Solar MOTs: are they a waste of time?

Where do you sit; are service and maintenance visits to a solar PV system a necessary and worthwhile exercise, or just a waste of money? This was the question posed to me today by a customer. My answer was: “Well that depends on the system, the installer and the way the system is behaving.”  

In my experience, service visits typically get arranged for five main reasons:

  • An obvious fault has developed
  • The customer is concerned about erratic or poor system behaviour
  • A scheduled arrangement
  • A change in system ownership
  • For no good reason

Faults do occasionally develop in even the best installed and specified PV systems. The most obvious fault is usually an inverter failure – if a system has only one inverter this can be seen by a complete drop in system output. Most customers spot such problems pretty quickly, however, I have known even complete failures to go unnoticed for fairly long periods of time. This has typically been due to a lack of system monitoring/metering, or due to overly complicated or poorly sited displays.

Poor system performance is sometimes difficult to spot; in my experience most customers pretty quickly get a feel for what is normal and can rapidly spot erratic behaviour. However, this gets more difficult on larger systems with multiple strings – where the loss of just one string can easily be missed. String level monitoring can detect such faults, but this isn’t installed on all systems. 

Here at Sundog we get fairly regular calls for service visits due to concerns over poor or zero output. While some are for our systems, most are for systems installed by others – often where the installation company is now no longer in business. Either way our approach is much the same: discuss the problem, try to winnow out any obvious problems over the phone; and if this fails, arrange a site visit. 

While a service visit to a misbehaving system is an obvious and sensible decision, setting up a routine maintenance plan is a more difficult choice. Of course, for some sites it is easy: large systems or systems on public buildings definitely warrant a maintenance arrangement (to sit along-side the other maintenance plans that these sites will operate). In my experience there is another category of site that definitely warrants routine maintenance: systems with multiple strings that don’t have string-level monitoring. As mentioned above, the loss of one string can easily be missed in these systems, moreover the failure of string fuses is a fairly regular occurrence.

Where we have a maintenance agreement this typically includes as a minimum: running a full set of string tests (Voc, Isc, insulation resistance); a set of combiner box/sub array tests; routine inverter maintenance (clean filters, scan error logs etc); a full suite of AC circuit tests and a visual inspection. On top of this we may also perform: IR (infra-red) scans of modules, junction boxes and switchgear; IV curve traces; and array cleaning.  The test set that is performed and the test interval are tailored for each site – with the customer choosing what is appropriate for the site and system.

Where a change in system ownership occurs, we will usually be asked to run much the same set of tests that we would perform for a routine maintenance visit. We also sometimes get asked to run these tests where a customer is concerned about the quality of their original installer…but that’s another blog.

So that leaves the final reason maintenance visits occur: for no good reason whatsoever. 

Sadly, I am starting to hear about service visits that I think are sold in this category – or should that be mis-sold? The feed-in tariff income from a typical domestic system does not warrant an expensive annual service. For a well specified and installed system, faults should be rare and should be easy to spot if they do. Yes a service every now and then (3-5 years?) is sensible and, of course, if there is the suspicion of a fault. But otherwise, let’s focus on giving our customers confidence in the quality, longevity and reliability of solar PV.