid. Grid parity depends on place, time and type of connection. For households in some countries grey electricity is already so costly, and sunshine so abundant, that the costs invested are immediately recovered . It is expected that that in a couple of years solar-powered electricity will in the Netherlands be able to compete very well with "grey electricity". When exactly this will be the case will of course strongly depend on whether the potential buyer finds the length of the cost recovery period acceptable.

Grid-connected
This refers to a network-linked system connected through the wiring in your own house to the power grid.

Grid parity
In this case, electricity from solar panels is as expensive as electricity obtained from the public grid.

Group
This is a collection of light points, sockets and switches that are interconnected and therefore form a group. This group has a common fuse with normally 16 ampère. To find out how many fuse groups you have, you can count the number of fuses or earth leakage circuit breakers in your fuse box, which will show you exactly how many groups you have.

Hybrid power solar panel
This is a solar panel that generates both electricity and hot water. Water is heated up through pipes in the panel. At the same time, this process cools the photovoltaic panel which further enhances the panel's output. This system is rarely applied in the Netherlands; it is more suited to southern countries.

Inclination
The angle of solar panels in relation to the horizon.

Insolation
The amount of sunshine a particular area receives during a specified period.

Kilowatt
kilowatt = 1000 watt, 1000 kilowatt = 1 megawatt, 1000 megawatt = 1 gigawatt, 1000 gigawatt = 1 terawatt

Kilowatt hour (kWh)
A kilowatt hour is a unit of work or energy. It is the amount of energy that is used when a power source has to supply 1000 watt (one kilowatt) for 1 hour. Kilowatt hour is often used, for instance on your electricity bill. An average household uses about 3500 kWh a year.

Kilowatt peak (kWp)
Maximum power generated under Standard Test Conditions. A kilowatt is 1000 watt.

Short circuit current
The (maximum) current which a solar panel generates as soon as the positive and negative poles are directly connected to each other. The current meets no resistance from an inverter or any other device. As a result, the voltage between the poles diminishes rapidly. See also Open circuit voltage.

Power grid
The power network to which practically all households are connected.

Metering company
Company that reads the energy meters. It is often a component company of the network operator.

Maximum power point tracker (MPPT)
This is the point at which the panels supply a maximum product of voltage and current. This is important if you install solar panels on two or more different roofs. In this case you will need an inverter which has a separate MPPT for each roof (or part of a roof).

Monocrystalline solar cells
These solar cells are made from one single silicon plate (monocrystal) that is sawn from one silicon crystal.

Grid-connected PV system
Grid-connected PV systems are connected to the power grid. The grid is used as a buffer for the electricity generated, and any surplus production is absorbed by the grid. If in a house fitted with solar panels more electricity is consumed than the amount generated by the PV system, the shortfall is supplemented from the grid. If there is a grid failure for whatever reason, the PV system switches off automatically for safety reasons.

Network operator
Company managing the electricity infrastructure. The network operator lays and maintains the cables, installs and replaces electricity meters, and where appropriate installs a gross production meter. In the past, the energy suppliers were responsible for network operation.

Nominal value
A value indicated by the manufacturer.

Inverter
An electrical device that converts direct current (DC) to alternating current (AC). Through the inverter, direct voltage is converted to the right voltage, i.e. 230 Volt alternating current.

Open circuit voltage (Voc)
The (maximum) voltage that a solar panel builds up as long the positive and negative poles are not connected to each other (open circuit). As soon as the poles are connected to each other (through an inverter, another device, or directly), current will start to flow and the voltage goes down.

Parallel switching
Method of switching several solar panels on one inverter, with each solar panel separately linked to the inverter. The tension between the two input sockets of the inverter is therefore not higher than that of the separate panels.

Photovoltaic (PV) system
This is a solar panel that converts light energy (solar energy) to electricity (solar power).

Polycrystalline (or multicrystalline) solar cells
These solar cells are cast, producing several crystalline areas during solidification.

PTC test
This term refers to PV USA Test Conditions, a test developed to be able to compare solar panels under real conditions (rather than in the laboratory). There are currently 10 413 types of solar panels that have been compared in this way. Manufacturers often boast of their STC test results, which are obtained in a laboratory.

Serial switching
This is a method of interlinking several solar panels on an inverter in which the negative pole of one panel is connected to the positive pole of the next one. Only the negative pole of the first and the positive pole of the last panel are led to the inverter. In serial switching, the voltages of the separate panels add up to the voltage of the string, in contrast with parallel switching where the voltage never exceeds that of the separate panels.

Inside a solar panel, the solar cells are usually also switched in series, with the positive pole of each cell connected to the negative pole of the next one.

