Solar Panel Power Diverters For Hot Water

It takes a lot of electricity to heat water, and this can result in high bills. Thankfully though, there’s a solution: solar panel power diverters. A solar panel power diverter takes surplus solar electricity and sends it to your immersion heater. Heated water can then be stored for many hours until it’s needed. This article will explain how power diverters work, how to decide if you should buy one, and how to choose a brand.

What is a Solar Panel Power Diverter?

Solar panel power diverter / immersion diverter / power diverter / solar immersion controller / immersion heater controller. Whatever you call it, it’s a small device that’s installed beside your hot water cylinder. Its purpose is to let you use PV solar panels to heat water.

Schematic showing solar panels which supply electricity to an immersion heater via a power diverter.  The provides hot water which is stored in a hot water cylinder (eg for a shower).
A solar panel power diverter uses PV solar panels to heat water

How Does a Solar Panel Power Diverter Work?

If you have solar panels for electricity then you’ll sometimes generate more electricity than you consume. A solar panel power diverter monitors this electricity generation throughout the day. Whenever it detects a surplus (you’re generating more electricity than you need), it switches on your immersion heater. However, it will only divert surplus electricity. Power diverters do this by throttling power to your immersion heater.

How Much Money Can You Save With A Solar Panel Power Diverter?

A solar panel power diverter can provide all or nearly all of the hot water you need from March to October. For the other months it should bring the water temperature up to lukewarm. Once lukewarm, it costs much less to get water the rest of the way to hot. Overall, you can easily save €120 per year for each person in your household. That could be €600 per year for a family of five. Savings will be especially high if your shower draws water from a hot water cylinder.

How Much Energy Can You Store in a Cylinder of Hot Water?

You might be surprised to know that hot water stores energy very well. In fact, a 160 litre copper cylinder can store around 10 kWh of energy. Just make sure it’s well insulated. If not, fitting a lagging jacket is a cheap yet effective upgrade.

Comparing the electricity stored by a solar battery to a power diverter.  5kWh for the battery; 10kWh for power diverter with a small hot water tank; 38kwh for a power diverter with a large hot water tank
Using PV solar panels for hot water lets you store solar energy until it’s needed

Should I Get a Solar Panel Power Diverter?

Probably yes. However, there are a few exceptions.

First, many heat pumps have in-built immersion controls that cannot operate side-by-side with a power diverter. Kensa heat pumps are a notable exception – they work fine alongside power diverters. It’s also possible to work around the issue by installing two immersion heaters: One for the heat pump and another for the power diverter. Aside from technical issues, heat pump owners might question the value of installing a power diverter, when they can already have cheap hot water from their heat pump. While it’s true that heat pumps provide low-cost hot water, a power diverter can take the workload off your heat pump during the summer months, prolonging its lifespan.

Second, it doesn’t make financial sense to install a power diverter if you’re receiving a high rate of export payment for your solar electricity. In this case, it might be more profitable to sell your surplus solar electricity than to use it for hot water.

Finally, for a power diverter to work you’ll need solar panels. Read Are Solar Panels Worth it in Ireland to find out if you should go solar. Or if you’re ready for a quote, then enter your Eircode below. All the installers on our network can include a power diverter in your quote:

Solar Panel Power Diverters and Batteries

A solar panel power diverter soaks up surplus electricity from your solar panel system. So does battery storage. This means there can be conflicts if the system components aren’t chosen correctly. However, with the right model of power diverter this won’t be an issue. You see, there are two main kinds of power diverter: “Zero-export” and “battery-compatible”.

The zero-export kind is great if you don’t have batteries. It captures every last Watt of surplus solar electricity for hot water. The downside is zero-export power diverters can conflict with battery storage systems. For example, zero-export power diverters will sometimes drain electricity from a battery.

Battery-compatible power diverters work well alongside battery storage. They manage this by letting a small amount of power export to the grid (~100 Watts) before kicking into action. This way, the battery has priority and gets to charge first. The power diverter then takes over after the battery has charged.

Can I Still Use My Immersion As Normal For Hot Water?

You might be wondering what happens if you don’t have enough surplus solar electricity for hot water on a particular day. Well, most power diverters come with a “boost” button. This lets you run your immersion using grid power if needed. That way, you can still have hot water even on days without much surplus solar electricity.

Using a Power Diverter With Two Immersion Heaters

A great way to use solar panels for hot water is with two immersion heaters. With this setup, you connect a power diverter to the lower immersion heater and grid electricity to the upper immersion heater. Hot water rises, so the lower immersion heater heats the whole tank using surplus solar electricity. Meanwhile, the top immersion heater only heats the top of the tank, making it much more economical to run. This means that if you do need to heat water using grid electricity, at least you’ll only have to use a small amount. You should also set the bottom thermostat to a slightly lower temperature than the top one to prioritise solar electricity over grid electricity.

Schematic of hot water cylinder with two immersion heaters.  The upper immersion heater is supplied with grid electricity while the lower one is supplied with solar electricity
With two immersion heaters you can be sure to never run out of hot water while also making maximum use of low-cost solar energy

Protecting Against Legionnaire’s Disease

Legionella is a type of bacterium that grows in lukewarm water. This is a problem because you can get an illness called Legionnaire’s Disease by inhaling shower spray that contains Legionella bacteria. To protect against Legionella, you should make sure to heat the entire contents of your hot water cylinder above 65 degrees C at least once per week. Most power diverters can be programmed to do this automatically.

How to Install a Solar Panel Power Diverter?

Solar panel power diverters should only be installed by qualified electricians. The instructions here are just for reference.

First, isolate everything from the electricity supply. Remove the old connections to the immersion heater/thermostat. Next, connect the power diverter to the immersion heater/thermostat. Connect the power diverter to the mains power using a dedicated radial circuit. Install the CT sensor near your electricity meter. Enjoy a nice solar-powered hot bath as the reward for your hard work!

Solar Panel Power Diverter Brands

While there are dozens of power diverter models available, some of the most popular options are listed below. The most important consideration when choosing a model is to make sure to get a “battery compatible” version if you have a battery system. It’s also worth getting a model with boost and timer functions. If in doubt, EnergyD recommends the myEnergi Eddi because it has a fantastic range of features and a good reputation for reliability. It also has battery-compatible and zero-export modes, making it suitable for just about every installation.

Solar iBoost+

Solar iBoost solar panel power diverter showing display read-out and boost button

A common power diverter on the Irish market. Features include:

  • Boost Button (Programmable)
  • Programmable on a weekly cycle to kill Legionella
  • Dual-immersion function
  • Battery-Compatible

SOLiC 200

SOLiC 200 Solar panel power diverter showing a simple design with only one button (boost)

A simple and popular zero-export power diverter. Features include:

  • Boost button (90-minutes)
  • Zero-export
  • Wired energy usage sensor

myEnergi Eddi

myenergi Eddi solar panel diverter showing graphical display and 4 control buttons

An advanced power diverter known for reliability. Features include:

  • Boost function
  • Programmable
  • Remote access via mobile app
  • Zero-export and battery-compatible modes
  • Recommended by EnergyD


If you have a hot water tank and solar panels, then you should probably get yourself a power diverter. EnergyD recommends the MyEnergi Eddi for its good reliability reputation and excellent features.