30-day solar update

I’ve been recording energy use and PV solar system output on my spreadsheet for 30 days now, so here’s a short summary.

Here’s my average daily use for last year for comparison:

Household use excluding hot water heating: 5 kWh—cost $1.26 per day
Off-peak (at night) hot water heating: 5 kWh—cost $0.83 per day
(Water is less because it’s on a lower rate)

Total use: 10 kWh—cost $2.09 per day
Add in service charge @ 92 cents per day
Total cost = $3.01 per day
(This is calculated on today’s prices—there have been 3 price rises since the beginning of last year).

Last 30 days on solar:

Household use excluding hot water heating: 2.2 kWh—cost $0.55 per day
Off-peak (still at night) hot water heating: 4.1kWh—cost $0.68 per day
(It’s lower because I’ve been more frugal with hot water)

Total use: 6.3 kWh—cost $1.23 per day
Add in service charge @ 92 cents per day
Total cost = $2.15 per day

So I’ve saved $0.86 per day on solar compared with last year’s consumption.

But—

The solar panels have generated an average of 13.4 kWh per day.
Since I’ve only used 6.3 kWh of this, 7.1 kWh per day has gone to the grid.

(Note that when the energy supplier FINALLY comes out and reconfigures the meter to show what has gone to the grid, I’ll be able to read it directly from the meter; no messing around with the spreadsheet).

So, that 7.1 kWh exported is worth 8 cents per kWh as credit = $0.57 per day.

So take that off the total cost of $2.15 per day and you get $1.58 per day.

Compare that with last years consumption of $3.01 per day and I’m saving $1.43 per day.

That’s worth $522 per year which (on this year’s bills so far), equates to about 6 month’s worth of energy bills, i.e. it looks like I’ve cut my energy costs in half. It will be even better if and when I can get the hot water heating changed from night to day.

Another way of looking at it is to express exported credits as a percentage of the daily supply cost (92 cents) and so exports covered 62% of that in the last 30 days.

Interesting things to note:

In the last 30 days there have only been 5 days when I used more power than the panels generated. I recorded the weather on those days as ‘heavy cloud/rain’.

Highest generation occurred on a ‘sunny all day’ day, and was 21.9 kWh.
Worst day was 3.8 kWh which I recorded as ‘heavy cloud all day’.

I’m constantly amazed when it seems like a dud day, with intermittent cloud, even rain showers, and the panels still produce over 10 kWh per day. Once the sun comes out, it doesn’t take long for the kilowatts to build up, especially in the afternoon, which is when the 12 panels on the west-facing roof spring into action (the remaining 8 panels are on the east-facing roof).

My system is a 3.9 kW system. I don’t expect 100% efficiency so never expect the output to reach that, but the other day I just happened to check the inverter as I was passing by, and it was reading in the high 3’s. I kept watching and got a shock when for a brief instant it went over 4 kilowatts. I didn’t think that would be possible, so I Googled (as you do) and found this:

Solar panel output is affected by cell operating temperature. Panels are rated at 25° C. Output can vary by 2.5% for every 5° variation in temperature.

If the panels are cool due to cloud cover and the sun bursts through the cloud, it is possible to exceed the rated output.

Which is exactly what happened. How cool is that!