Wednesday, October 8, 2014

Power consumption at my house


We have a normal house, and have not invested greatly in energy efficiency, but we are very careful with our power use. We have spent about $2500 installing a solar hot water heater, and have bought some energy saving LED light bulbs, and are very aware when we use power. All of our appliances are at least reasonable when it comes to power use, and we have installed a small (2 kW) solar PV system (we paid $5000 for a good one, but similar systems are available for $2000). We are now in a situation where we are exporting (selling) about 3x the power we are importing (using), and I expect to have negative power bills (ie. the utility pays us). I have estimated our ROI for this investment to be about 17.5%. We do not have a generous tariff on our solar PV (we get 24 c/kWh when we sell, and are charged 35 c/kWh for what we buy). We buy 100% GreenPower. I think this makes the case that high electricity bills aren't a necessity, and that you have the power to do something about it.

Our power consumption

This is a short post which shows graphs of the power consumption at my house. There is nothing fancy about it -- I just take regular meter readings myself so that I can monitor the system's performance. These graphs should stay up-to-date as I add new data. I show these data so that you can see what is possible in a very normal Australian house. We don't have fancy passive-house design or lots of expensive modifications. We just don't use a lot of power because we're careful. Our house was built in 1955 and is unassuming. We do not use gas at all (well, we have a gas bottle on the BBQ which we use a handful of times each year).

We have a standard grid-tied solar PV system. This means that any power it produces, that we are not using then-and-there, is exported to the grid. The amount of power exported is measured. If we are using power (eg. electricity in the house) while the system is producing, then only the excess power is exported (ie. the difference between what's generated at the panels and what we consume). If our consumption exceeds what we're generating, then we're importing power.

The first graph shows daily consumption (get power from grid, red) and solar PV export (send power to grid, green). I don't take a meter reading every day, so where I don't take a reading the graph shows an average since the previous meter reading. Notice the sharp change at the beginning of September where the electric storage hot water system was switched off. This reduced our daily power consumption by about two thirds (from to 3 kWh/day). Note that I live in Australia -- winter is from June to August!

The second graph shows our cumulative consumption and solar PV export. Seasonal variation in exported solar PV production is very evident (green line, from May to August). The production decrease in winter looks worse than it actually is because this shows not the generated power, but the exported power (which has our instantaneous use subtracted from it, which is higher in winter). Also note that the angle of power consumption changes at the beginning of September, when the electric storage hot water system was switched off. Our 2 kW solar PV system is mounted flat on a tiled roof. I don't know the exact angle, but it would be less than about 30 degrees, which is sub-optimal for winter production (because of the angle of the sun in winter, they would produce more power if they were at a steeper angle -- 60 degrees in Adelaide). They do and are unshaded all day.

This article was written by Angus Wallace and first appeared on

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