A Solar Battery That's 75% Cheaper Than a Tesla Powerwall

Everyone seems to be talking about the Tesla Powerwall and other batteries for home energy storage.

Or, whether it's even worth pairing a solar battery like the Tesla Powerwall with their solar photovoltaic (PV) system.

But I'd like to introduce a simpler and much cheaper way to store energy: hot water.

Please Note: PV solar panels are those that turn sunlight directly into electricity. PV panels are not to be confused with solar hot water, which I am not discussing here.

Hot Water as Solar Energy Storage

Most people are familiar with storing energy in the form of electricity in a battery. We all use mobile phones with lithium battery storage on a daily basis. Maybe this is why solar batteries and the Tesla Powerwall 2 have such appeal: we appreciate how they work!

You can also store energy in other forms, such as hot water. And hot water is the biggest energy user in most households. Of course, many homes already have a hot water tank, but probably don't see it as 'energy storage'.

It's time to change how you look at your hot water tank.

You can effectively store excess solar PV power as hot water using any electric hot water tank. However, one problem with electric storage hot water tanks, is that they have a very high input power. Their power draw is typically 3.6kW, making them a poor match for many solar power systems, especially on a cloudy day.

This is why I recommend installing a heat pump hot water system if you have solar panels. Then, program it to operate during the day (say 9am to 4pm or similar) so it can 'recharge' directly from solar power.

Heat pump hot water systems only consume between 0.5kW and 1kW when operating. This makes them a perfect match for solar power, even on cloudy days.

Tesla Powerwall 2 vs Heat Pump Hot Water

A heat pump hot water tank stores about the same amount of usable energy as a Tesla Powerwall 2, but costs over 75% less. It's also about three times more energy efficient than other hot water systems.

Hot water obviously can't run your other appliances. The key here is to understand that hot water is a major energy user, and upgrading to a heat pump is far cheaper than batteries at the present time.

Here's how the numbers stack up:

  • A Tesla Powerwall solar battery costs around $14,000 (installed) and stores 13.5kWh of energy as electricity.
  • A quality heat pump hot water system costs around $3,000 (installed) and stores about 13kWh of energy as hot water.

A heat pump hot water system uses around 4kWh of electricity to 'recharge' this 13kWh of usable hot water. So not only is a heat pump far cheaper than a battery, it's also a more efficient way to use and store energy.

How Much Energy is Stored in Hot Water

To measure how much energy is 'stored' in hot water, you need to understand the following.

The specific heat capacity of water is 4,184 Joules/kg ˚C. This means that "water has to absorb 4,184 Joules of heat for the temperature of one kilogram of water to increase 1 degree celsius (°C)" (link).

To calculate how much energy it takes to heat a certain volume of water you can use the following formula:

Energy (J) = mass (kg) x specific heat capacity (J/kg ˚C) x temperature change (˚C).

Let's say water is supplied to your property at around 15˚C. A Bosch Heat Pump that we used to sell has 270 litres (ie. ~270 kg) of storage capacity and it's factory set to 56˚C. This means the tank contains the following amount of stored energy as hot water, once heated up:

Energy (J) = 270 kg x 4,184 J/kg ˚C x (56 - 15)˚C

Energy = 46,316,880 J (Joules)

Energy = 46.3 MJ (Megajoules)

Energy = 12.9 kWh (1 kWh is equivalent to 3.6 MJ)

    Tesla Powerwall 2 vs. Heat Pump Hot Water 'Solar Battery'

    Putting all the above together we have:

    Product Tesla Powerwall 2 Lithium Battery Bosch Heat Pump Hot Water Tank
    Recharges Using Electricity Electricity
    Stores Electricity Hot Water
    Energy Storage Capacity 13.5kWh ~13kWh
    Energy Efficiency 90% (for each 1kWh of electricity you put in to the battery you can get 0.9kWh out) 330% (for each 1kWh of electricity used, you get approx. 3.3kWh of hot water)*
    Cost ~$14,000 (installed) ~$3,000 (installed, after rebate)
    Source Tesla Bosch**

    * Don't worry, this is not in defiance of the laws of Physics. A heat pump (like your refrigerator or AC) uses electrical input to transfer heat from ambient air into the water. This is what allows it to achieve 3.3 factor efficiency. This is known as its Coefficient of Performance or 'COP'.

    ** This particular Bosch heat pump hot water tank is no longer available in Australia. However, the numbers are similar for brands of comparable size and quality, such as Stiebel Eltron.

    So... Don't Install a Tesla Powerwall!?

    This post is not meant to be against home battery storage. It's not even anti- Tesla Powerwall. After all, there are many electrical loads which are well suited to solar batteries such as the Tesla Powerwall 2. But it is pro- energy efficiency and common sense.

    My point is really to consider the economics of battery storage. And what else you could be doing to store (and use) energy in a more efficient way.

    I have shown above that storing energy (in the form of hot water) with a heat pump is both far cheaper and more energy efficient than lithium batteries. This will likely remain the case for many years to come.

    If you are considering a solar battery, and you don't already have an efficient home, including efficient electric hot water storage, you're probably doing things in the wrong order.

    Want to learn more about how to save energy and reduce costs?

    Looking for more economical ways to save energy? Here are some of our best selling energy efficiency products:

    By Ryan McCarthy |

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