Maximising Your Solar: How The Eddi Device Can Power Hot Water The Smart Way
- Ben Henderson
- Dec 19, 2025
- 5 min read
Updated: Dec 23, 2025
If you already have solar panels, you know the feeling on a sunny day when your system is generating more than your home can use. You might export that surplus back to the grid for a small payment, then buy energy back later at a higher rate. The Eddi device gives you a smarter option. It diverts spare solar into your hot water cylinder so you store energy as heat, reduce imports, and make your solar work harder for you.
In this guide, you will learn what Eddi is, how it works, typical savings, and how to pair it with a battery and smart home tech for the best results. You will also get simple steps to maximise the value of your solar investment across the year.
What is The Eddi Device & How Does it Work?
Eddi is a smart power diverter from myenergi that sits between your consumer unit and your immersion heater or other resistive loads, such as towel rails or underfloor heating. It constantly measures your home’s power flow, compares generation with usage, and when it detects export, it routes that surplus into your immersion heater instead of sending it to the grid.
A few key points about how it operates:
It uses proportional control, not just on or off. If you have only 300 W spare, Eddi will send 300 W to your immersion and ramp up and down in real time.
It prioritises self consumption. House loads are always met first. Only genuine surplus is diverted.
It works with or without a battery. With a battery, you can set priority rules, for example, charge the battery first during the day, then heat water, or the other way round.
From your side, you simply set target temperatures and schedules in the app. Eddi takes care of matching spare solar to your hot water needs automatically.
Why Heat Water with Solar Energy?
Heating water with mains electricity or gas adds up. A typical UK home uses around 3 to 5 kWh daily for hot water. If your immersion is powered mainly by surplus solar, you can cover much of that energy for free.
You can use solar energy to heat water in two common ways:
With a diverter like Eddi that powers a standard immersion heater.
With a dedicated solar thermal system. These can be efficient in summer but are separate systems. Eddi is simpler to add if you already have PV.
For households with a hot water cylinder and immersion element, Eddi is an easy win. It often costs less than adding several extra panels, yet it lifts your self consumption rate immediately.
Typical Savings & Payback
Savings vary with roof size, immersion tank size, household routine, and your import and export tariffs. As a rule of thumb:
In spring and summer, many homes can meet most of their daily hot water demand with diverted solar. That can be worth 20 to 60 pounds per month depending on usage and tariffs.
In shoulder months, Eddi can still cover a meaningful share of hot water, reducing boiler run time or immersion imports.
Over a year, customers often see several hundred pounds of savings and a payback period commonly in the range of 2 to 4 years. If your export rate is low relative to your import rate, payback trends faster.
These are indicative figures.
When we survey your system, we estimate output, diversion potential, and likely savings based on your tariff and hot water routine.
Eddi Plus a Battery, What is The Best Order of Priority?
If you have or plan to add a battery, the right priority depends on your goals.
To minimise imports, charge the battery first, then divert to hot water once the battery has reached a defined state of charge. This ensures you have stored electricity for evening use when rates are higher.
To minimise boiler or gas use, prioritise hot water during strong solar hours, then charge the battery with remaining surplus. This can be ideal if you are moving away from gas or you want the cylinder hot for
evening showers.
Many homes blend both strategies. For example, set the battery to charge to 60 to 80 percent by mid afternoon, then let Eddi take surplus for hot water. If your battery includes time of use charging overnight, you can top up the tank in the morning with cheap rate power as a backup.
We can help you tune set points and schedules so your system adapts across seasons.
Smart Control & Integrations
Eddi integrates within the myenergi ecosystem and can coordinate with devices like the Zappi EV charger. That means you can set sensible priorities automatically. For example, top up the car to a minimum miles target, then heat water, then export.
Useful tips:
Use boost only when needed. Keep manual boosts short so you do not flatten your solar potential later in the day.
Set temperature bands to avoid frequent cycling. A wider band reduces on off switching and still gives you reliable hot water.
Review the app data monthly. Check how much energy was diverted and adjust priorities with the season.
If you are considering smart EV charging with surplus solar, have a look at the zappi ev charger for seamless integration.
How to Maximise the Value of Your Solar Panel System
Getting the most from solar is about using more of what you generate at home.
Here is a simple plan:
1. Fit a diverter like Eddi to soak up daytime surplus. This captures a major hidden saving.
2. Add battery storage when it suits your budget. Even a modest capacity can cover your evening peak and reduce imports. Explore options if you want a flexible home storage battery that works well alongside
diverters and EV chargers.
3. Shift usage to sunny hours. Run washing machines, dishwashers, and slow cookers late morning to mid afternoon.
4. Tidy your tariff. Pair your system with a time of use tariff so you can charge a battery cheaply overnight in winter, then let Eddi take solar for hot water on brighter days.
5. Optimise with data. Most inverters and the myenergi app provide clear charts. A quick monthly review will reveal easy tweaks.
6. Maintain your kit. Keep panels free from heavy debris, check cylinder thermostats, and confirm that firmware is up to date. Small checks keep performance on track.
Is Eddi Right for Every Home?
Eddi is ideal if you have:
A hot water cylinder with an immersion element.
Regular daytime solar surplus.
A wish to lower imports and reduce gas or electric water heating costs.
If you have a combi boiler with no cylinder, Eddi will not be a fit unless you add a cylinder. In that case, you might prioritise a battery first to boost self consumption.
Why Our Customers Choose This Setup
Our customers want simple, dependable gains from their solar. Eddi delivers that. It is tidy to install, needs little attention, and starts saving from day one. For many households around Milton Keynes, Leighton Buzzard, and across Buckinghamshire and Bedfordshire, a diverter plus a battery has become the sweet spot for value, comfort, and control.
If you are early in your research, you can learn more about options for solar panels and installation to see how a diverter fits into a complete system. Local readers exploring upgrades might also compare choices for a fox ess battery if storage is on your list.
Summary
Eddi is a clever addition to your solar setup. It detects spare generation and diverts it to your immersion heater so you heat water for free rather than exporting. You can expect strong summer savings, useful gains through the rest of the year, and a clear route to faster payback on your panels.
Combine Eddi with a battery and smart scheduling, and you will maximise self consumption, lower bills, and enjoy reliable hot water without fuss.
If you would like help designing or upgrading your system, we are here to advise on diverters, batteries, and EV charger integration so your home uses as much of its clean solar energy as possible.
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