Monday, May 4, 2015

bits that fall through cracks

As I've described in how much do we use and renewable energy as investment, we have made an effort to reduce our power consumption, without going to absurd lengths. We regularly export about 8 kWh/day to the grid from our 2 kW solar PV system, and draw about 2 - 4 kWh/day from the grid. This is changing as we approach winter (read on).

As I said quite bluntly in Solar PV: opinions, merits, challenges, I am a believer in using the grid where it exists, because a battery storage system must be greatly oversized when off-grid to cope with the worst possible conditions -- conditions that are encountered only rarely [1]. Despite this, Australian utilities seem determined to cause grid defection: the process where people decide that they're economically better-off without the grid and leave it. I believe this will be a bad thing for Australia, because a lot of investment has already gone into the grid, and this will be wasted if people defect from it [2].

However, I'm also unhappy about our patterns of power use as they are. We don't have any fancy monitoring installed at our meter, I just take regular meter readings. I usually take a meter reading just before going to bed at about 10pm. Sometimes I check the meter again in the morning, just to see what has been used overnight. Without any intervention, we used about 1 kWh overnight.

Overnight power use

What's using all this power? Here's what I estimate/measure:
Consumption (W)
microwave0 (5)
mini oven0 (5)
fridge (continuous equiv)33
clock radio5
master bedroom cd player5
kids' bed cd5
old laptop5

Our Electrolux ETM4200SB fridge is one of the most efficient consumer fridges (we bought it second-hand for $500), and uses about 800 Wh daily, when the fridge's environment is about 24 C.  800 Wh/day is the same as 33 W (continuous equivalent) [3]. All the other values in the table are standby power consumption that I measured myself with a plug in Watt-meter. If you add all these values up, and multiply by 12 (hours), you get about 1 kWh used overnight. Note that I've counted the microwave and mini oven as having a standby power (also known as a phantom load) of zero -- that's because I switch off the microwave at the wall.

Clearly, if we want to draw less power from the grid, this is what needs to be reduced. Particularly at night when solar PV isn't producing. On windless nights, those electrons are supporting coal power, even though we buy GreenPower [4].

The fridge is the big one, and it would be good to get a fridge like the ozefridge, that can "store coldness" for use overnight (so that it doesn't use electricity at night time), but it's too expensive to justify right now -- if we were considering going off-grid it would be a no-brainer though (because storing energy in batteries is less efficient, with greater maintenance, than storing the energy as coldness in the fridge). 
I will install proper switches on the supply cables to the two CD players, and I want to put a timer on the office computers and router (so that they're properly off at night time). EDIT: I have found that the timers consume a significant amount of power and are unreliable, so I have just been switching things off or unplugging them.

Doing this reduces our overnight power consumption to about 0.5 kWh overnight (10pm to 7am).


(Note that the resolution of these measurements is 0.1 kWh, so they are a bit approximate)
  • I turned off the router and the two CD players overnight. Instead of using 1 kWh overnight, we used 0.7 kWh.
  • Then I turned off the study computers and washing machine too, which reduced our consumption to 0.6 kWh overnight (I bought cheap powerboards for the study PCs (~$10 each) that have a switch on them that turns off the whole board)
  •  If I turn off the router overnight our consumption is about 0.5 kWh overnight.
  • We've unfortunately needed to run a night-light for the kids, which is using almost 0.1 kWh overnight
In looking to save power, this is a significant saving for us (about 20% of our total grid draw), for very minimal effort and no sacrifice.

Solar -- Winter

This is a sun path diagram. It slows the path of the sun through the sky in Adelaide across the year. The upper green line (top of the yellow area) is the path taken at the Winter solstice. The lower blue line (bottom of the yellow area) is the path taken at the Summer solstice. The red line is the path taken on May 5th (today) -- you can see it is not far from the Winter solstice path, even though the Winter solstice is nearly two months away (this is because the path the sun takes is a sinusoidal curve, and the time of greatest rate-of-change is behind us (that occurs at the Autumn/Spring equinoxes) -- the rate of change occurring near the Solstices is small (for example, if you pay attention to the time of sunrise and sunset, you'll notice it changes most rapidly at the equinox, and most slowly at the solstice).
Sun path diagram. The original is here

Below are meter reading data that I have collected. In these data, look at the date and the pattern of solar PV production and electricity consumption is clearly variable as the seasons progress. In particular, the decrease in solar PV production (green line) during Winter is marked.
Also obvious is the large change in grid-draw (red line) that occurred on September 1st, 2014. That was when we switched off our electric storage hot water heater and went to solar hot water.
There is also a further reduction that occurs in early February 2015 -- this is the reduction that resulted from my targeting of phantom loads! It is subtle on this graph, but is clearer on the next graph which shows the cumulative data. The switch to solar hot water is also apparent in this graph, as an inflexion point at September 1st 2014, and a second inflexion point is visible at early February 2015. This shows that the savings from our reduction in phantom loads are significant.


Once the main areas of energy reduction are targeted,  it is very worthwhile to reduce phantom loads, particularly overnight. For essentially no effort, we're saving money every day, and reducing our support for coal and gas fired electricity.

