Since the feed-in tariff (FiT) began, domestic PV installers looking to maximise returns for their customers have normally installed to a 4kWp ceiling. The Distribution Network Operator (DNO) process is simple and retrospective and the rate difference in the next FiT band has meant that returns were more favourable. Another issue that caused confusion in the industry was whether or not this 4kWp FiT ceiling should be defined by the modules on the roof or by the capacity of the inverter which could limit the output. This anomaly was recently clarified when OFGEM published their guidance, explicitly stating that the FiT limit is dictated by the array and not the inverter, the OFGEM statement can be seen here: How Solar PV Total Installed Capacity (TIC) is calculated in the Feed-In Tariff Scheme. However, this potentially creates a problem for the industry as 250Wp modules are becoming increasingly more rare due to advances in technology and inevitable improvements in solar panel efficiency. 260Wp modules and beyond are now becoming the norm and as the size of a 4kWp system is easily calculated as 16 x 250Wp modules, it is not so easily calculated in 260Wp modules. Achieve faster payback with 4+kWp systems Making the system calculation is not such a big problem as the difference between FiT bands has now been reduced so much that the returns from a system larger than 4kWp are now very similar to a 4kWp system. Take a look at the following options on a south-facing, 30 degree pitched roof in Birmingham, with a kk value of 929kWh/kWp calculated according to the MCS guide*. Option 1: 16 x LG250 modules could be installed, providing 4kWp for £7,000. Total benefit (FiT + Electricity Savings) in first year is £844.28. Assuming no electricity inflation, this gives a payback of 8.29 years. Option 2: 16 x LG260 modules, providing a 4.16kWp system, which is just over the FiT band. The system price increases slightly due to the higher cost of the modules, so is £7,172.48 in total. Total benefit in the first year is slightly less because of the lower FiT rate at £823.94 a year, but payback is still good at 8.7 years. Option 3: 18 x LG260 modules, resulting in a 4.68kWp system using the same inverter. There are some small additional costs for extra rail and clamps, plus the extra time required to install two more modules, but as the main system cost is the modules, the system price would be about £500 more than the 16 x LG250 system, so £7,500 in total. The total benefit in the first year is then £926.93, resulting in a payback of 8.09 years The 18 x LG260 system therefore gives higher yields and a faster payback than 16 x LG250 modules. *We have assumed 50% deemed self-consumption, 14.9p/kWh generation tariff under 4kWp, 13.5p/kWh generation tariff over 4kWp, 4.64p export tariff and electricity prices of 11p/kWh. NB: The owner of a PV array larger than 4kWp does not receive the higher FiT rate up to 4kWp and then the lower rate FiT for anything over 4kWp. The FiT rate received is based upon the output of the whole array. Sticking to G83 and the benefits of under-sizing the inverter The other principle reason that PV installers stick to 4kWp systems comes down to the inverter, as larger inverters mean a G59/2 application must be made instead of a G83. However, this does not need to be an obstacle. We can happily install 18 x 260Wp modules with a 16 Amp inverter and need only make a G83 application. The inverter is under-sized in relation to the PV array, but this is actually a good thing. Below are some simulations from PVSyst, again based on an installation in Birmingham under the same conditions as listed above: Option 1: 16 x 250Wp modules on a Fronius TL 3.6 is simulated to generate 3,510 kWh/yr**, which is 878 kWh/kWp installed Option 2: 18 x 260Wp modules on a Fronius Tl 3.6 is simulated to generate 4,141 kWh/yr**, which is 885 kWh/kWp installed **PVsyst calculations are typically less than the MCS generation values, because it uses more conservative Meteonorm data. The kilowatt hours per kilowatt peak has actually increased for the oversized array. This is because even though the system is trimming electricity during ideal sunshine conditions (which rarely occur in the UK), the rest of the time the array is generating less than the maximum. Inverters operate at higher efficiency when under-sized – meaning that for most of the time a 4kW inverter with a 4kWp array will not be running as efficiently as a 4kW inverter with a 4.68kWp array. The inverter is running at maximum capacity in the second system and therefore at maximum efficiency for more of the time. So, not only can we happily install systems over 4kWp on a 16 Amp inverter and therefore still only need to make a G83 application, but the benefits of under-sizing means that the system will perform better! In conclusion The industry has become used to designing and installing systems that are 4kWp or just below, but now that the FiT rates have changed we can now update our approach and actually install beyond this limit to have larger and better-performing systems. As under-sizing inverters is good practice in general, we can still install these systems under G83 and get better performing systems. When installing exactly 16 modules stick with 250Wp modules. But for systems from 1 to 15, 17 and 18 modules in size then if space allows, we recommend using 260Wp module
Posted on: Thu, 03 Oct 2013 22:27:56 +0000
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