The inverter is at the heart of every solar pv array and arguably more important that the actual solar panel. There are basically three types of inverter available for the domestic market.
As you have probably learned from previous research the solar panels that will make up a solar electric solution for your home or business convert sunlight into DC, or Direct Current, electricity. But your household equipment uses AC, or Alternating Current. So a method of converting DC from to AC is required and this is called an inverter.
There are 3 basic types of inverter:
They tie into the end of a ‘string’ or row of photo-voltaic panels, typically up to 8 or 10 in a row, and convert the total generated DC electricity into AC. These are also refered to as a ‘central’ inverter. There may be one or several such strings per system in order to keep the DC voltage produced to a manageable level, normally below 600 to 800 volts, so that the inverter does not “over-volt”. If there is more than one string they are usually combined in ‘parallel’ and the output passed through the inverter to feed AC current direct to your main supply box. This solar-generated supply is then used by you or fed back into the grid and credited to you as part of the Feed in Tarrif. Central or String inverters are the normal answer for unshaded roofs and combine cost effective performance with the convenience of one central point of connection and ease of maintenance.
These are smaller (hence ‘micro’) inverters attached to each individual solar panel, and convert the generated DC current right there at the panel into AC. Each panel’s individual AC supply is then combined and fed to your home or business the same as that from a string inverter system. The downside is that you are introducing AC electricity directly onto your roof whereas either pole of a DC cable will not in itself produce an electric shock. However the live side of an AC connection certainly will wake you up. In addition larger systems can have voltage drops across the longer distances that are more significant with AC than with DC. This means that a micro-inverter system may lose more power transmitting the converted AC electricity than a DC transmission which is then converted at the end by a string inverter.
Optimisers made by Solar Edge are arguably the most efficient solution for roofs with partial shading. An optimiser is fitted to each panel which when connected produces a check voltage of one volt. Once optimised from the central inverter they independently produce the maximum voltage available from each panel to the inverter. The advantage of this system is that each panel will produce the maximum power available individually of the others, which may be suffering shading or uneven degradation from age. To explain further we must look at the MPPT – this is a slightly technical area. For a solar panel to work at its most efficient, one of the considerations is for the voltage to be adjusted based on the amount of light hitting the panel. And voltage is handled by the inverter. This means that the performance of the solar panel is reliant on accurate Maximum Power Point Tracking, or MPPT, by the inverter. So it follows that MPPT performed at each panel by a optimiser will yield better overall system results than MPPT done by one string inverter for the whole system.
So much for the basics, how do you decide which to use?
A micro-inverter failure will stop the production of electricity from the panel it is fitted to, whereas a string inverter failure will stop the entire string, resulting in a more significant or complete loss of electrical supply. There is an alternative related argument here, and that is that micro-inverters offer many more potential ‘points of failure’ in a system. That said the Solar Edge optimisers carry a 25 year warranty.
Panel orientation may not be the same for all panels in your system to give best performance, but with a string inverter you will typically limited to two opposing roofs. For example East and West when central inverter with twin tracking capability (MPPT) can be used. With an optimiser or micro-inverter system you can orient the panels individually on any number of roofs for optimal output.
If you have tall buildings or trees near your roof, shading will become an issue. With a string inverter if one or two panels get shaded it can affect the entire string by as much as half of its total performance. With optimisers or micro-inverter panels it is the only the shaded ones that are affected while the rest of the system keeps performing. However, if the shading is only slight, say around a chimney, it may be possible to alleviate this to a large degree by splitting the array into two strings and use a twin track central inverter. This will have the effect of allowing the second, unshaded string to operate as normal until the first, partially shaded string returns to normal operation.
As always there is a cost to be considered and both Micro Inverters and Solar Edge optimisers are more expensive than a central inverter due to the number of components used. In order to try and decide on the cost effectiveness of each system we ran a computer simulation test using an industry standard program (PV Sol) on an Aurora 3.6 kW twin MPPT central inverter, Enecsys Micro inverters and Solar Edge Optimisers.
Using sixteen Phono 250 watt panels on a South facing roof based in Uttoxeter we obtained the following annualised output.
The Aurora Power One central inverter produced 3,457 kWh per annum.
The Enecsys Micro Inverters produced 3,487 kWh per annum. An increase of 29 kWh per annum giving a slight gain of £4.97 FIT.
The Solar Edge Optimisers produced 3,602 kWh per annum. An increase of 145 kWh a gain of £24.27 FIT
If you take a 20 year view the Solar Edge Optimisers will produce £485.40 in extra FIT, whilst the Enecys Micro Inverters would only produce £99.40 in extra FIT over the period.
In conclusion then it would appear that in normal unshaded conditions a central inverter is the most economical method of producing AC current and in shaded conditions the Solar Edge Optimisation system is the preferred route.