In this article, we will look at how an existing hot water tank can provide an alternative to adding a battery storage system, and in turn save the homeowner a considerable amount of money, & installation time, whilst reducing not only the homes/business electricity consumption, but also reliance on natural gas, and bills. 

With astronomical increase on bills effective from April 2022, with the increase of the energy price cap, getting the most out of your solar system is critical. 

Solar PV energy generation often does not match the energy demand of a typical home during a typical day. A ‘Due South’ system’s energy generation profile will look something like the below on a nice bright day, with a slow and steady gain in power, peaking in the middle of the day, and then a slow and gradual decline. This graph relates to a period in March. You’d see a more gradual and wider bell curve during peak summer months, with longer, brighter days. 

Moments of cloud cover, shading or rain will cause the power generation throughout the day to be more varied, the graph below shows a dip at 13:45pm where the system loses a considerable amount of power and then improves, before tailing off for the day. 

Below is a typical home’s energy demand profile over a given day. The average energy consumption in ‘standby’ i.e when basic things such as fridges/freezers/lights/heating functions is around 250watts. In the morning, kettles and other equipment cause a spike in demand, followed by a ‘wash cycle’. As we move through the day, a dishwasher may cycle, and for dinner the oven may be used, with an increased load as more lights are put on during the evening/darker moments. 

Let's now look at this with the overlay of energy generation of a 1.6kWp system. 

As you can see, there are large parts of the day where the home’s energy demands outstrips (greater than) supply. This is represented by the blue shaded area. Equally, areas shaded in red, with a white backdrop show times where solar pv generation is greater than the home’s demand for energy. 

In a grid tied system, this energy would normally be fed back to the grid, and you may receive a small credit for this surplus. 

There are a number of solutions to better manage the energy you make from your Solar PV system, to ensure you are either storing it for use when you need it, through the use of a battery, or diverting it to your hot water tank immersion heater, in order to heat your water. 

We will take a look at both scenarios here, and look at the pro’s and con’s of both, and then investigate further how to ensure the system is operating efficiently. 

1. Battery Storage

Advantages

• Optimize energy efficiency in home, store excess energy and provide it when the home needs it most. The below shows the state of charge of an ESS (energy storage system) battery. You can see the state of charge of the battery is slowly depleting during the day, and then when the home’s energy usage spikes it begins to more rapidly deplete. When the home’s demand reduces, the start of charge of the battery begins to climb quickly, as the solar PV generation kicks in. 100% SOC is then held even against a backdrop of demand from the property, until Solar production starts to drop off, and the batteries then have to feed the load of the home. Throughout the entire cycle, the home is able to avoid drawing on grid energy and is entirely self sufficient/off grid. 

The battery reached its lowest point of (SOC 6%) at 10:20am, and then the solar system production began to outstrip demand, and the battery quickly regained a charge to 100% at 14:38pm. 

• As with most improvements to an energy system, you are then going to reduce your energy bills, through greater self independence. 
• Reduce carbon footprint further, through reduction on grid requirements for energy 
• One of the major advantages (provided your system is set up correctly) when having a battery installed, is a backup if the grid goes down. Many of the latest inverters have a ‘power point’ , which can be wired to a number of critical circuits in your home, to run essential loads in the event of a total grid failure. One example is the Fronius Gen24 inverter which comes with such functionality. 

https://www.fronius.com/en/solar-energy/installers-partners/products-solutions/features/backup-power-function

Drawbacks

• The first drawback of a battery system relates to price, and up-front investment, additional cost of installation, as you will often need a superior inverter with the additional technology for battery management, along with additional metering to control the system. 
• You’ll need suitable space for the battery to be located, away from excessive sources of heat, although they can often be placed outside if space permits. 
• Another drawback comes down to size, often a small battery is likely to do little toward balancing out some of the peaks of energy demand, and with the introduction of electric cars, the battery on a car, far out strips the size of the average battery in a solar system. Vehicle to grid systems whereby the car battery is able to feed the home’s energy demand, is going to improve things certainly, but for now batteries remain an expensive way of ensuring you are able to capture that excess solar PV energy, and avoid feeding it back to the grid.
• Batteries as with any electrical system also have a ‘lifespan’ and overtime the efficiency of the battery will reduce. Typically you can expect approx 5000 charges/dis-charges with a Lithium pack, so at least 10-15 years. 

2. Immersion heater feedback 

The second/alternative option we want to explore is fitting a device which monitors how much energy you are exporting back to the grid, and ‘sends’ that energy to an electrical coil in your hot water tank, and in effect diverts any excess electricity you have, to ensure it's used in your home or workplace. 

