Should I Install a Solar Hot Water Diverter?

If you’ve got solar panels, you might be wondering whether a hot water diverter still makes sense.

The answer has changed in recent years.

It used to be a clear “yes”. Now, it depends heavily on how you heat your water and how your tariff works.

What does a diverter actually do?

A hot water diverter takes surplus solar electricity and sends it to your immersion heater, storing energy as hot water instead of exporting it.

This was very effective when export payments were tiny.

Today, electricity — even exported — has meaningful value.

The key shift with modern tariffs

Most TOU tariffs now include:

• Higher export payments (often ~10–15p/kWh or more)

• Cheap off-peak electricity (sometimes ~7–20p/kWh overnight)

• Expensive peak periods (often 30p+/kWh)

So instead of just asking:

“Can I use my solar?”

You now need to ask:

“What is my solar electricity worth — and when?”

If you have a gas boiler

Without a diverter:

• You export solar → earn ~10–15p/kWh

• Your boiler heats water using gas → costs ~5–7p/kWh

• 
  

With a diverter:

• You use solar for hot water

• You avoid using gas

• But you lose export income

• 
  

The result:

• You give up ~10–15p

• You save ~5–7p

  
  

Conclusion: you’re typically worse off.

It can still make sense if:

• You don’t have mains gas (e.g. oil, LPG, or direct electric heating)

• You value maximising on-site solar usage for non-financial reasons

  
  

But purely on cost: exporting is often better.

If you have a heat pump

This is where things become more balanced.

A heat pump is very efficient:

• 1 kWh of electricity produces 2.5–3 kWh of heat

  
  

So hot water from a heat pump is already relatively cheap.

Without a diverter:

• You heat water using electricity (often shifted to cheap periods)

• Effective cost of hot water:

  • Typically ~8–12p per kWh of heat (depending on tariff and efficiency)

    
    

With a diverter:

• You heat water using immersion (less efficient)

• You “pay” by giving up export (~10–15p/kWh)

  
  

The key insight: With a heat pump, you’re replacing an efficient system with a less efficient one.

So:

• Value of exporting solar ≈ 10–15p

• Cost of heat pump hot water ≈ 8–12p

  
  

Conclusion: That’s close — often slightly in favour of exporting.

When a diverter might make sense with a heat pump

• You don’t have a battery

• You export a lot of midday solar

• Your hot water runs during expensive daytime periods

• Your heat pump struggles to produce hot water efficiently

• You want to reduce compressor use in summer

  
  

When it usually doesn’t

• You have a battery (this changes everything)

• You can heat water overnight at cheap rates

• Your heat pump is well-optimised

• You’re on a tariff with strong export payments

  
  

The bigger picture: what TOU tariffs reward

Modern tariffs are designed around timing and flexibility.

The highest value actions are:

1. Shift usage to cheap periods (overnight)

2. Avoid peak import times

3. Export when prices are high

4. Store energy (battery or thermal — but intelligently)

A diverter is a passive solution — it doesn’t optimise for timing.

Conclusion

• Gas boiler: Diverter is usually a poor financial choice

• Heat pump: Diverter is borderline / situational→ Often close to break-even, sometimes slightly negative

• With a battery (any system): Diverter is generally not worth installing