Solar Hot Water System Installed on a roof

Energy performance comparison for hot water heat pumps and solar hot water.

It’s difficult to compare COPs between heat pumps and solar hot water systems because it depends on when you are measuring them.  A solar hot water system using only 30W for the pump will be returning up to 2kW of heat energy to the cylinder, giving a COP of 67.  But the electric element is required to boost the system some of the time.  So averaged over the year an Apricus solar hot water system will reduce electricity use for water heating by around 75%, giving a COP of 4.

A Reclaim CO2 hot water heat pump also has an efficiency range while operating which averages out at around a COP of 4.  So the systems are very comparable and we would expect both to reduce electricity use for water heating by at least 75% on an annual basis.

The other factor to consider is how the energy that is running the system or doing the boosting is provided.  The Reclaim has to use electricity to operate so if the household has PV panels with at least 1.2kW of spare generation during the daytime then the heat pump could be run pretty much entirely off their self-generated electricity, making it extremely efficient in terms of net energy use by the household.  Alternatively, there are good electricity tariffs out there which give up to 3 hours of free power a day and again the heat pump can be operated in this time making it extremely cheap/free, for example, Contact Energy Good Nights tariff.

An Apricus solar hot water system will need boosting at times of low solar gain (cloudy weather and shorter days in winter).  This can be provided by electricity but not from PV panels because they generate at the same time and have parallel limitations on generation in poorer weather.  The electricity for boosting can be done on an off-peak or free time of use tariff if the household is willing to limit boosting to these times.  Having a slightly larger cylinder helps with storing enough hot water for this arrangement.  Alternatively, an Apricus solar system can be boosted with a wetback especially in a rural setting for pretty much free hot water year-round.

So the bottom line is that both systems will deliver around the same reduction in total energy use.  The deciding factor is how the remaining energy is provided and which system suits the households specific needs.

Higher COPs (coefficient of performance) equate to higher efficiency, lower energy (power) consumption and thus lower operating costs.