Our electricity consumption

I just did a couple of calculations of our electricity consumption.

For the first calculation I used the total number of days our system has been running (1,695) and the total energy consumption (4,765 kWh) to get 2.81 kWh/day.  But we have been away for some of that time (say three months), so I subtracted 90 days = 2.96 kWh/day.

Even then, there has been only one person here for some of that time, maybe around 5-6 months, so 2.96 is an under-estimate of our average energy usage.  More relevant is the fact that until around late 2010 we were living in what is now the office, so fewer lights, no load from the fridge for most of that time because we used a gas fridge, and for much of the time, no washing machine. When we did get a washing machine, quite late in our shed-living days, it was a very old one that was about to be dumped by the neighbour of a friend, so had very high energy consumption.

We don’t keep a note of our meter readings (though we should), so it isn’t possible to track changes in electricity use over time. But I do have meter readings for today and two weeks ago, so can do the sums for that period – it comes out at a surprising 4.57 kWh/day.

Why surprising? Well, almost all our lights are 5W LED downlights, and they are set up in “banks” or as individual work bench lights so that their usage can be minimised. (Though they are rated at 5W, an energy meter on a bank of five in the office shows that their average power usage is 6W per LED).  This is probably mostly a result of losses in the 50W transformers that we use (each of which has a maximum of 10 LEDs connected to it).  Even with no lights turned on, each transformer uses 4.4W continuously, probably through hysteresis in the iron core, so for the four transformers that are constantly on we are using (losing) just over half a kWh per day.

Another reason for the higher than expected power use is that this is summer, and we have been having heat waves off and on for weeks, so the fridge has probably been running much more than usual.  It is a Vestfrost with separate compressors (and separate doors) for the fridge and freezer components, so is pretty economical.  When we bought it, it was way ahead of even the most economical of the commonly available brands.  However now there are quite a few models with similar storage space that are, on paper at least, are equal or better.   It’s set up in front of the pantry, so there is plenty of space behind it, and the gap between the top of the fridge and the ceiling is 2-3 metres, so there is plenty of vertical air circulation.

Our 1.98kW solar array on the roof of the "office"

Our 1.98kW solar array on the roof of the “office”

We aren’t too worried about this level of energy use on our part, because we are on off-grid solar.  All of our power comes directly from solar panels during most of sunny days, and from a large battery bank during the night or cloudy parts of the day.  The battery bank consists of 24 two-volt 650 amp hour gel batteries.

Our solar PV system has a design load of 5.4 kWh/day – based on our estimated usage at the time of 3.95 kWh/day.  Since most of the power we use comes directly from the solar panels (fridge, washing machine, tools and computers are operating mostly during the day when the solar panels are producing much more then we use at any one time), our current 4.57 kWh daily usage represents a much smaller load on the batteries and leaves us plenty of leeway within our design load.

We manage our batteries so that they don’t go below 80% capacity, and even with this conservative management approach we need to turn on the generator to recharge the battery bank fewer than three times per year on average.  With gel batteries and this low level of draw-down we expect to get 15 years or more out of our batteries.

Though I say that we aren’t too worried about our level of electricity consumption, we do try to minimise our use, just because it feels right – and because one of our objectives in living where and how we do is to demonstrate a range of approaches to sustainable living.  I’ll deal with other ways in which we strive for sustainability in other posts.

In the meantime we can get some satisfaction from the fact that our level of electricity consumption is nowhere near the average 13.9 kWh/day of two-person households in our postcode area.

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