The modules produce direct current. Your electrical devices require AC power. That doesn’t fit yet! In order to be able to use the solar power in the house (or to feed it into the grid), it has to be converted into alternating current.
This is exactly what inverters (also called voltage converters) do. At the same time, they control and monitor the entire system.
- Which inverter do I need for my solar system?
The inverter depends on the power of the connected solar modules. The available power starts at two kilowatts and goes into the megawatt range.
Typical grid-connected house roof systems have at least one inverter, but often several – depending on the number of modules, system performance, configuration and inverter type.
5 kW for typical private DIY solar power system
10-20 kW for commercial systems (e.g. hall or barn roofs)
500-800 kW for use in solar power plants.
These three types are the most common types of voltage converters in a private photovoltaic system:
String Inverters – Are the cheapest. Control a string of solar modules connected in series.
Multi-String Inverter – Controls multiple strings of solar panels connected in series
Module inverters – Are built directly into the individual modules. Control power more efficiently.
Experts recommend an inverter with an efficiency of 96 percent or more for a single-family house – PV system. Modern devices even have an efficiency of up to 98 percent.
When calculating your yield, bear in mind that the maximum efficiency specified by the manufacturer is rarely achieved. Rather, they should be based on the so-called European efficiency.
1.1. Which voltage converter do you need for small systems?
Solar panels and charged batteries are known to supply direct current. However, normal technical devices in the household require alternating current. There are also these so-called voltage converters for mobile and small systems, which convert the direct current into alternating current – from 12V or 24V to the required 230 volts.
There are the large inverters for your PV system on the roof of the house or small inverters or voltage converters, which are often used in mobile homes, boats or when camping, so that normal household appliances, radio, chargers, etc. can also be used there. Let’s take a closer look at these smaller systems.
The normal alternating current in the household changes in the form of a sine curve. Similar to how a siren wails between loud and quiet.
Not all voltage converters generate exactly such a sinusoidal AC voltage. Some devices generate a current that is more like an on/off beep – this is then called a square wave.
A distinction is made between voltage converters which supply current with:
Real sine wave
Modified sine wave
Square-wave voltage (rather rare and less than 100W power output)
The question now is whether the devices you use need a pure sine wave inverter. The following table gives an approximate overview of which voltage converter works when:
Square-wave voltage sufficient:
- Incandescent bulbs (not LEDs)
- Commercially available coffee machines (no pad or capsule machines)
Modified sine wave:
- Notebooks / laptops
- Electric household appliances
Pure sine wave
- Battery chargers
- Compressor refrigerators
- Switching power supplies
- Led Lamps
- Pad or capsule coffee machines
- Electric toothbrush
If a device requires a pure sine wave but does not receive it, the following errors sometimes occur:
Power supplies get warmer than normal
Humming power supplies
Notebook trackpad spins
Battery chargers do not fully charge
The following applies: the worse the sine curve, the less reliable the operation of some devices. The smoother the sine wave, the better.
Also: And the smoother the sine curve, the more expensive the voltage converter.
- How much power does my voltage converter have to have?
This depends on the required device performance.
Hair Dryer: 1200W
Use time: 10 minutes
With 1200W from the 12V battery you need 100 amps for 10 minutes calculated 17Ah (amp hours).
If the solar battery used has 120Ah, about 15% of the storage capacity of the battery is used.
In the case of gel, AGM or wet batteries, no more than 50% of the storage capacity should be used in order to optimize the service life. That would be 30% instead of 15%.
Incidentally, the voltage converter used should not be operated with the specified maximum power. This is only intended to compensate for voltage peaks that occur when some devices start up. So pay attention to the specified “continuous output power” of the voltage converter. This should be sufficient for the required wattage of the devices used.
If in doubt, buy a device with the next higher performance level. The advantage of this is that you have a power reserve if you want to connect other devices.
