In a perfect world, UK roofs would all face south, with a 35° pitch and no shading in sight. But we don’t live in a perfect world. As solar installers, we have to find clever ways to maximise the potential power from roofs of all make ups, orientations and shaded spots.
The most common suggestion to do this is with micro-inverters or power optimisers, which are designed to improve performance at the panel level. But there are risks, such as high costs to access and repair them if they fail, and sometimes you’re better off with a regular string inverter.
So which is the best solution? Let’s focus on the most popular brands, and compare SolarEdge optimisers vs Enphase micro-inverters.
Before diving into optimisation, it is worth being aware of a ‘traditional’ solar PV system and how a string inverter can be effective in mitigating shade and generating a high output. The combination of bypass diodes, half cut cells, and advances in global power point tracking (GPPT) means that in certain shade and designs, a string inverter (such as brands like Fronius, Growatt or Solis) can exhibit a very similar percentage loss due to shading compared to an optimised array.
In a traditional configuration, the solar PV modules are wired in strings (series connections of panels) and the strings are wired to one central inverter via maximum power point trackers (MPPT). The MPPTs will continually adjust the voltage and current from the panels to achieve the maximum output of the strings whilst the central inverter will handle the inversion from DC to AC.
This configuration is nearly always our recommendation when the panels are installed on one roof face and there is little to no shading. However, where shading is more extreme or there are complications with multiple orientations, the question becomes: is SolarEdge or Enphase a better choice?
Panel optimisers, such as those manufactured by SolarEdge, and micro-inverters manufactured by Enphase have a few similarities that aren’t shared with standard string inverters:
The main difference between SolarEdge and Enphase systems is the function of their specific module level power electronics:
Micro-inverter system:
Power optimiser system:
Enphase and SolarEdge’s differing module level power electronics tend to lead to a very similar level of performance; however the way that they operate requires different design considerations.
Enphase:
SolarEdge:
These design considerations ultimately make Enphase a more flexible solution as they can be successfully utilised in any scenario, whereas SolarEdge systems have to be more thoughtfully deployed. However, one of the main advantages of a SolarEdge system is that its oversizing can be very useful if wanting to avoid prior grid permission or if receiving an underwhelming grid offer. The only downside to oversizing is that in spring and summer it is highly likely that the system will suffer from clipping, as the maximum output is governed by the inverter rating as opposed to the panel rating.
Image: Enphase.
Due to the simplicity and cost effectiveness of a standard string inverter, this tends to be our recommendation in most situations, except when shading or multiple orientations become too complicated for a string inverter to manage.
In a scenario where a string inverter isn’t applicable, our recommendation would be to utilise Enphase micro-inverters largely due to their flexibility and apparent greater reliability. When installing module level power electronics at roof level, a key parameter is reliability, as if a failure arises this can cost in the region of £1,000 (allowing for scaffolding and labour) to replace.
Spirit has installed SolarEdge power optimisers on over 450 sites since 2011 and in this time period, roughly 10% of sites have experienced an optimiser issue, with just over 60% of these sites experiencing multiple optimiser faults (though these don’t all require replacements). Unfortunately, we are not in a position to quantify the Enphase reliability but anecdotal evidence seems to suggest that they have a lower failure rate. We would propose SolarEdge, however, if the site would benefit from significantly oversizing its panel rating to the inverter. The most likely case when this would be applicable is if the DNO response significantly limits the system size below the desired.
If you would like any advice about what inverter is most suitable for your installation, please get in touch with our team on 0118 951 4490. You may also like to download our contractor's guide to PV: