Solar Edge / Tigo, Are They Worth It?

It would be surprising if you work in the UK solar industry if no-one had tried to sell you anything made by Solar Edge or Tigo recently. I’ve been approached numerous times by the manufacturers themselves and by their distributors.

If you haven’t seen them yet and don’t know what they are, these devices are MPP trackers that operate in individual modules. They are being touted as the next big thing in PV efficiency, and as a favourable alternative to micro inverters. They look very similar to the connection box fitted to the back of PV modules already, and actually several module manufacturers are starting to offer modules with the technology already fitted in place of the standard connection box (UpSolar and Trina as the most notable). They can however be fitted at the point of install or retrofitted afterwards (if you want to take the whole array off again and re-install it once these are stuck to the modules).

The way that they work gets a bit technical, so I won’t go into it in great detail (Although feel free to ask if you’re interested) but essentially they take each module on its own and ensure that it’s working at close to the peak efficiency that it can given its individual conditions (Irradiance / temperature etc) by changing the operating voltage of each module. Most importantly this means that the problems traditionally associated with module miss-match across an array are largely eliminated, so shading, poor module grading, temperature differences and such similar things won’t drag the efficiency of the whole array or string down which can be a serious problem, particularly on shaded arrays.

They also allow the modules to be individually monitored and to be isolated if required.

Ok, so far so good. I really can’t argue that these things seem to be a fantastic idea. The question begs however, are they worth the extra cost. Clearly these things aren’t going to be free so if you were to buy them, would you ever see the benefit or would they only payback in 30 years or so after the array itself was history?

From the advertised literature for Solar Edge for example, they say that for a heavily shaded array you would get around 25% extra yield, and for an unshaded array you would get around 2% extra yield. These are Photon figures from case studies so should be reliable. To my mind however, that doesn’t really tell you much past that they’re unlikely to be worthwhile if you already have a well positioned and designed array.

The difficulty I see here is that if you’re getting 25% extra yield from a heavily shaded array, that may still be an additional 25% onto a yield that was never going to be worth it in the first place. How do you quantify the improvement? well, one issue here is that the software tool recommended by the manufactures to do simulations using this technology is PV-Syst, which is expensive and not terribly user friendly. It’s a very powerful tool and should give you good results for these types of systems but it can take quite a long time to model anything, which will eventually add to the cost of the systems delivered.

If you take the cost of the optimisers to be approximately £50 each as installed then for a 16 module system, the additional £800 or so for the array install is adding something like 15% to the cost of the install at present. Ok, the inverters are cheaper as a result of not needing their own MPP trackers, but not by much (About £40 in this example compared to an SMA equivalent)

Let’s do a simple worked example using some back of an envelope figures.

First let’s look at the really big Central Inverters.

Firstly the unshaded case

If we take a 4kWp array costing £6,000 installed. It’s in a fairly good location with a good aspect so gives an annual yield of 950kWh/kWp this would give a payback period of about 6 years and an 20 year IRR of 18.9% (Assuming 3% RPI and 5% electricity price inflation)

If we installed the Solar Edge system at an additional cost of £800 we would get a yield increase of 2% giving 969kWh/kWp. in this case you would increase the payback to 6.6 years and the IRR would reduce to 17%. Ok so not a great deal in this case.

What about the shaded case? If we assume that the same array is fairly heavily shaded and so would give an annual yield of 600kWh/kWp then we would get a payback without the optimisers of 9.4 years and an IRR of 11.5% With the optimisers (Assuming the 25% increase in yield) the system would yield 750kWh/kWp giving a payback of 8.6 years and an IRR of 12.9%. If these figures were correct then they would be worth installing for this system but it would be very site and system specific.

It essentially seems to me that for a traditionally designed and installed systems in a decent location, it’s never going to be worth the extra expense. For sub-optimal systems with shading issues it may well be worth it, but if you don’t have PV-Syst and the time or expertise to construct detailed simulations then it will be difficult to justify the expense and the customer may end up with a PV system that never performs as well as they thought it would.

This example is clearly a bit rough and costs will come down, especially when they’re integrated at point of manufacture. The problem will still remain that it will be difficult to quantify the benefits unless better software tools are developed. Eventually however I’m sure that PV*Sol will include support for these devices, or that the manufactures will develop their own.

Ok, now on to the interesting part, at least to my mind. What devices of this type open up are a large range of non-traditional system design options that would have been at best impractical before but that should be relatively simple now. The main things are:-

• – Different module orientations and pitches on one inverter
• – Different module sizes and types on one inverter

One of the largest expense for any PV installation is mobilisation. The process of selling, designing, and sourcing the array and then of getting the equipment and installers safely up onto the roof to fix it all there. There are clearly economies of scale then to be had in maximising roof coverage once you’re on the roof.

Installations like those shown above have always been possible. You would just have to use individual inverters for each aspect that the modules were in. This can make the project very much more expensive both in materials terms and the labour required to install them. With optimisers installed you can just use one (Or a few) large inverters making the whole thing much simpler. It’s even possible to install different sizes of module onto one inverter so you could maximise roof space where it is limited by filling in all the corners with smaller modules. That wouldn’t have been possible at all before except by using micro inverters.

This use for the technology does make sense to me because it opens up buildings for PV installation that wouldn’t have been economic before.

However. We still run into the problem of how exactly you quantify that without software tools that can cope with the fine resolution offered by this technology. You could just use PV-GIS-CMSAF modelling, or the new MCS guidelines which don’t take different technologies into account at all, but they’re a bit rough, and the new MCS shading analysis doesn’t work for systems like this as it compensates for the large miss-match losses caused by shading which should be eliminated here.

Ok, I think I’ve waffled on for long enough now. Time for a conclusion.

In my opinion, these are a brilliant innovations who’s time hasn’t quite come yet. The cost is improving but still too high, and I think most importantly, the industry standard simulation tools have yet to catch up with this technology so the benefits are very difficult to quantify. Once these things catch up. I think this will become ubiquitous in the PV industry. I certainly see them as a far better alternative to Micro Inverters.

If you have any questions I’d love to hear from you, please leave a comment or send an email.

Images courtesy of Tigoenergy.com