Hotwater: Make it efficiently

Question: In Brisbane, Australia what’s up with hotwater?

Method: These data are mostly from Brisbane houses from March to September 2023. Units are kWh/day mostly. Typical family of 3 but varies.

Answer: read on.

Does it matter?

The best unit costs 1kWh/day and the worst 9kWh/day. These have similar usage. That’s about 30c versus $2.75 per day. That’s about $100/year versus $1000/year.

So yeah, it matters.

Hit me with the numbers!

Ok, sure:

Electricity consumption (kWh/day) for a range of hotwater systems

What does that mean? Let’s take a few:

  • hfs01a: Solahart 300L on-roof solar, no timer. Two kids. 1.8kWh/day ($200/year). That’s pretty good.
  • hfs02a: As above, maybe 30% less hotwater use. 0.8kWh/day ($100/year). That’s great. This is the effect of a $50 timer. More on timers below.
  • roop / arn: 300L storage units. 3 occupants. Poor insulation. 7kWh/day ($750). That’s expensive.

Geez, too much information. What’s the answer?


  • Buy a solar hotwater system with tank on roof and add a timer.
  • Expensive but apply for rebate. It will payback in 3 years.


  • Solar pumped or heat pump
  • Expensive but apply for rebate. It will payback in 6 years.


  • Use a electric storage
  • It’s cheap to buy but expensive to run


  • Insulate your pipes and tank well.
  • Keep electric storage if you have solar. Add a timer.

What was that about the timer?

If you have a solar hotwater system, it needs a timer:

Six months of the same hotwater system in the same city with similiar usage. Why are they different?
Zoom in to one day. The blue system doesn’t have a timer. The left hand peak is at 5am, so the water is hot when the sun rises. The green system has a smaller peak at 3pm: it has used the solar panel to heat up. It uses less electricity.