If you are considering buying a PV system or an installer looking to understand the options available for different PV projects, we hope this guide will help with that process.
Solar panel inverters could be referred to the brains of the Solar PV system. Essentially, the inverter converts the Direct Current (DC) electric from the PV panels to mains power and Alternating Current (AC), which is suitable for running household appliances such as fridges and TV’s etc. Solar Panel inverters offer a number of other functions to the user such as, allowing you to monitor the performance of your PV system.
For more information on Solar Panel Inverters click here
Grid tied inverters are the most common type and are designed to work when connected to the mains power supply only.
These inverters are certificated to G98 for inverters less than 4kW and G99 for those over.
Off-grid inverters are designed to work without grid connection and tend to be more expensive than grid tie inverter solutions and would normally work in conjunction with other pieces of equipment to store the power. These inverters would tend to have different voltage characteristics and therefore should not be considered if a mains supply is part of the installation.
Grid Tied Inverter Options:
String inverter where specified, can be supplied for either single phase or three phase grid connection depending on your incoming supply. Most domestic installations will be single phase and most commercial installations, three phase.
You can run single phase inverters on each phase of a 3 phase supply but you cannot run a 3 phase inverter on a single phase supply.
String inverters are designed to be more efficient as they can be connected to several panels linked in series, which raises the incoming DC voltage. When the panels are arranged like this they are known as a ‘string’.” Each string of panels is connected to the inverter, which transforms the DC electricity produced by the panels into appliance friendly AC electricity. The inverter is sized to the total power of the panels installed but consideration has to be taken in the design of the system, so that the PV panel string voltage and current is suitable for the inverter
When considering a grid tied inverter you will see that the manufacturer label will say if it has 1, 2 or more MPPT’s (Multi Point Power Tracking). These MPPT’s are the DC connection point from the PV panels and is the part of the control system that maintains the power to the grid when the DC power from the panels fluctuates depending on the weather and orientation.
Standard string inverter technology has been used for over 20 years. It is a tried-and-tested technology, but has its drawbacks in certain circumstances. A string of solar panels will only produce as much electricity as its least productive panel. If one or more of your solar panels is shaded during any part of the day, the power output from that entire string would be reduced to its level. For this reason, if your solar panels are installed facing different directions and its difficult to run them on two MPPT’s, then optimising the panels is a method of improving their performance. Some manufacturers (Solar Edge for instance) produce a different type of inverter which has no MPPT but allows installation in a single string with electronic high frequency control and optimisation which allows panels to be switched in and out of the string on an individual basis to maximise output.
Multiple strings can be run from one inverter, for example a 3.6kw inverter with two MPPT tracker allows each set of panels to be stringed (say 14 panels) as 2 string of 7 separately. This works well for East, West orientation systems. For a straight South facing system, a single mppt tracker would be suitable.
However, with the advent of higher wattage panels, it is possible that it is necessary to connected two string in parallel into a common MPPT input to meet the inverter operating criteria.
String Inverters are typically the cheapest option but may not necessarily offer the best ROI as it depends on installation factors such as shading and PV panel positions.
In the UK, brands such as Solis, Growatt, Fronius and SMA Sunny boy are leading brands and have a well proven product. Quality Chinese manufacturers have made a name for themselves by making robust and efficient inverters with very low failure rates.
Terms you may come across:
To learn more about Solis Inverters click here
String Inverters with optimisers work in the same way as a sting inverter but with the benefit of optimisers attached to each panel. The benefit of this optimiser is that it regulates each module ensuring the maximum power is harvested from each module. This helps negate the effects of shading on the PV array maximising overall array output. Typically optimisers can produce anywhere between 5% - 20% more power depending on the amount of shading at the site.
Customers also consider installing solar panel optimisers as they offer greater safety in the unlikely event of a fire. The is due to a switch which can be activated to isolate the module at module level, preventing the Direct Current (DC) entering the building.
Optimisers also offer you the ability to monitor your PV array at module level. The advantage of this is you can see how each module is performing in real time and are useful for monitoring any performance issues before the loss of any significant generation.
Inverters with Optimisers are typically more expensive than string inverters, however often they can improve the ROI of the system due to the increase in performance.
The two major brands on the market are SolarEdge inverters, who alongside their inverters, offer a number of smart home solutions and EV charging points. The other brand is TIGO whose optimisers are designed to work with any string inverter.