Voltage
Voltage or tension is the difference in electric charge. For instance, the positive and negative poles of a battery have a different electric charge. By connecting the poles to each other, a current starts to flow from the positive to the negative pole. The current continues until the electric charge of the two poles is the same and the tension has disappeared.

A tension also arises between the positive and negative poles of a solar panel as soon as light falls on the panel. Unlike a battery, however, the solar panel cannot get depleted; as long as light shines on the panel, the tension between the poles continues and the current can continue to flow.

STC (Standard Test Conditions)
The output of a solar panel is determined through testing under standard conditions, on the basis of 1000 W/m2 solar energy and a solar panel temperature of 25°C.

String
A number of series-switched solar panels.

Tolerance
The maximum deviation from the values indicated. Example: a 200 Wp solar panel with a +/- 5% tolerance can supply power between 190 and 210 Wp. It is therefore preferable to keep the tolerance as low as possible to ensure optimum operation of a string of solar panels.

Power
The "strength" of a solar panel. The (nominal) power is calculated by multiplying the (nominal) current by the (nominal) voltage.

Full load hours per year (Wh/Wp)
The amount of energy (Wh) which a system generates, divided by the nominal power (Wp) of the system. In other words: the system's output converted into the number of hours which the system needs under standard test conditions (STC) to generate that quantity. Dividing the output by the nominal power makes it easier to compare systems of different sizes with one another. The number of full load hours per year depends to some extent on the quality of the system and to a large extent on the number of hours of sunshine in a year. In the Netherlands, it is assumed that for an average system there are approximately 850 full load hours a year. This means that a 100 Wp system generates about 85 000 Wp a year, which is equal to 85 kWh.

Volt
Unit of electrical potential.

Watt
Unit of power.

Watt peak (Wp)
Watt peak (Wp) is the nominal power of a solar cell or solar panel tested under standard STC conditions. By taking about 87% of the number of Wp, you get the number of kWh generated.

Alternating voltage
Form of voltage in which one pole is alternatingly positive and negative in relation to the other pole. The alternation between positive and negative voltage is gradual according to a sinusoidal curve.

Watt hour (Wh)
Unit of energy. One Watt hour is the amount of energy consumed for instance by a 1-Watt lamp when that lamp is on for one hour. A 0.5 Watt lamp that is on for two hours of course consumes exactly as much energy. In general. therefore, the number of Watt hours is the product of the nominal power expressed in Watt and the number of full load hours (hours "on full power"). A 240 Wp solar panel that generates electricity at nominal capacity for three hours has therefore produced 3 x 240 = 720 Wh, or 0.72 kWh.

Solar cell
A thin, usually square platelet that converts sunlight into electrical voltage. On a normal solar panel, several dozens of solar cells are arranged in rows and serially connected to one another.

Solar collector
Device designed to capture sunlight for practical use. Strictly speaking, "solar panel" and "solar collector" mean the same thing and sometimes the terms are used interchangeably. In common parlance, however, the term "solar collector" is reserved for solar thermal systems (solar heat) and "solar panel" for photovoltaic systems (solar current).

Solar boiler
A system to heat water through the sun's heat. A solar boiler usually consists of a solar collector that captures heat, a transfer fluid that passes on the heat and a boiler vessel in which the hot water is stored for subsequent use.

Solar cell
In a solar cell, photovoltaic conversion occurs under the impact of incident light. In this process, sunlight is converted to electricity. Solar cells consist of semiconductor material such as silicon. Under the influence of light, negatively charged electrons are released which move away and leave positively charged "holes". Separation takes place leading to a side with positive voltage and a side with negative voltage. If these two sides are connected to one another, electrical current (direct current) starts to flow. An inverter converts it to alternating current (of the electricity grid).

Solar collector
This device is used to produce hot water in the collector with solar heat.

Solar thermal
The use of solar radiation to heat something up. It contrasts with "photovoltaic" which uses sunlight to generate electric energy.

Solar panels
Solar panels that generate electricity are composed of photovoltaic cells (PV cells). The capacity of a solar panel is expressed in Watt peak per m2. Solar panels lie on the roof and are often blue or brown. Using solar panels, you can sustainably generate part of your electricity. Via solar panels sunlight is converted to electricity so that your household needs to have less recourse to fossil fuels. As a result, your household emits less CO2.

Solar power installation
The main components of a solar power installation are a number of solar panels and an inverter. Besides, equipment is needed to affix the solar panels to your roof (usually aluminium rails) and cables are needed to connect the panels to the inverter and the inverter to the fuse box. In the fuse box, additional groups are installed together with a production meter to measure how much you generate.

Solar power
Solar power is electricity that is generated by sunlight and the sun's heat. This is possible with solar panels. A solar panel (solar cells) can convert solar energy to electricity.