Solar Micro-Inverters: Efficiency vs Sizing

It is with great interest we at the QSA are starting to see the questions of Solar Micro-Inverters vs String Inverters coming to the fore more and more. So here’s our take on the question of Solar Micro-Inverters vs String Inverters.

The current comments being bandied about by sales staff include the following:

1. Micro-Inverters are more efficient, (True Micro’s are around 99% and Strings are around 95-97%)
2. Micro-inverters have longer warranties (True Micro’s with 25-year standard warranty are available)
3. Micro Inverters produce A/C power so use cheaper cable (True, however, where exposed to sunlight it must still be mounted in conduit)
4. Micro Inverters will allow for easy expansion of the system at a later date (True in regards to work involved but FALSE in practicality)

Comments on Items 1-4.

1. We like solar Micro-inverters they are more efficient, however, they are also a lot more expensive, so we only recommend them when there are shading issues involved or there is insufficient space to put up the minimum number of panels to operate a string inverter.
2. No arguments on the warranties, however when you compare what you’ll pay for the equivalent warranty period in adding to a string inverter the string inverters invariably come out at a lower cost and with most people moving house every 5-8 years why pay for someone else’s warranty if you don’t have to.
3. Cheaper cable is not a major factor in the fitting costs of a system, using quality components is the only consideration here, just ask those people fitted out with the non-compliant DC Isolators that suffered house fires as a result and ask them if they would have preferred to have got the slightly more expensive system from a reputable company rather than the cheap guy down the road or the sub-contractor working for the TV advertised companies who saved on his installation costs.
4. Expansion –
   •  In QLD expansion is limited by size of the inverter, so the current maximum in Queensland Residential Applications without special consideration/permission for micro-inverters would be 4.95 kW cumulative output.
   •  Where the panels each have their own inverter this effectively limits you to only 4.95kW of panels too.
   •  A 5 kW string inverter will likely have between 4.6 – 4.95kW output, however this can accept an increased capacity of up to 6kW in panels (CEC recommends up to 20% overloading).
   • This overloaded 6 kW panel system into a 5 kW string inverter will produce more power than the equivalent 5 kW solar micro-inverter system at a lower overall cost.
   •  In QLD Energex is set to limit systems to 3kW after Sept 30th and inverters with over 3 kW total output will have to be fitted with reactive technology.
   •  It is possible that that micro’s will go up in price because of this reactive technology requirement.
   •  Reactive compliant string inverters are already available at the same price.
   •  If a 5kW string inverter is already fitted this will not be a problem as new regulations are not retrospective and more panels can be added up to the CEC recommended max of 20%.

In light of the above information we can only conclude that the current spate of sales staff pushing micro inverters are doing so for the following reasons:

• Higher commissions are being paid based on the higher overall cost of the solar micro-inverter system
• The higher efficiency rating of the units is being used as a tool on an uninformed customer base that is still coming to terms with the new technology in order to get a sale based on a false point of difference
• The sales person or company is either unaware of the upcoming change in regulations or ignoring them in order to get a sale.

Our advice:

Only opt for Solar Micro-inverters when shading is a major problem, or when insufficient roof space exists to fit enough panels to power a string inverter.

If looking to maximise your power we suggest using a 6kW panel system into a 5kW string inverter rather than a solar micro-inverter system that will cost more but still produce less.

If looking to fit up a smaller system and expand later on, fit a 5kW inverter and 3kW of 

panels now and add more panels later.

http://www.qldsolaradvisor.com.au/solar-micro-inverters-efficiency-vs-sizing/