[1] Another alternative is that off-gridders accept that sometimes they run out of power, and use candles for light, fire for cooking, and nothing else. This is fairly widely accepted by rural off-gridders, but something tells me that city folk will be less inclined to accept this.
[2] Note the idea of a proposal being "economical" -- I don't believe that economics adequately captures many of the most important elements in a decision, and that this is due to economic externalities (economic costs being imposed on non-players). Let's take an example. It is widely considered that cheaper to install a larger solar PV system and use the excess electricity to heat water. It costs less money. This is because solar panels are artificially cheap (their cost to society is much higher than the price paid for them -- I won't substantiate this claim here, it's an article to itself, but there are many such articles written already). In comparison, the monetary cost to install a solar hot water system (that uses sunlight to heat water directly, without converting it to electricity) is higher but the cost to society much lower.
This shows that "economic considerations" are not necessarily indicative of overall merit.

[3] I have considered playing with the fridge's thermostat. My idea was to run the fridge much colder during the day, then raise the thermostat so it didn't work as hard at night time. This would make a dent in our night-time consumption. I haven't done anything with this idea yet (there are clearly food-hygiene considerations here!)

[4] This is probably a somewhat contreversial statement. We buy 100% Greenpower, so in theory our power is all sourced from renewables. However, if there is a windless night, then any power consumption increases the electricity spot price, which aids coal/gas fired power stations (currently there is almost no storage of renewable energy in the grid).

This article was written by Angus Wallace, and first appeared at


  1. Hi Angus,

    Well night time usage is often cheaper because the coal fired generators are technically quite hard to speed up and/or slow down, so all of that electricity has to be used regardless. As an example of the problem, I once had a 600W wind turbine installed here for a month or two a few years back. It was an interesting experiment because there were times that the turbine produced electricity, the batteries were full and I didn't use the excess energy in the house. This is a problem because the resistance inherent in the batteries is used by the wind turbine to help slow the blades spinning too fast in high winds (when they're producing energy). If the charge controller decides that the batteries are full and disconnects the wind turbine, then the blades can spin way too fast and the turbine may self-destruct. Literally, the thing will blow up and this is also a serious problem for the commercial wind turbines.

    Anyway, the charge controller directs the excess energy into a "dump load" so that the blades on the wind turbine don't spin too fast and self-destruct. A dump load is essentially a resistor which converts the electrical energy into heat. It works sort of like an electric radiator. However, in my case the dump load once became so hot I thought that it was going to catch fire and it left scorch marks on the wall - so the simple thing to do was get rid of the lot of it all (sold on EBay) and add a few more solar panels which can be disconnected from the batteries with no issues at all.

    Now, spare a thought for the people and systems that have to match the grid supply to the grid demand without anyone ever giving them a second thought. The same problem occurs just on a massive scale, so night-time usage is not good from a CO2 perspective, but from a systems perspective you may be able to understand why night-time rates are cheaper.

    As an interesting side note, Norfolk Island has a lot of grid connected solar PV and once the entire island produced so much electricity that supply completely outstripped demand and the system packed it in. Completely dead.

    Running out of power off grid is not cool. The reason for this is that batteries are chemical reactors and they don't store electricity in the same way that a water tank stores water. The weird thing about them is that the more often you take them below 70% full, the more damage you do to the batteries and thus they will have a shorter working life. So, the less you use a battery the longer it will last. It sounds strange having such a high capacity (i.e. I have 10 days’ supply in the batteries here), but you can't really use that full supply. I rarely take the batteries below 70% full. And it got there the other week too.

    Systems are very complex beasts and it is a real pleasure to see that you think in those terms.

    PS: The standby current here is 25Wh (60% of which is used by the inverter). Don’t feel too bad about the night-time light I use one here too because it so dark otherwise!



  2. Hi Chris,

    Yes, coal provides most base-load in Australia (for exactly the reasons you state) -- that's why I don't want to use base-load electricity! ;-)

    Regarding your wind turbine, I love em, but micro-wind power just doesn't make sense in an urban environment. I'd love to get together with all my neighbours and buy a 500 kW turbine that was 30 m tall. I wonder what else you could have done with the energy dump from your turbine. Heat and cool your house?

    The grid is a beast to manage. I've done some reading on AEMO, and there's a lot to it. That's why I don't buy the argument that renewables will complicate the grid and introduce instability -- the grid is so complex -- it can take it!

    It's very interesting to hear your thoughts on batteries (thanks for your final comment last week too) -- I have no personal experience using them off grid. I am very aware of the depth-of-discharge considerations though, and that putting energy in them is not like filling a rain water tank! I'd love to see a commercialised fly wheel as power storage -- there are some interesting developments in that area. For now, I want to keep experimenting with using less electricity. I have some ideas that could possibly allow one (with some minor lifestyle changes) to need only 2 - 3 kWh of (usable) storage to be off-grid. Btw -- I've made a rocket BBQ/oven -- will write a post soon ;-)

    I'll have to read up about Norfolk Is -- I haven't heard of that.

    That's interesting about your inverter's standby charge. I hadn't thought about that. Maybe I should switch off the inverter at the meter overnight and see what difference that makes. I'm watching with interest Tesla's battery storage -- it'll be interesting to see where that goes. There was an intersting pair of articles at about the sustainability of PV and battery storage. Have you seen them?

    Thanks for your thoughts, interesting as always :-)

    Cheers, Angus


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