Can you heat hot water with solar?

Fast read

A hot water diverter is a device that detects surplus solar power in a home not being used and diverts it to heat hot water, rather than sending it back to the grid. This maximises the self-consumption of solar energy.

Direct PV hot water heating directly powers an electric hot water heater, typically using heat pump technology, with DC power from solar panels without conversion to AC. However, once the water is heated, excess power is wasted. Most homeowners prefer to power their entire home, including hot water, using a grid-connected solar PV system.

To enhance savings, some install a timer to heat water during peak solar production. However, for optimised efficiency, a solar diverter is recommended, which can also divert power to other appliances.

What is a hot water diverter and direct PV hot water?

In homes with electric hot water heaters, the hot water heaters are often the appliances with the largest use of electricity. This is at least 25% of the power used within the average home. So if we can find a way to make hot water generation more efficient, then we also have a clear path to lower electricity bills. As such, when installing solar panels on your home, it makes sense that you would want to use solar power where possible to heat your hot water. This is much more cost-effective than using power from electricity. There are a few different ways to use Solar PV panels to power an electric hot water system.

The most practical and efficient solution to this dilemma often comes down to the employment of a technology known as hot water diverters. Hot water diverters are instrumental in leveraging excess solar energy, which is otherwise fed back into the grid, to heat water. This not only optimises solar energy usage but significantly curtails electricity costs in the long term. So let’s look at the various options.

Direct PV hot water heating

This is where solar panels are installed to power an electric hot water heater, usually with heat pump technology. In this case, the DC power from the panels can directly power the heat pump element without being converted (via an inverter) to AC power that could be used in the house.

These hot water systems have been used in areas like Western Australia, where grid-connected solar power is limited to 5kW capacity on single-phase homes. It can make sense to have additional PV dedicated to powering the water heater as it’s not connected to the grid. This technology is nevertheless not so popular in other states. 

These hot water heaters have also an electric booster element that would be able to power from the grid when there is not enough electricity from the panels to heat the water via solar PV  during cloudy periods and rain etc. 

In much of the country and for many homes, these direct solar hot water systems would not make sense as they take up valuable roof space for panels that are only used for heating the hot water. Once the hot water is heated, the additional electricity from the panels goes to waste as it cannot be used in the rest of the house.

Moreover, these systems do not possess the ability known as ‘catch power,’ which is the ability to harvest surplus solar power when available. This lack of catch power feature results in a lower level of efficiency and savings as compared to other alternatives. So what else is on offer? 

gas boiler
The old central gas boilers, which run all day to keep the water at a certain temperature are on the way out 

Hot water heating from grid-connected solar systems 

The more popular option for most would be to install a solar panel system big enough to power the whole home, including the hot water generation. In many cases, the easiest and cheapest way to set this up is to have a timer on the water heater so that it turns on and runs in the middle of the day when the solar PV system is generating plenty of electricity and can power the heating of the hot water. Then set the time to turn on one more time in the evening, when tariffs are low, for a boost up and one would have found a way for one of the cheapest hot water generation methods. 

While this works well when the sun is shining and the solar system is working full throttle, during poor weather and during the middle of winter, this method carries the risk that the water heater will be to a large extent powered from the grid at full electricity rates. So in summary – yes significant savings will be achieved, but it is not perfect. 

This system doesn’t have the catch power capability either, which implies that it can’t automatically harness excess solar power to heat water whenever such surplus power is available. This contributes to its limitation in achieving optimal energy savings and reducing grid dependency, especially during less sunny periods.

residential hot water tank
The ideal way to maximise savings and self-consumption of solar is by what’s called a solar diverter

Solar diverter

A solar diverter is an electronic device that can detect when there is additional capacity of solar power being generated that is not being used in the house and divert that electricity to heat hot water right at that time – rather than send the power back to the grid. 

As the amount of electricity from a solar system can fluctuate on many days with clouds passing over and depending on the time of the day, the diverter technology can adjust the supply of electricity to the water heater based on the amount of solar being generated throughout the day. 

If clouds come over or the weather turns bad and the amount of solar drops, the diverter can stop supplying electricity to the water heater, and pick it up again, when solar power production resumes. The diverter can more efficiently supply electricity to the hot water heater from solar power and minimise the need to heat the water from the grid.

The concept of catch power is seamlessly integrated into the solar diverter technology. Catch power has the ability of the system to capture excess solar power, which would otherwise be wasted or fed back to the grid, and use it for heating water. This feature significantly augments the efficiency and cost-effectiveness of the system by ensuring that a substantial amount of solar energy is utilised, thus lowering the reliance on the grid.

Notify of
Inline Feedbacks
View all comments

Find your local installer