More power of Energy Efficiency

21 11 2012

Just when we thought we could not lower our electricity consumption, along comes the next “bill”…..

Honestly, I was blown away……  never in a million years did I ever think we could get under 2 kWh per day, without even trying.  The only thing that occurred during this billing period is that the old upright freezer in which we kept the goat meat was converted to run as a fridge at 10°C for the cheese……  but all the same, the graph on the back of the bill shows we were going downhill for an entire year.  And there was I telling people we “only” consumed 2.4 kWh/day.  I am baffled.  Yes, the little freezer could quite conceivably use half a kWh/day running at 20 below, but this doesn’t really make sense.

The new bills are a little more sophisticated than they used to be now.  That neat little graphic that compares your consumption with that of other households “in your area” is eye opening.  Two person households “in our area” consume 1,255 kWh a quarter, while we only used 166…….  I’m justly proud of that.

What was disappointing though is that we only exported 903 kWh of excess solar generation, which over 90 days is a neat 10 kWh/day.  From 3.5 kW of PVs, that’s less than 3 hours of peak power.

This is due to my ageing SunProfi inverter putting a lot of power into the backup batteries, and the new PVs running very inefficiently when the weather’s hot, rarely producing more than 1800W from their rated 2200W…….  it’s because of a little understood phenomenon called thermal de-rating.

Panels are rated at X kW @ 25°C.  Some manufacturers must be ashamed of their Temperature Effect figures, as they don’t tell you what they are.  But they normally range from 0.2% to 0.5% per degree C above 25°C.  A panel would have to just about be in Antarctica to run at 25°C in full sun!

So if it’s 35°C outside (and it often is here), your black panels on the roof could be running as hot as 70°C, a temperature difference of 70 – 25 (degrees) = 45 degrees.  If your de-rating number is 0.5% (worst case scenario), then your panels’ power output could be de-rated by 45 x 0.5 = 22.5%.  On 185W panels like ours, that means they actually only produce 145W, so 12 of them would only produce 1740W……  which is pretty close to the truth!

I could complain, but I’m waiting for my $460 cheque…….

For advice on how we achieved this, visit this page…..