Don't Waste Your Money on Battery Storage Until You Have One of These

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

But I'd like to introduce a smarter and much cheaper way to store energy.

Hot Water as Cheap & Efficient Energy Storage

Much like storing electricity in a battery, you can store energy in a tank of hot water. And as hot water is the biggest energy user in most households, this energy will be readily consumed. 

The Bosch Heat Pump Hot Water Tank which we are now selling does exactly this. It can be programmed to operate at the most cost effective times for your household. And it's energy efficient, using just 600 Watts of power to operate. This makes it a perfect match for solar power, even on cloudy days.

Tesla Powerwall vs Bosch Heat Pump Hot Water

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

Of course, hot water can't run your other appliances. The key here is to understand that hot water is a major energy user, and upgrading to the Bosch Heat Pump is far cheaper than batteries at the present time.

To put this in context:

  • A Tesla Powerwall battery costs around $12,000 (installed) and stores 13.5kWh of energy (as electricity).
  • A Bosch Heat Pump costs around $3,000 (installed, after rebate) and stores about 13kWh of energy (as hot water).

Due to how it works, the Bosch Heat Pump only uses around 4kWh of electricity to 'recharge' this 13kWh of usable hot water. So not only is the Bosch 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. Now, we know the Bosch Heat Pump has 270 litres (ie. ~270 kg) of storage capacity and that it is factory set to 56˚C (which you can adjust). 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. Bosch Heat Pump for energy storage

    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 $12,000 (installed) $3,000 (installed, after rebate)
    Source Tesla Bosch

    *Don't worry, this is not in defiance of the laws of Physics. The Bosch unit is a heat pump (like your refrigerator or AC) so it 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 Post is Not Anti-Battery Storage - It's Pro-Energy Efficiency (& Common Sense)

    The above post is not explicitly anti-battery storage. After all, there are many electrical loads which are well suited to batteries such as the Tesla Powerwall 2. 

    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 Bosch Heat Pump Hot Water Tank is both far cheaper and far more energy efficient than lithium batteries. This will likely remain the case for many years to come.


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