String inverters have quickly become the inverter of choice for many US developers and engineering, procurement, and construction firms for projects up to several megawatts in size. This popularity is due to several key advantages string inverters have over central inverters for smaller systems. However, some important implications for Operations & Maintenance (O&M) costs are not being taken into account. While in most cases, the additional O&M costs associated with string inverters are not such that they would change the owner’s decision to use them—an unexpectedly high O&M bill could be an unwelcome surprise when the project’s financial performance is analyzed over its operational lifetime.
String inverter advantages
Before discussing the O&M implications of string inverters, let’s review the advantages of string inverters that have led to their rapid adoption:
- String inverters are replaced instead of repaired, so less-skilled workers can do the replacement and there is much less likelihood of a prolonged outage.
- When a string inverter fails, the capacity that goes offline is far less than in the case of a central inverter failure. This reduces the urgency of getting the string inverter back online.
- String inverters typically provide more detailed monitoring capabilities. This could potentially reduce the electrical scope of the preventive maintenance and represent some savings.
Most of these advantages are accurate portrayals of string inverters in operation. However, there are a few caveats that should be mentioned. Though the string inverter replacement itself may not require labor skilled in troubleshooting inverters, a licensed electrician is still required to safely isolate the inverter—reducing the likelihood of saving costs in labor. Though a string inverter going offline should be less urgent and require a slower response time than a central inverter, not all customers have adapted to this reality and so O&M providers are pressured to respond quickly to many more outages because of the additional points of failure.
String inverters – The whole O&M story
Various aspects of string inverters actually make the total O&M budget higher and not lower than equivalent service for central inverters.
A few of these are likely temporary:
- As a relatively newer technology, our experience has been that string inverters have higher failure rates than central inverters. We think over time this issue will go away with future generations of string inverters.
- Some customers have not yet adapted to the fact that since string inverter failures will happen more often (because of more points of failure) but affect less production. In order to keep pricing and service levels equivalent to what they’ve received with Central Inverter services, solar owners will need to adapt to the fact that a string inverter warrants a lower priority than a central inverter because of the lower impact on production.
Aspects that make string inverter O&M services more expensive and are less likely to go away in the near future:
- More points of failure means more visits to the project site.
- Unlike with a central inverter warranty, string inverter warranties only provide the project owner with the replacement string inverter. Their O&M provider must provide the labor to do the replacement.
- String inverters typically weigh in the ballpark of 150 pounds each and they must be replaced, not repaired. This means that some combination of multiple people and mechanical lift assistance is needed every time a string inverter needs to be replaced, which is more often since there are more of them. Each inverter failure now costs a multiple of what it would be for a central inverter failure.
- Many of the small projects where string inverters are typically considered ideal are either roof mount or carport projects. In most cases, installers are locating the string inverters on the roof or elevated in the carport structure. Replacing these elevated string inverters requires anything from a scissor lift or fork truck for easy applications to a crane for more extreme roof heights. This adds additional cost to every string inverter replacement.
O&M providers for distributed photovoltaic assets are grappling with how to safely and cost-effectively replace string inverters given the challenges highlighted. Our own analysis and experience suggest that the costs associated with replacing string inverters greatly outweigh the benefits that string inverters can provide in reducing the scope of preventive maintenance required.
We’re not proposing that string inverters shouldn’t be used, but instead that their disadvantages from an O&M perspective shouldn’t be swept under the rug. We’ve frequently encountered project owners who don’t take into account the additional O&M cost associated with string inverters and installers who don’t consider the O&M implications of where they are siting string inverters in their installations. Therefore, when calculating the cost/benefit of string inverters or preparing a proforma for a project, the higher O&M costs associated with string inverters should be incorporated and weighed against their advantages.