So pay attention to quality when buying. With very cheap pure sine converters, the sine wave is sometimes not as clean as stated. That causes technical problems and you just get annoyed.
- Tips for use in PV systems
The inverter is a central and important component of the photovoltaic system. Ask the Solarteur (solar installer) what happens if the device fails. How fast can the manufacturer react?
A cool location for inverters of photovoltaic systems is the basement or another adjoining room. It is best installed next to the meter cabinet or security box. The mounting location should be cool and dust-free with constant humidity. Advantage: In the basement, the devices are easy to reach, read and check.
The inverter should have good power control behavior. The larger the MPPT voltage range, the better. Also pay attention to the noise levels of the device. Installed in living rooms, the level of your inverter should be around 40 decibels.
- How expensive are inverters?
The inverter accounts for around 10% of the investment costs.
Prices for an inverter depend primarily on the power required. One currently calculates:
Per kW inverter output / approx. €200
E.g. 5 kW inverter approx. €1,000
If several inverters are required (depending on size and shading), the costs increase.
There are inverters for grid-connected PV systems and for stand-alone systems. They are divided into single and three-phase, as well as inverters with and without a transformer.
Single-phase inverters are usually used in a small PV system. If possible, transformerless inverters are installed. They are somewhat smaller and lighter than transformer devices and are more efficient.
When installing solar panels, the load on the inverter should be considered. Under heavy loads, the devices heat up, which can lead to a momentary reduction in performance. However, whether installation in a cooler place makes sense – for example in the basement instead of in the attic – depends on the individual case: the voltage level of the generator is decisive, because it determines the power loss occurring in the DC lines.
If you use thin-film modules in your solar system, have your installer confirm the suitability of the selected inverter.
- What can an inverter do?
The colloquially referred to as “inverter” is actually called a grid feed device, or NEG for short, or inverter, because it fulfills more functions in a grid-connected system than just “inverting” direct current to alternating current.
3.1. convert electricity
The conversion efficiency is important here. It indicates how much direct current comes out as alternating current. Modern inverters achieve efficiencies of around 98 percent.
The NEG / the inverter regulates current and voltage in such a way that the solar system inverter outputs the highest possible power (maximum power point or MPP tracking). To do this, the device quickly and precisely sets the operating point on the current-voltage characteristic of the module string.
3.2. Monitoring faults
The inverter monitors:
the energy yield of the PV system and shows any faults
the supply network to which the system is connected
If the public power grid fails, it switches off the solar power system in a fraction of a second for safety reasons. The NEG records and saves operating data and error messages and makes this data visible on a display.
3.3. Monitoring & data logger
Inverters enable the control and monitoring of all parameters, operating data and yields. Some of the data can be read on a display.
It becomes more convenient if the data is sent to an app, software or an online platform. You can then e.g. View your investment performance from your computer. This makes it possible to read off the operating parameters on the PC or by remote query. So-called data loggers are used for this, which are connected to the inverter via an interface.
If all modules are evenly irradiated by the sun, an inverter is used.
If the modules are attached to roof surfaces with different orientations, such as south and south-west surfaces, then separate inverters are used for the respective module surfaces.
- Inverter types & technology
4.1. micro inverter
Module inverters are located directly on the module, usually installed in the module junction box. This is a DC-DC converter that adjusts the voltage so that the connected module is operated at its maximum power point (MPP).
They are offered for solar module outputs from 100 to 1400 Wp. An isolating transformer is used for safety.
Since each module inverter works independently, there is no partial shading.
This is why module inverters are used when individual modules are shaded during the course of the day.
Suitable as an addition to existing systems or as an entry with fewer modules (balcony system)
Disadvantages: More expensive to buy
4.2. String inverter / string inverter
String inverters are the most common inverters in photovoltaics today. They are connected to several solar modules (string) connected in series via a cable.
Output: 500 to 2,000 Wp
Modules in series – The performance of the system depends on the weakest module.
