Heat Pump Hot Water or Solar Hot Water in Melbourne?

We’ve sold and installed hundreds of efficient solar hot water systems in Melbourne and surrounds in the last two decades. Over the last few years we’ve been selling less and less solar water heaters and more and more heat pumps. Why is this?

Despite anecdotal evidence from some uninformed parties, (good quality) heat pumps are very well suited to cooler climates. Some studies have shown that the further South you go the more efficient they are compared to solar hot water with the break-even point somewhere around Canberra, depending on your local micro-climate. This makes sense when you think about the details. Solar hot water relies on direct solar radiation, therefore it can only operate during daylight hours and is impeded by cloud cover. The days get significantly shorter in Winter as you go further South and average daytime air temperatures get colder. You also tend to get more cloudy days and cold winds. All of these factors reduce the efficiency of solar hot water systems significantly as there is less energy input from the sun and higher heat losses.

In Summer in Melbourne close to 100% of your hot water can be provided by a  good solar hot water system but you’ll be using an inefficient and power-hungry booster to provide most of your hot water over the Winter. The end result averaged over the year is usually around 60-80% solar input with the rest being made up by the booster. This does not match up very well with a solar power system, as it’s output will also be lowest in the middle of Winter, which is why solar hot water has traditionally been combined with a wetback wood heating system in stand-alone situations.

A good quality heat pump on the other hand is only minimally affected by the time of year. If the air temperature is a bit colder then it will just run for a bit longer to reheat your tank and they can run at night, on cloudy days and in sub-zero conditions. Their efficiency typically results in 70-80% energy savings year-round, making them equivalent or better than a solar hot water system with more predictable costs and better matching to a solar power system.

Matched with a decent solar power system, a heat pump can be close to 100% solar powered : the heat it is extracting from the air is provided by the sun and the electricity required to move it is supplied by your solar panels. This means greater environmental savings on top of the financial savings.

Another argument in favour of a heat pump is that solar panels will give a better return on your roof area. Prime solar real estate on your roof should be covered with PV panels, as they generate energy whenever the sun is out. If you’re not using it then it will be exported for your neighbours to use. A solar hot water system on the other hand will spend the long Summer afternoons just idling and overheating because it’s already done it’s job and heated your tank in the morning. The extra solar panels you can install in it’s place will likely generate enough energy to power a heat pump hot water system.