Wednesday, May 11, 2011

Solar year 3 ends

When we got photovoltaics (PV) on our roof 3 years ago I, like many people, didn't understand how it all worked. I really wanted to so I wrote a spreadsheet (Solar Tracker) to help work it out. Once a week I take readings from our inverter and power meter and enter them into the tracker. It has grown from a very simple thing to a vast sprawling monster spreadsheet which calculates way too many things, but I love it. I know how much power I use, generate, feed in, can predict bills, etc.....Anyway, our solar year runs from late to April so the exciting results are just in, so here we go. This chart shows weekly usage and generation (printed and scanned, still working out blog technology) and a summary below of stats.

The chart shows our usual pattern of higher usage in winter with low generation, then a cross over in spring to lower usage and higher generation. I'm pretty proud of the fact that we have continued to use less power than we generate with a very small system (1.1 kW Kaneka thin film amorphous grid connected, facing due North). Our usage is about a quarter of the average for West Australia. It was nice to get a credit of $167 for the year after 2 years struggling to pay our $20 bills! The reason for the credit is the introduction of a net feed in tariff to WA in August 2010. Under this incentive program the state government and utility company pay 47 cents per kWh fed to the grid, which is more than double the 22 cents we pay to buy in the power. Since we feed in 77% of what we generate we do quite well out of this program and next year our bill should be a credit of about $220. The feed in tariff is guaranteed for 10 years which has cut the payback time of PV from about 15 years to 5 to 8 years for most people. BAU means business as usual, so what our bill would have been without PV (so we saved $600 this year). Smart Power is a system whereby our power provider charges different tariffs at different times of the day. The advantage of this used to be greater for us but is lessened by the feed in tariff, but we still gained a bit.

The beauty of having 3 years of data is that we can compare this year with previous ones. Here is a chart of our cumulative usage in years 1 to 3.

This year we scraped in using slightly less power than last year (probably thanks to our holiday!). Our year before PV we used 6 kWh a day and we cut that by 25% in year 1. Then we cut our usage by about 8% in year 2 but it's getting harder now. I just found out that our answering machine uses 0.3 kWh a day, which might not sound like much but it's 7% of our consumption. If you try to call me and can't leave a message I'm afraid that's why (it's going to the op shop I think). So hopefully we'll be able to cut our usage to 4 but I think it will get harder and harder. One great thing about taking readings is that we can see the results of our behaviour change, it's a positive feedback loop which reinforces what we are doing. Before PV I didn't have a clue how much power we were using, even though it told me on each bill. By the way, if you're not sure how much you're using, your bill will probably say how many units you're using and a unit is 1 kWh. If you have PV, your utility company will estimate how much you're using based on the size of your system. If you want to know exactly you need to do some maths and take some readings. Take a reading form your inverter, which should tell you something like E total (numbers of Kwh generated since day 1). Then take a reading from your power meter, there should be 2 figures, 1 positive (taken from grid), 1 negative (fed to grid). Then take another reading a week, month or year later. Subtract the latest figures from the first ones for each reading and you'll have 3 numbers: kWh generated, Kwh taken from grid and Kwh fed to the grid. Then do the following equation:

Kwh used= kWh from grid + (kWh generated-kWh fed to grid)

Power use is very seasonal so to get the most accurate figure get a year's data. Now on to generation:

Perth is probably the best place in the world for PV generation since we get an average of 8 hours sun a day (it sometimes amazes me why there aren't panels on every roof!), so we are lucky in that respect. PV systems usually over perform in their first year, so there's no surprise that year 1 was our best year. The interesting thing is that year 3 was better than year 1. Most PV systems lose efficiency as they get older, so I was expecting this to happen with ours. I think it shows that the weather we've had is probably masking the PV cell efficiency drop off. We've had a very dry year which obviously leads to less cloud cover and higher solar radiation.

We now take weekly readings of gas and water consumption, kms travelled by car and cargo bike and also egg production. So stand by for more geeky installments. Embrace your inner nerd. Geek, c'est chic.


  1. aah - this is such cool info! I so wish we were tracking our generation and usage but I am sad to say that is a wee bit too far down our list of things to do but I know if we managed it I would geek out with you. That is an amazing fact about your answering machine. We have started flicking most things off at the mains overnight - have you considered that? it is quite easy for us as the board is by the bathroom so clean teeth and flip the switches.

  2. I am glad to read your post!! pretty informative..thanks for providing such a nice post. Keep sharing more.

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