Is your new solar system producing the power output it should?
A very common question I get from people who have just got a new solar system is: “Now the solar panels are on my roof, how do I know what the solar system output should be?”.
The solar system owners are usually a bit concerned because they have bought, for example, a 5kW system but their inverter is telling them that they only got 4kW of peak power yesterday! Where’s the missing 1kW? Most people expect a 5kW solar system output!
Here’s an email that I got last week from Hans that is a perfect example:
Thanks for your advice. Much appreciated.
I had a system installed via you, all very nice and well installed on Thursday 15/3 and very pleasant people to deal with.
5 KW , 20 x 250 W ET panels with a Sunnyboy inverter.
During the test we have measured at peak at 14:20 4.7 kW and we were very pleased. After that day we had some hotter and more sunny days but the capacity of the system always stays below 4 KW at peak. never go above it, strange.
Today a hot clear day 25kWh at peak 4kw.
May I have your opinion about this and can i expect to reach the 4.7 KW again?
What Solar System Output should Hans be getting from his 5kW system?
A fact that isn’t advertised widely in the solar industry is that most solar systems in Australian conditions will usually peak at only 80% of their specified peak power.
This is actually completely normal in almost any industry that sells hardware that has associated specs. Think about the last time you bought a car with a specified fuel consumption from the manufacturer. Did your car actually ever achieve that fuel efficiency on a day-to-day basis? Thought not!
Solar panels systems usually reach only 80% of their specified peak due to “system losses“. So what are these system losses, and how can you work them out for the solar system sitting on your roof?
In Hans’ case he is using ET Solar 250W solar panels. He hasn’t told me the model number, but there are 3 matching panels in my Solar Panel Comparison tool. I’ll assume he is using a ET-P672250 and use those specs in the calcs.
The 5 Losses in Every Solar System
1. Manufacturer’s power tolerance. (1%)
All panels have a power tolerance. In the panels that hans is using, this is -1%/+3%. So worst case is that the specifiedpower output is actually 1% lower than spec.
2. Temperature Loss. (10%)
I wrote an entire blog post on solar panel temperature losses. To cut a long story short, solar panels don’t like to be hot. Most solar panels lose about 10% of their rated power on a 25°C day, more if it is hotter. Let’s assume 10% for this estimate.
3. Dirt (5%)
When your solar panels are put on your roof, airborne particulates like dust will settle on the panels’ glass. These particulates block the amount of sunlight reaching the solar cells behind the glass reducing your power. The reduction in power from particulate build up typically lies in the 5%-15% range. Hans’ panels have just been installed so we’ll assume only a 5% loss.
4. Wiring Losses (voltage drops) (2%)
All the solar panels on your roof are interconnected with wires, then a long pair of DC wires connects the final solar panel to your inverter. All these wires have a small electrical resistance, which means the electricity flowing through them will suffer a voltage drop. This will reduce your power proportionally, typically by around 2%.
5. Inverter Efficiency (4%)
Everything goes through your inverter so the inverter efficiency will directly affect your system output. Hans is using an SMA Sunny Boy inverter. If he has a modern, transformer less model, he can expect an inverter efficiency of about 96%, giving a 4% loss.
Multiply all those together (see the pic on the right) and you are looking at 20% total losses, giving a real world peak power of 4kW – which is exactly what Hans was seeing.
But what about the 4.7kW he saw on the first day?
That was probably down to 3 of things. Firstly Hans tells us that the days following this reading were hotter. So there would have been higher temperature losses later on. Also, after the first day, a thin, almost invisible layer of dust probably quickly settled on the panels increasing the dirt losses from a starting point of zero. Thirdly, the panels can initially degrade a percent or two in the first day or so, before they “settle in”, after which they should degrade a lot more slowly (about 0.5% per year). Take all these into account and it would explain why Hans has not seen his 4.7kW peak power since.