Typically optimisers come with a 20 year warranty, where as the string inverter will come with a warranty of 5 - 10 years.
To learn more about SolarEdge click here
Microinverters are essentially mini inverters designed to convert the DC to AC at module level. Typically for every solar panel, you have a micro inverter. However there are dual micro inverters, meaning 2 panels for every 1 microinverter.
Microinverters offer module level monitoring, enhances safety as the inverter is outside the building and the ability to run small single panel systems.
Please bear in mind due to G98 certification you will need to have a gateway installed with your microinverter no matter the size of the array.
Microinverter system are the most expensive option but typically come with the highest warranty of 20 years.
Click here to learn more about Enphase Microinverters
Hybrid Inverters are designed to work with batteries. They consist of an inverter converting the DC to AC and a battery manager. The battery manager will dictate which batteries the hybrid can be installed with.
Currently there are single phase hybrid inverters ranging between 3kW to 5kw and three phase hybrid inverter ranging between 5kW to 10kW
One option to consider when installing an inverter, is to have a hybrid which can have a battery installed later if the additional cost of the battery is an issue at the time of installation.
If you are looking to retrofit a battery you can consider an AC coupled or DC coupled unit, which can be added to any inverter and is simply a battery manager with an internal inverter in the case of the AC system. Or the power is returned back through the PV inverter where a DC system is installed.
In the UK well known Hybrid brands are Solis, SolaX, SolarEdge, Victron and SMA.
Click Here for more information on Solax Hybrid Inverters
Pure Sine Wave Inverter:
A pure sine wave inverter converts DC to AC. A sine wave is a continuous wave that describes a smooth repetitive oscillation. Conventional AC power is produced by rotating machines and is mathematically described as a sine wave. It is the ideal waveform for the transfer of AC power and with low harmonic distortion. Pure sine wave inverters produce power which equals or better than the home. It uses sophisticated technology to protect even the most sensitive electronics. Pure sine wave inverters can power just about any electronic without risk.
Typically, pure sine wave inverters come with an in-built AC plug socket. For a Sine Wave inverter to work effectively, you need to match the inverter to the loads you are pulling, including your peak load.
Pure sine wave inverters are available between 300W to 5000W and are the ideal solution for running PV systems in remote locations.
They are most associated with being run from a DC supply (usually battery) 12V, 24V or sometimes 48V for conversion to 230 Volts AC.
Typically string inverters come with a minimum of 5 years and most manufacturers offer warranty extensions to 10 or 15 years for a fee. The cost will vary from manufacturer to manufacturer.
String Inverters with Optimisers.
Typically, there are two elements of the warranty of the string inverter, which come as standard with a 5 to 10 year warranty. The optimisers usually come with a 20 year warranty.
Microinverters usually offer a 20 – 25 year warranty.
Ideally a string inverter will be installed in a plant room, garage or utility room, where there is good ventilation and easy access. In some circumstances, the loft maybe used if there is not a suitable alternative.
Microinverters and optimisers are usually installed underneath the panel on the roof. However, sometimes they may be installed in the loft for ease of access for maintenance purposes.
It’s a good question, and the answer is there is no right or wrong solution. If you have a south facing roof with no shading issues, a string inverter would work just fine.
If there are any shading issues and you would like more in depth monitoring, then the Inverter with Optimisers would be more suitable. However, this would require a greater initial capital investment.
All inverters in installed in the UK need to be either G98, G99 or 100 certified.
It is also worth checking that the inverter has all relevant CE certification.
In certain situations the Distribution Network Operator (DNO) will allow you to install a PV system with a restricted export to the grid. To be able to do export limiting, the inverter you install must be G100 certified. Typically, the inverter will only need a couple of extra components to export limit, such as a CT clamp and a modbus meter. If, say you have a 6kWh system and only allowed 3.68kWh to be generated onto the grid, the inverter will sense this and will modulate the power to restrict the generation onto the grid to the maximum allowed. This maximum output is adjustable when the inverter is installed.
There are a number of Solar Inverter brands in the UK that have established themselves as reliable solutions offering great value for money.
To learn more about solar inverters or if you have any further questions please do not hesitate to contact NuVision Energy Ltd suppliers to the Trade on 01404 891693 or email firstname.lastname@example.org
If you are looking for an installation please feel free to contact us and we can recommend an installer
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