Monday, October 10, 2016

2 years on: System performance

We've now had our solar hot water and solar PV system for 2 years. I thought it was worth commenting on them.

Solar Hot water (main)

This system is detailed here and here. Based on my meter readings, I estimate it has saved us 4500 kWh (just over 6 kWh/day), which is worth $1350 on the standard tariff (we avoid the off-peak tariff because it supports fossil fuel power, even when buying Greenpower) At this rate, the system will pay for itself in about another 18-24 months (4 years total from new).

The tank looks a little weathered, but still in very good shape. Beacuse  it is low pressured, I am confident it will last many years. I partially shade the panels (with an old cotton blind) in summer, to stop it boiling, and we lose very little water from it.

Kitchen Hot water

This system performs poorly in winter. It's nice to have instant hot water, and it was a good learning project, but I wouldn't recommend people do this if they have other solar hot water. At some point I might re-purpose this system into a backyard shower.

Solar PV

Our system has produced almost 10 MWh. Since we got the new input/output meter we've exported about 7 MWh to the grid (which means we've self-consumed 3 MWh of our solar PV). We get paid $0.24 / kWh for exported power and pay $0.32 / kWh (including the GreenPower premium) for imported power. So, the return on our solar PV has been $2600, or about 1/2 the cost of the system.
As of the end of September, we lose the $0.24 feed-in-tariff, and will be paid about $0.08 / kWh. That will reduce the return on the system cost. In another 2 years, we'll have got back an additional $1500 at these tariffs. I think we'll pay back the cost of the system in about 3 years (5 years total from new).


Our 2 kW solar PV system has produced more electricity than we use almost every day (about 50 exceptions in two years) since it was installed. I've shown the economics here, because that's of interest to some people, but it's a great feeling to look at the roof and see the power we harvest. It's strange how people carefully consider at the economics of solar PV, but not of cars.
At some point, I think we'll install a hybrid battery system. I like the idea of self-consuming more of our power. But I don't think we'll try to go fully off-grid. I think that doesn't make sense in the city (firstly, we'd need a much larger battery to provide for the small number of days when we lack solar PV and secondly it has a bit of a gated-community feel to it). I might also install a second solar PV system. That will have much worse payback than the first, but that's ok.


  1. Hi Angus,
    Top work and those are good numbers. I spotted what looked like a fully functioning solar hot water flat panel at the tip the other day... Go figure that one out. Yup, people are way nutty about solar PV. For a while now, I've been recommending it as a gift to the environment rather than speaking about the economics of it all and that seems to tone down the worst offenders. Would you consider installing a bigger solar hot water system than your present system?

  2. Hi Chris,

    That's a good score. Because many collectors are coupled to mains-pressure tanks, it is hard to stop leaks. If you instead couple it to a low pressure tank, you can fix a leak with a silicone gun :-)
    With regard to solar PV, do you mean that people see it as a silver bullet, or that it's a waste of time? (sorry, not totally clear). My opinion (FWIW) is that the faster we roll out solar PV the better, but we need to cut consumption even more urgently.
    I would definitely consider a bigger HWS, though we don't need it at the moment. We boosted it once this winter for an hour (2.4 kWh used). Having said that, my wife does sometimes shower at work, but that's mostly because she rides a bike. I'm comfortable smelling a little bit :-P Perhaps as the kids get bigger, they'll take a renewed interest in personal hygiene... at the moment they have a couple of baths a week. I'll play that one by ear... another option is to construct a closed-cycle shower -- such as the OrbSys shower (which has the benefit of saving water too).
    Cheers, Angus


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