Put Solar On It! (2)

Solar power has been going gangbusters since my previous post under this title (2014) and an update of it is well overdue. This isn’t it, however. What I want to do here is talk about domestic solar power, and specifically its advantages here in North Queensland, via four small projects which came out of our own move from one suburban Townsville house to another two years ago.

I will go from smallest to largest.

Hall Lighting

The new house is a low-set, 1950-ish cement block home pleasantly surrounded by trees. That makes it much darker than our old high-set home, and its double-fronted layout means that the central hallway gets no direct natural light at all.

We had to choose between running lights all day, every day, and putting in a small skylight. Initial quotes for a skylight (Solatube, basic model) were around $750 with, of course zero running costs for about 10 hrs/day of adequate light, 365 days/yr. Could we do better?

The lights that were in place were three incandescents, about 40 W each, so they would burn through about 1 KWh/day. At 30 c/KWh, running costs would be about $100 per year, so the skylight would be better value over ten years. Sounds good, doesn’t it?

But if we swapped to LED lights, power consumption would drop to 15 W and the running cost would drop to $16 p.a., and that lovely all-natural piped daylight would cost us far more over ten years than nasty-fossil-fuelled electric light. Sigh.

But if we notionally dedicated one corner of one rooftop solar panel to lighting the hall with our LEDs, running costs would drop to zero. That’s better!

The automatic gate

Lazy, perhaps, but we wanted an automatic gate on the driveway, replacing the sagging 1950s item. The gate installer gave us a choice: one small solar panel on the gatepost with a back-up battery in a box at its foot, or mains power from the house.

The solar option (panel, battery and installation) was about $150 cheaper than running mains power through the ceiling to the nearest corner of the house, down the wall and then underground across five meters of garden and garden path. It has worked beautifully for the year since we put it in and costs us nothing at all to run.

The hot water system

Our old house had a passive solar hot water system on its roof when we bought it. It worked brilliantly. We turned on the booster about ten days per year for nearly thirty years. The total energy cost in that time was nearly nothing and the only maintenance was roughly what we would expect for any hot water system, i.e. replacement of the tank (twice, I think) and the panels (once, when the rusted frame fell apart when the workers took it off to replace our asbestos roof).

The new place had an off-peak mains storage hot water system and our first thought was to replace it, asap, with a passive solar HWS like our old one. Our second thought was to look carefully at the options:

  1. What we had: old-fashioned (resistive) electric storage HWS, running on (a) off-peak mains power or (b) solar PV.
  2. One of the new high-efficiency heat-pump electric storage systems, again running on (a) off-peak mains power or (b) solar PV.
  3. Passive solar on the roof.
  4. On-demand electric (or gas – but we knew we didn’t want gas). This couldn’t be on the off-peak tariff but if solar PV was installed it would use solar power during the day and mains power after dark.

Working it out took some time because of the trade-offs between installation costs and running costs. I ended up calculating lifetime (ten-year) costs for four technologies over three different scenarios (mains power only, solar without a battery, solar plus battery) and found that solar power, with or without a battery, made more difference than the type of HWS.

Solar PV power knocked the passive solar HWS out of contention. The on-demand HWS was competitive with the storage systems in any house with solar PV, but free (solar) electricity meant that the extra cost of the heat-pump (rather than resistive) storage system was not justified.

Our decision, therefore, was to keep the old HWS as long as it lasts and run it on solar power. Upfront costs? Notionally an extra panel or two on the PV system. Running costs? Notionally about 6c/KWh for the electricity which went into the HWS instead of the grid. Practically, not enough to worry about.

Solar PV

Here again we were influenced by our experience at our previous house but found that things had changed significantly in the ten years since we installed 1.5 KW on our roof for $3500.