(We are in no way affiliated to this product) it just works… 

The Solariboost has a CT (current transformer) clamp which gets placed around your positive cable that feeds your home’s electricity. This is linked to a small transmitter, which is sending information to the main ‘iboost’ which is directly connected to your immersion heater, controlling how much power should be used. 

The advantage here is that you are then able to use all that extra energy you are using to heat your hot water, and avoid burning gas to do just that. The iboost is relatively inexpensive, and of course there are other models that perform a similar function, but we found with the simple display, it’s a useful bit of kit. 

In this particular installation, the iboost was fitted with the rest of the solar equipment in the loft nearby a hot water tank, but provided there is a live supply suitable rated up to 3kw, it can be placed anywhere in proximity to the tank. 

There are a number of drawbacks to this system which I want to explore in further detail, as I do feel they open up the options for looking at more intelligent systems, for better managing one’s extra energy output. 

There are typically 2 main ways of heating hot water in a home using natural gas which we need to explore as a pretext. 

1. Combi boiler (on demand) when hot water is requested, the boiler is initiated, and heats the water instantaneously. The drawbacks are often around flow pressure, and this system is designed for smaller dwellings where multiple hot water consumers are unlikely to be used at the same time. This may be partly overcome with additional pumps, but depends on the heat output of the boiler. 

The solar Iboost is only suitable for systems with a hot water tank, i.e not a combi boiler setup. 

1. The first drawback or challenge I have found when using a system such as this, is that it's quite a challenge to program the temperature control on an immersion heater. These are typically not digital, and don’t provide a good level of feedback. What’s important here is to ensure that the temperature is not set too low, otherwise the iboost will always see an open circuit (i.e the water is hot enough) and won’t send excess energy to it. By increasing the temperature of the thermostat on the immersion heater, you are improving the likelihood of a closed circuit (i.e the immersion heater circuit is closed, and looking to heat the water, i.e the command is (please send electricity) 

This takes some fiddling, too hot, and you risk overheating the water in the tank which comes with its own dangers and concerns, too low, and you risk wasting the excess energy you have generated. I feel both the iboost and immersion heater probes themselves could have improvements to better match and set up the temperature of the coil. 

2) The second drawback that I found when playing with the system relates in a similar way to how solar energy production and demand are often at different times of the day. Typically households may use hot water in the morning and the evening when solar production is at its lowest point. During the middle of the day, hot water demand is likely to be at its lowest point, many of today’s household consumers such as washing machines and dishwashers have their own internal heating elements to deal with hot water demand. 

What this therefore means if that if you are relying on solar energy to heat an immersion heater probe, to provide you with hot water, it's often at the wrong times, and you’d find in the morning your water wasn’t warm enough, and whilst excess solar PV energy in the day would heat the tank up sufficiently, for earlier showers, later in the evening the temperature of the tank would start to drop and you’d then be reliant on the gas again to heat your water. The graph below illustrates just this, the red area’s show demand when excess energy to heat the hot water is unlikely to be present. 

3) A third and final drawback or concern relates to control. Most homes will have a method for controlling when the ‘gas boiler’ is set to heat hot water. This may be through an intelligent system such as ‘heat miser’ or a more basic thermostatic control, with a few set points during the day. 

The problem is these systems are not intelligent. In an ideal world, if solar production is likely to outstrip the demand of the home, you’d like to keep your hot water off for as much as possible during the day, to ensure the set point of the hot water tank is low, and therefore the excess solar PV is used to heat the water up. However, if in doing this, you turn the system off, and then on a cloudy day, or high usage day of energy at home, you don’t send much energy to the immersion coil, you are left with cold water. As the systems are unable to communicate often, you are left trying to balance having the hot water on enough to ensure when it comes to shower time, you are not left cold, but also ‘off enough’ to ensure you are utilizing the excess PV energy. Coupled with setting the right temp on the thermostat it's an incredibly tricky endeavor. 

Having tested initially, we did however find the following results in a single month period. 

Having seen relatively poor results in March, I have tweaked the immersion heater temperature control upwards, to ensure the system is pushing back to the grid less, and heating the hot water more. I will in due course report on subsequent results. 

Conclusion 

Battery storage solutions are a fantastic way of reducing your dependence on the grid, and storing excess solar energy for use when you need it most. If you cannot quite afford the investment, and provided you have a hot water tank, an iboost or similar device will help you retain some of that excess energy, but as we address in detail, its a real balancing act, and it may drive you quite simply round the bend trying to eek out the most of your watts generated. With either solution you’ll be reducing your grid dependence, and using a cleaner source of energy production, leading to a cleaner greener future. 

Thank you