• By Cliff Mail

Adding a Tesla Powerwall battery to a domestic solar system


Rod & Christine Brown of Kerikeri were early adopters of solar and after 7 year’s of running a 4.2 kWh solar panel installation they found that a good deal of the energy being produced was being fed back to the grid.

On sunnier days, particularly in summer, their solar array was generating as much as 26 kWh of power in a day. They do not even go close to using that amount of energy during the hours that it is produced. As a result, that surplus energy was being fed back into the grid generating a mere 8 cents per unit (kWh), well below the cost of importing power from the grid, which they had to do once the sun went down.

So they decided to again be early adopters and became one of the first owners of a Tesla Powerwall battery in N.Z making their purchase in November 2017. Since then, they have been happy to be feeding their surplus solar energy into the battery rather than into the grid.

The Powerwall has a capacity of 14 kWh and a maximum draw down of 5 kilowatts and it has already had a significant effect on their power costs.

Rod & Christine's household electricity usage

Since installing the Powerwall their drawdown of electricity from the grid has fallen dramatically. In the year ending March 31st 2018 their annual power bill was only $20.5 (including a $200 dividend from Top Energy).

The total energy drawn from the grid is greater than shown because the switch board has been split so that; the 5.5 kW ducted heat pump (their only form of heating), electric hot water boost for their solar water heating and the oven are not fed from the battery.

On this basis they find all normal household circuits are easily run off the battery. The maximum draw down with the toaster, kettle, lighting circuits, TV and use of the water pump is about 3kW - well within the capability of the battery and without requiring a compromise in their activities.

One important feature of the battery is security of supply. Since commissioning it in November 2017 there have been 21 power outages totalling 25 total back-up hours from the Tesla battery.

For Rod and Christine, it was an expensive purchase but security of supply and low or zero monthly energy costs were enough to convince them to make the switch.

With the price of batteries continuing to fall, the addition of storage capabilities to your solar system will become more enticing.

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