Musings on Intermittency

4 07 2021

Today is a great day for writing about intermittency. Tasmania is dishing out typical winter weather. It’s cold, wet, and….. very dark! Not Dark Mofo kind of dark, but no sunshine to speak of dark…. while writing all this, it’s started raining in earnest, and the solar input has dropped below 8…! You’ll notice too that there’s no wind either, so having a backup wind turbine would be of no help at all. Luckily, inside the house is nice and warm. Firewood is not intermittent.

As you can see from the above photo of my weather monitor, the midday solar input was a mere 37.2W/m². Full sun is 1000W/m², therefore we’re getting about 3.7% of full capacity meaning our 2000W solar array is producing maybe 75W. That’s the best we’ll get all day, I don’t even have to walk to the power station to know that today is a zero Solar energy day…….

its clear from what people say to me that they simply don’t understand intermittency. Sure, they realise that clouds may come and go during the day, but I’m talking about solar’s real Achilles heel here. Its inability to make reliable electricity a complex civilisation can run on.


For us personally, seasonal intermittency is a real issue. Last year. I wrote about this and the way I reconfigured our battery bank to make the most of the prevailing conditions. We had three zero production days last winter, we’ve already had at least five this year. It’s a real pain in the proverbial……

While he was here visiting recently, our son Alex rigged up a system that allows me to monitor the off grid system via a cool app on my phone without having to walk up the hill. It’s still work in progress because the signal in the house is too weak, but the phone app is way superior to reading the instantaneous readout on the MPPT’s display….. It even comes up with a monthly log of the system’s performance, like this….:

This is a screenshot from my phone, pretty cool, hey…. The white numbers show the daily generation in kWh/day. You’ll notice lots of zeroes, and quite a few near zero. 0.2 and 0.8 really don’t cut it when we need/want 2.0+

When your battery bank stores 10kWh and you get seven zero days literally in a row, you’re in trouble. Because if your fridge is off for two days, when it comes back on it’ll consume a lot more than its normal daily load to cool right back down again.

All my friends were saying, get a generator…. After all, when I did my diploma in renewable energy technology all those 25 years ago, my teachers warned me that it was impossible to live off grid without one. Making me, the still naïve learner wondering why it wasn’t possible to shake off the shackles of fossil fuels….

So i started looking at gensets online and found this baby on eBay. It’s the same Chinese brand as my big chainsaw and concrete vibrator that have done me well so far. I figured if I can live with 2kW of PVs, I should manage with a 2kW genset, especially one that can run for an hour on half a litre of petrol, a whole day’s requirement. Three or four hours running this a year won’t break my carbon footprint, I told myself…. Plus at $300, it wasn’t going to break the bank either. I started it four or five times, and the bloody thing broke. I kid you not. Something went wrong, I couldn’t pull the starter fast enough to reach ignition speed. It’s gone back and it could take as long as four weeks to get satisfaction.


There are none. This problem is caused by the tilt of the Earth’s axis, and I defy anyone to solve this. But it doesn’t stop people telling me that, for instance, increased PV efficiency is coming to the rescue. I’m told (constantly, mind you…..) that 30% efficient solar cells are on their way. The solution you have when you don’t have a solution…..

our PVs are ~18% efficient or they were when they were brand new five years ago. So 30% efficient ones should generate roughly 50% more electricity. Sounds impressive, but on those zero days, 150% of 0 is still 0. The best we would have got during that bad period would have been 1.0kWh which is still way insufficient.


Then, to rub salt into the wound, the end of June saw seven days straight of bright sunshine that of course completely recharged the batteries. So much electricity was generated I was looking for any excuse to consume it, Using my 2kW drop saw to make architraves, pumping lots of water in my newly finished bathtub, listening to music on YouTube via the TV and vacuuming everything in sight… That’s the trouble with a hyper efficient house…!!!

By the end of the sunny period, our batteries were full by 10:30am, and it was all going to waste. If we had 30% efficient modules, we would simply have wasted 50% more power, and for longer to boot…..!

Complex civilisation running on 100% renewables will have to simply hibernate in winter! Considering that half the world’s population lives at least as far from the equator in the northern hemisphere as we do here on the other side of the world, that’s a lot of disappointed people no longer enjoying economic growth….

Am I game to turn the TV on tonight to watch Jack Irish? Fingers and toes crossed……