We talked to a couple of big installers. They both wanted to put a now-standard 6 KW system on our roof, for $5,500 – $6,000, regardless of our actual needs. (I calculated 4.5 KW would be more than enough to cover present needs, including the HWS, and with capacity to spare for an EV in the future.) A smaller local company with a word-of-mouth recommendation was more sensible, proposing a 3 KW system. They installed it for $2500 (which was less per watt as well as less overall), all done in a single day.

solar panels on house roof
Our new solar installation

It’s worth noting the price drop over the decade between installations, from $2.35 down to $0.85 per watt, but the other thing that struck us was that the installers now treat it as an absolutely routine process – “We’re doing one a day,” etc – with all the advantages of speed and efficiency that brings.

We’re only going to get a small feed-in tariff, of course, so there’s no advantage in paying for a bigger system and hoping to make money by exporting our additional surplus to the grid: at present rates we would take about ten years to recoup the extra $2,500 up front. That’s significantly longer than for a correctly sized system, which will pay for itself in less than five years. (Our installer calculated 2 years and 8 months for ours but it would be natural for him to be a tad optimistic.)

What about batteries?

The three installers we spoke to were unanimous in confirming our feeling that batteries were not yet cost-effective; payback time is still too long, while the technology is still improving and costs are dropping quite fast.

What about other additions?

For me, EVs are the potential game-changer. If we can buy an EV which both charges from our roof and feeds electricity back into our house when needed, we have a win-win scenario. Or if someone starts a business repurposing older EV batteries for household use when their capacity drops below (e.g.) 80% then we will keep them out of the waste stream for a few more years and have another win-win scenario.

These considerations also apply to our potential need for more rooftop capacity for an EV in the medium-term future, and in fact the installer suggested that we might as well think about a completely separate additional system when that time comes, taking advantage of the new technology at the time.

The idea surprised me but it actually parallels what happened to other new technologies: instead of one huge motor running all the machines in a factory via overhead transmission shafts and pulleys, we have one for each machine; instead of one huge computer running the nation, as in dozens of 1950s SF projections, we have thousands.

Hmm. Solar farms are suddenly looking prematurely old-fashioned. We’ll wait and see. Meanwhile, we’ll keep on putting solar on anything that needs it.

12 thoughts on “Put Solar On It! (2)”

    1. …but the revolution powers ahead on all fronts:

      A pilot program that begins today will see up to 1,000 concession holders offered a free solar system in exchange for forgoing their South Australian government energy concessions. The state government says eligible low-income homeowners could save hundreds of dollars per year through the Switch for Solar program. Residents in 17 north-eastern Adelaide suburbs can participate in the $4.25 million scheme, along with people from Goolwa and Hindmarsh Island, on South Australia’s south coast.


  1. We’ve just received our first power bill since the panels went up. It looks like we’re saving a little more than $500 p.a. but the period of the bill bridges the installation date so it’s hard to be sure.

    1. Our next bill has arrived, and we’re nicely on track: we’re exporting twice as much to the grid as we’re drawing from it. The feed-in rate (8 c/kWh) is well below the cost (22 c/kWh) so we’re still paying Ergon, not the other way around, but we’re not paying much at all.
      What about the other projects?
      Our storage hot water system is thriving on solar power, although we are giving it a short early-morning boost with mains power to make sure our showers are hot enough. Our fancy electric gate has been absolutely trouble-free, and our mostly-solar-powered hall lighting is fine.
      And we’re still thinking about an EV.

    1. I’ve just seen an advertisement on facebook (which means it’s aiming for the bottom of the market) for batteries “from $3980” to recoup the (quoted) 20c/kWh difference between the feed-in tariff and the cost of mains power at night. On those figures, and assuming a 5 or 6 kW PV system, the pay-back time is still ten years.

  2. A hidden cost of the solar-powered automatic gate is that it relies on a battery and batteries have limited lifespans. That one, in fact, lasted three years. (We have just replaced it after noticing that we couldn’t open the gate in the early morning but needed to wait until sunshine hit the panel.) The replacement cost, $100, means that the real running cost is $30 per annum, not zero. That’s not a problem, but it’s a reminder that a Tesla Powerwall or its equivalent will also have a recurrent (sorry about the awful pun!) cost.

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