Solar vs Heat Pump Energy Savings

Let’s first touch on some basics with regards to solar water heaters and heat pumps.

Solar water heaters use the radiation from the sun to generate heat. The size of the solar panel will determine how much energy can be collected from the sun. So, if we for example have a 3m2 solar panel connected to a 150L geyser this might give us 150L of 60°C water at the end of a warm sunny day but, during cooler days with less sunshine, it might only be able to heat the 150L to 35°C.

In this case we would need an electrical element to heat the water further. If we have a solar panel that is only half the size (1.5m2) we would only get out half the energy and an electrical element will need to do the rest.

Also if we assume we have 150L of 60°C water at the end of a warm sunny day and we use half the hot water in the evening, the water will more than likely be cold in the morning and so if someone would like to shower in the morning, an electrical element again will need to heat the water to a useable temperature.

From the above it should be clear that solar water heaters do rely on electrical elements to provide hot water at all times.

A high efficiency properly sized solar system (double the geyser size compared to a normal electrical heating installation) installed in a place like Johannesburg could provide almost all the hot water required if the user is willing to use hot water only in the evening or only in the morning. If you however install this same system in Cape Town you would typically not be able to get more than a 60% saving. The reason for this is that Gauteng has a very constant solar irradiation level all year round while Cape Town has very high levels of summer irradiation and very low levels in winter.

However, the sad truth is that most solar systems in South-Africa are undersized and will provide much less than a 50% saving on the water heating bill. We see so many houses where families of 3 or more are living having just a 2m2 solar panel on the roof connected to a 150L geyser. Most likely the company that sold it to them promised them big savings but it is simply physically impossible.

Please also note that if a high efficiency properly sized solar water heater gives a saving of 50% on your water heating bill this will not result in a 50% saving on your total electrical bill unless the geyser is the only electrical device in your house.

Domestic hot water heat pumps work slightly different. The heat pump uses a small amount of electricity to extract a lot of energy from the surrounding air. A heat pump is also using the energy from the sun but only indirectly and so it can work day and night, winter and summer. The efficiency of a heat pump is called the COP. A COP value of 4 means that the heat pump produces four times as much thermal energy as what it uses electrically – in other words a 75% saving on the water heating bill.

Unfortunately the COP of a heat pump is dependent on the ambient and the water temperature and so, in a practical domestic hot water system using a high efficiency heat pump, a more realistic annual COP value is 3 – in other words a 66% saving on the water heating bill.

A high efficiency heat pump like the ITS-5HDP super takes about 1.5 hours to re-heat a 150L geyser, which is used in most households in South Africa. This enables you to always have hot water at a fraction of the cost no matter when or how much water you use.

My neighbours are a family of 4 (husband, wife and 2 young boys) that use water moderately. Measurements we have done on their 200L geyser show an average consumption of 16kWh/day. They would like to save on their electricity bill and the most cost effective way is to install a solar water heater or a heat pump.

Let’s first look at the solar water heating option. On a 200L geyser most solar installers will put a 2.4m2 solar panel (flat plate or equivalent evacuated tube). A top of the range high efficiency 2.4m2 solar panel that is perfectly installed will provide a maximum thermal output of 7.3kWh/day (based on an average solar radiation of 23MJ/ m2/day and some typical thermal pipe losses).

At an Eskom tariff of R2/kWh this will equate to a saving of R32556 over 5 years (assuming a 10% annual electrical tariff increase). A high quality solar system like this will cost them about R18500 for a retrofit system (solar panel connecting to the existing geyser) and about R25 000 for a complete system (solar panel plus new solar ready geyser).

With a high quality domestic hot water heat pump working at average South-African ambient temperatures you will conservatively get a COP of 3. With a COP of 3 they would on average save 10.66kWh/day and therefore R47585 over 5 years. A heat pump like this would cost them about R25000 fully installed.

The Jones’s are also a family of 4 but they are more liberal with their water use. They are using an average of 30kWh per day on their 200L electrical geyser. Since a 200L geyser can only store 200L of hot water, connecting a bigger solar panel on it will not linearly increase the saving. But the Jones’s do have people in the house during the daytime and so we can go for a bigger 4m2 high efficiency panel. This solar panel will give an average of 12kWh/day and therefore a saving of about R53500 over 5 years. The 4m2 solar system will be about R4500 more than the 2.4m2 system pricing given above.

Should the Jones’s decide to rather go for a domestic hot water heat pump, we can use exactly the same unit size of for my neighbours. Again working on a very conservative COP of 3 then the Jones’s will save 20kWh/day and therefore R89195 over 5 years.

From the examples above it can be seen that even for a family that use water conservatively the heat pump will pay for itself in just over 3 years and provide a much bigger long term saving than a solar system. The solar panel savings calculated above also assumes that the solar collector panels are mounted Solar-North with the optimal inclination. Variances in the facing and inclination of the solar collector panels will decrease the systems output.

With the Jones’s the heat pump will pay for itself in about 1.7 years while the solar system will take about 3.5 years.

The life expectancy of a heat pump is about 10 years. Solar water heaters is often stated to have a life expectance of 20 years but do keep in mind that it really is only the panel that is expected to last that long. Many solar water heating system uses a controller and pump and these will reduce the system life expectancy to around 10 years. Thermosiphon solar heating systems (no circulation pump and differential temperature pump controller) should be the life expectancy “winner” if one ignores the hot water storage tank needed in all systems. Both solar systems and heat pumps need to be serviced annually to ensure optimal performance. Servicing is basically just cleaning the system and making sure everything is working correctly.

A solar system can provide a bigger saving than a heat pump but for that the solar system needs to be oversized and water usage patterns needs to be adjusted. Typically you need double the volume of hot water that what you would need for a normal electrical geyser or a heat pump system. In coastal areas like Cape Town however the winter irradiation is much lower than summer irradiation. This means that even if your solar system was sized for 100% of your solar usage in summer you will have only about half the thermal output you need in the winter and the electrical element would need to do the rest. Winter is also when people use the most hot water and so a heat pump would almost always be a better solution in areas like Cape Town.