Are Hot Water Timers and Controllers Still Worthwhile With Solar Batteries?
By Aniket Bhor on in Battery Storage For Solar
For any Kiwi home with a solar power system, the goal is to consume maximum energy from its solar panels while minimising grid power usage, especially because solar buy-back rates are never equal or even close to the retail rate of grid energy.
As such, it makes sense to shift most or all of your water heating consumption to solar, as heating water makes up around a third of your total energy needs. Typically, your water heater only has a temperature sensor that turns on and off to maintain the temperature. This process will not discriminate between grid and solar power, and you may end up paying more if a significant portion of the energy comes from the grid.
Hot water timers and controllers (also known as solar diverters) are great options to maximise your solar self-consumption and import less grid power. But if you already have a solar battery, which is the ultimate solar self-consumption device, do you really need timers and controllers? Let’s understand timers and diverters in the context of solar self-consumption and find out if they’re worth buying if you already have a solar battery.
Hot Water Timers
A hot water timer is an extremely basic electronic device, especially compared to a solar battery or diverter. A timer can be programmed to run only during hours of solar power generation, such as 11 am to 1 pm. This will ensure that your water heater uses maximum solar power to operate.
Besides that, timers don’t have much ‘intelligence’. But the good thing about hot water timers is their low cost. A decent device will cost you $200-$300 with installation, and will certainly save you more than its cost over its lifespan.
It is hard to say exactly how much money a timer can save, because different homes have different hot water consumption patterns. But for example, let’s say your timer costs you $250, and it increases your solar self-consumption by about 5 kWh per day. If the difference between your electric rate and your solar export rate is about 15 cents/kWh, you are saving about $0.75 per day on average. This means you recover the timer’s cost in about a year, which is a good payback period.
While this makes financial sense, you cannot ignore the fact that there is a possibility for shifting your grid consumption to solar further, potentially saving you even more. Let’s take a look at solar diverters and see how.
Solar Diverter
As the name suggests, a solar diverter ‘diverts’ part of your solar energy to certain appliances. And one of the most common appliances that receives energy from a solar diverter is a water heater.
The heating element in a water heater is capable of generating varying amounts of heat based on the time and amount of power it receives. Most other appliances in your home can’t do this - for example, your 200 W TV won’t operate on low brightness or on half the screen if it receives just 100 W. But your water heater is pretty flexible, taking and using whatever amount of energy it gets in a continuously variable manner.
Solar diverters are a bit smarter than timers. They can grab whatever surplus solar energy is about to go to the grid and ‘divert’ it to your water heater. This makes your hot water tank somewhat analogous to a battery - storing energy for later use. However, energy stored in your hot water tank is always thermal energy (unlike the far superior electrical energy in a battery).
Here’s a schematic showing the working of the Fronius Ohmpilot, a popular solar diverter:
Cost-wise, a solar diverter is notably more expensive than a timer. A reliable unit can cost you $1,000 or more. While that sounds a bit high, diverters also boast several useful features that justify their pricing to some extent.
For instance, most diverters come with remote monitoring with a mobile app. You can also pair some diverters with a solar battery and send surplus energy to your battery if your hot water needs are already fulfilled. Some diverters also allow you to set a boost timer to use cheaper grid power during off-peak hours.
Whether a solar diverter saves enough money again depends on your energy consumption patterns. If you are exporting a good chunk of your power to the grid, a diverter would help save decent amounts of money. If you are already using most of your solar power in your home, spending $1,000 or more on a diverter may not be a good idea just to save a few cents every day.
Here is an example case presented by Catch, a solar diverter manufacturer:
In this example, the unit saves you an additional $465, which means your $1,000 investment can ideally be recovered in just over two years, which isn’t bad considering you can save hundreds of extra dollars every year after that. Now let’s talk about solar batteries.
Solar Battery
Solar batteries are the most superior form of energy storage, because they store energy in electrical form, which can then be converted to whatever form you need and not just heat. Think of this as a currency - your employer pays you money that can be used to buy whatever you wish. If your employer starts paying you in a specific form, like grain, you’ll probably call the employment office.
Similarly, diverters and timers can only offer some added heating, which is nothing but stored energy in thermal form. You can’t run your lights with hot water, just as you would with a battery. But of course, a good battery will cost $15,000 or more, and that means timers and diverters are still more desirable to most solar homeowners.
The real question, as we have it in the article title, is not if you should buy one of these three, but if you should buy a timer or diverter when you have a battery. To answer that, we think that modern batteries can do pretty much everything a diverter or timer can do. For example, many batteries allow you to prioritize loads, allowing you to heat your water before any surplus flows into the grid.
Moreover, the main goal of a diverter or timer is not to heat water with surplus solar energy, but to prevent surplus solar energy from going into the grid, and that itself is the main goal of batteries in New Zealand.
Therefore, we don’t think a timer or diverter makes much sense if you already have a solar battery or have plans to install one. But also remember that buying a battery just because you want something more capable than a timer or diverter might be a mistake.