Oversizing your solar array

Oversizing your solar array

Oversizing your solar array in comparison to the size of your solar inverter can be a great way to generate the most kWh from your solar power system at the best price. It sounds counterintuitive when you first hear it; installing a 6.5kW solar array on a 5kW inverter seems like an inefficient way to design a solar system, but rather than using the term oversizing, I’d suggest it’s more like optimising or supersizing your system. There are boundaries and limits to how far you can go, but in many cases I think it can be a good idea.

Firstly I must mention that this isn’t appropriate with all solar inverters, so before you do anything, check with the inverter manufacturer (or your local solar installer) to make sure their inverters are designed to cope with oversizing your solar array. Green Energy Options has experience in oversizing solar arrays and can advise of which inverters are most appropriate.

Why Oversize?

The price of Photovoltaic (PV) solar modules has dropped dramatically over the last few years, so it is now much cheaper to add additional solar array capacity. The idea is that rather than putting money into additional inverter capacity put your dollars into extra panels and you’ll see a better energy harvest and better financial result.

To explain why, I’ll start with the nameplate rating of a solar PV panel; a 260 Watt panel for example is a power output that’s achieved in a lab under Standard Test Conditions (STC) that is at 25 degrees Celsius and 1000W/m2 of irradiance. In reality the panel will rarely put out 260W of power because of system losses. For much of the time the panel’s operating temperature will be more like 40-65 degrees and heat inhibits the panel’s output a little, cloud cover can also mean the system won’t be performing at i’s peak or the angle of the sun in relation to the panel can contribute to a reduced output. We also need to bear in mind that over time the silicon in the module will degrade and dirt and dust build up can reduce output too.  Under these circumstances where a solar panel or a solar array isn’t performing at its peak output very often, the top end of the inverter’s output range is rarely used.

Oversizing Enphase micro-inverters

Enphase Energy recommends oversizing panels in relation to their micro-inverters, they recommend using panels up to 270 Watts on their 215 Watt micro-inverter. Below they show in a graph that the losses from ‘inverter clipping’ are mostly below 0.5% (when the panel output is above the peak inverter capacity). This is by far outweighed by the additional production that you get from having a larger panel, up to 12% increased output. In essence the same theory goes for larger string inverters too; Fronius state that their Galvo range of inverters can be oversized by 100% of the inverters nameplate capacity, as long as other conditions are met and their warranty is still upheld.

Enphase Oversizing Graph

Enphase_White_Paper_Module_Rightsizing

Clean Energy Council Guidelines

One thing that needs to be kept in mind is the Clean Energy Council’s (CEC) design and installation guidelines, where they state that ‘In order to facilitate the efficient design of PV systems the inverter nominal AC power output cannot be less than 75% of the array peak power and it shall not be outside the inverter manufacturer’s maximum allowable array size specifications.’ In other words the array size cannot be more than 133% of the nominal inverter power rating.

Powercor is the electricity distributor for Western Victoria, this includes Geelong, the Surfcoast, Bellarine and some western suburbs of Melbourne. Powercor have a 5kW inverter limit for single phase connections. In a scenario, where you are a high energy consumer and would like to install a larger solar array to maximise the benefit of your solar system, we can theoretically oversize your solar array to 133% or 6.65kW and still be within the CEC guidelines.

Analysing Costs

An analysis done by our Friends at AC Solar Warehouse show that a high quality 260W solar module paired with an Enphase M215 micro-inverter (peak output 225 Watts), would see clipping of 0.01125kWh on the best day of the year. They went on to say that there was approximately 12 weeks of the year that saw some clipping and that this worked out to be about 0.987kWh of lost production. Offsetting at $0.30c/kWh it works out to be a loss of 29.6 cents per module. This amount is pretty small, particularly when you way up the alternative cost of a larger capacity inverter, there’s no benefit in selecting micro-inverters that increase the cost of the system by $30 per module, in order to harvest an extra 29.6 cents per year. That’s a 100 year payback.

Oversizing and inverter life

Just one final note, oversizing your solar array means that the inverter will be running at its peak output for a larger portion of its life. This can cause components to heat up and run hotter and in some cases have a shorter life expectancy. The good news is that many inverters have systems in place to manage excessive heat build-up and protect heat sensitive components; most will even shut themselves down in extreme cases. Good inverter placement allowing appropriate cooling and ventilation can help to optimise inverter life expectancy, talk to your solar installer about the best place to position your inverter.

For any questions about optimising, supersizing or oversizing your solar array please feel free to give us a call on 1300 931 929.

Aaron Lewtas

Green Energy Options