All strands must be of the same length, have the same number of modules / the same angle. =>can lead to wasted space on the roof
The direct current outputs of the modules have high voltages. => Risk for installers and firefighters
No monitoring possible for individual modules – remote maintenance is therefore not possible.
4.3. Multi-string inverter
Single or three phase inverter equipped with more than one MPP tracker for multiple strings (even different) of 400 watt solar panel.
4.4. Renogy technology with power optimizer
The Renogy inverter in combination with the appropriate power optimizers offers a number of advantages. The Renogy power optimizer is a DC/DC converter that is attached to each PV module by the installer or pre-integrated, replacing the normal PV junction box.
Benefits of the power optimizers are:
Increase energy yield by finding the point of maximum power output (MPPT) for each module individually.
fixed string tension
Orientation, tilt and module type in the same string are more flexible
Monitoring of the individual modules & monitoring
thus improved, cost-effective maintenance at the module level
Since the voltage regulation is already done, the inverter only takes over the conversion of direct current into alternating current. Advantage: less complex inverter, therefore cheaper and more reliable.
4.5. Central inverter
Central inverters are large voltage converters for large roof systems. The devices are usually housed in a separate room. The advantage is the high efficiency. Central inverters only have one MPP tracker, despite their greater power. They are ideal for large systems with a homogeneous generator.
Disadvantage: In the event of a fault, the entire system section is out of order.
Power: 20 kW to 1,000 kW.
4.6. Inverter for thin-film cells
Thin-film cells will probably play a bigger role in the future than they have in the past. However, these modules place special demands on the inverters because their electrical properties differ from crystalline modules. Thin-film modules, for example, offer a wide range of voltages. Therefore, the inverters must be dimensioned for a higher voltage range.
4.7. Hybrid inverter
This is a combination of inverter and storage (internal or external). It is used when an uninterruptible power supply is required.
4.8. MPP Tracking – Maximum Power Point (MPP)
The maximum power point (MPP) of a string (or module) is the point at which the solar module produces the highest output and depends on:
Since this point in the solar system changes constantly over the course of a day, technology is required to optimize the system.
The MPP tracker in the inverter does this work! It calculates the MPP based on solar radiation, temperature and individual module properties and regulates the photovoltaic system in such a way that the highest yield is achieved.
There are now also inverters for solar systems that are partially shaded at times. The MPP trackers are then specially tailored to this (see SMA information sheet). The functions of the MPP tracker are taken over by microcontrollers – manufacturers have their own algorithms and software for optimizing the MPP tracking.
- What is important when wiring the voltage converters?
It sounds mundane, but they are not to be neglected: the cables of the photovoltaic system.
Only lay solar cables. Because direct current is used, in contrast to alternating current, high safety regulations must be observed.
Only single-core cables may be laid.
The cables must be double-insulated, extremely weather and temperature-resistant, halogen-free and UV-stable.
Keep in mind that the photovoltaic cables in your system should last at least 20 to 30, but possibly even 50 years. When making the offer from the solar system installer, make sure that the cables are identified as solar cables.
With a well-sized cable cross-section, power loss is kept low. A cable cross-section of at least 4 mm2 is recommended, but this depends on the wiring.
The longer, the higher the resistance of the solar cable – and the greater the power loss through the solar cable alone. This is not a problem for a single-family house. You should therefore not be stingy with buildings in which a longer distance has to be bridged.
5.1. Connections of the photovoltaic system
And when it comes to the connections of the solar generators, it is also important to place value on the very highest quality. Because here, buying cheap doesn’t just mean buying twice: Before that, there is extensive troubleshooting and then time and work to fix it. About the connection types: There are screw terminals, screw connections, spring terminals and plug connectors. Experts recommend the latter as particularly practical and safe.
During installation, ensure short cable distances between the solar modules, connection box and inverter. If the inverter is in the basement, the solar cables can be routed down through cable ducts or other supply channels such as closed chimneys or along the outside wall of the house.