Oversizing solar arrays – or designing a solar system so that the PV array has a higher peak capacity than the inverter – is an increasingly popular way of increasing system yield. Nigel Morris, CEO of Roofjuice, explains the ins and outs of oversizing – and undersizing – a PV system.
I am often asked about the relationship between solar panel power and inverter power.
Now, logically, if you have (say) 3,000 W of solar panels on your roof, you would expect a 3,000 W inverter to be the perfect match, right? Not necessarily!
There are a number of issues to consider which might determine the ideal inverter size.
1. Peak power
The first and most important issue to remember is that the rating of our 3,000 W solar panels is a peak, or instantaneous, rating. Solar panels produce different amounts of power depending on their orientation and angle towards the sun, their temperature, the density of the air, and so on. So, for consistency they are all produced, tested and rated under “˜Standard Test Conditions’, which happen to be 25Â°C, 1,000 W per square metre of sunlight and an Air Mass Density of 1.5.
I describe this to customers as being similar to a 100 hp engine rating. A 100 hp engine will only actually produce 100 hp under a certain set of circumstances (temperature, tuning, fuel type, throttle position, etc). Any change to these conditions or wear and tear and your engine won’t produce 100 hp. It’s the same for solar panels.
Second, the peak available sunshine actually occurs for a very short period of time during the day (around 1-2 hours). Either side of that time, of course, the output is less.
So, when you take into account seasonal and weather variations, non-optimal conditions, efficiency losses, DC to AC losses and the natural degradation of components, you are extremely unlikely to ever see 3,000 W from your 3,000 W solar system.
A rough rule of thumb I use (after two decades of looking at data) is that you will typically see around 80 per cent of the peak output rating as a real peak power – so around 2,400 W in this example.
“But that’s a rip-off,” a customer might say.
Well, if it’s sold honestly and clearly, it’s not, because this is disclosed. More importantly, solar design is more about daily energy than instantaneous energy and we have great standards and design rules for calculating energy output from solar systems that take this into account.
2. To upsize or downsize?
Occasionally you will see solar systems that have oversized inverters, for example a 3,000 W solar array with a 5,000 W inverter. This is sold as a feature to allow the upgrading of a solar array in the future. It’s a reasonable approach, assuming you can find the panels to match in a few years, the engineering is done right to avoid losses and the customer doesn’t mind spending money on something they may or may not use in the future.
Quite often you will also see undersized inverters, for example a 3,000 W solar array with a 2,400 W inverter. This is sold as a feature (sometimes referred to as overclocking) to fully maximise the inverter capacity and save on costs, and is also a reasonable approach. However, the engineering and design needs to be spot-on or you can damage the inverter. The Clean Energy Council design rules specifically take this into account and allows overclocking of up to 30 per cent within its guidelines. \
So, you can quite reasonably upsize and downsize, within the rules, depending on what you are trying to achieve. But importantly, the inverter needs to be able to handle it.
I quite regularly sell systems that are downsized (on the inverter) for cost and performance maximisation, and where the components are ideally matched. For example, a 315 W (DC) LG Neon solar panel matched to an Enphase 250 W (AC) inverter. This is oversized by 21 per cent, so well within the rules, and as described above it actually makes outstanding use of the capacity and avoids any wastage. However, in perfect solar conditions for small periods of time this can lead to clipping where (for example) the panel wants to produce more energy but is limited by the inverter.
I am often asked about the amount and value of lost energy due to clipping in overclocked systems like this and there are two versions of the simple answer. First, although you may lose some energy when and if you clip, it will be for a short duration. As the guys from AC Solar Warehouse wrote in a paper on the topic recently, the value of that energy (assuming it was offsetting at 30c/kWh) is about “29.6c per solar panel per year”, or $2.96 per year on a 10-panel system.
Second, by having lots of solar energy available earlier in the day, the solar output curve is actually fattened out in the mornings and afternoon a little, so there are gains to offset the losses too. The inverters ramp up faster in the morning and also ramp down slower in the afternoon (see image above).
So, what you may lose at the peak you will gain on the fringes, and in inclement or overcast weather you can potentially gain even more because the peak solar outputs are lower.
All up, the summary comes down to this: overclocking, when done intelligently and with the right equipment, will in all likelihood increase or at worst equal the energy from a system that is not overclocked.