If you have been following this blog since 2017 you may recall we moved into the Perth house in May of that year. Initially we had very few household effects whilst we waited for our possessions to be delivered from storage. In August of 2018 we had a 6.4kW solar array fitted to the roof of the house. This certainly lowered our bi-monthly electricity bill. However it did annoy me that we were paying roughly six times the cost of a unit of electricity (kWh) from the utility company than they were paying us for the surplus power we generated and returned to the network.
This afternoon I sat at the pc and went back through the data in the electricity bills from May 2017 until the end of 2019.
Our electrical consumption for the period May 2017 to Dec 2019
The blue lines show how little electricity we were using when we first moved it. However note it was steadily increasing. The very high peak in Jun-Aug 2018 is an aberration. This is the period when the solar array was fitted and the utility company billed us for the new solar meter, etc in kWh equivalents. They then didn’t credit us for the electricity we sent back to the grid until the Oct-Dec billing period which is why the red line is so low for that period. The green line is our consumption for 2019 and is relatively flat. Obviously the bill is lower because most of the time during daylight we are consuming the electricity we generate.
Last year is the only period where we have 12 months of accurate data and I’ve used this to calculate the average daily kWh’s we purchased from the utility company.
A battery of 8kWs would probably meet our needs but 10-13 kWh’s would provide some spare capacity for a very dull day.
What is the cost of purchasing electricity from the utility company? It’s broken into two parts. The actual units of electricity we consume from the grid, which is a variable charge and a supply charge, which is a standard daily rate.
The Jun-Aug and Oct-Dec data for 2018 should be ignored.
It’s obvious the 2019 data is the lowest of the three year period. The average daily cost is $1.40 with the fixed daily supply charge being just over $1 of that amount.
The Tesla Powerwall 2 residential battery has a capacity of 13.4kW and costs $10-13,000 installed. If we were to install the Powerwall the payback period would be 9286 days or 25 years ($13,000 ÷ $1,40). The battery has a guaranteed life of 10 years. Even an 8kW battery has a payback period of 15 years.
Therefore, with our current solar power setup it’s not financially viable to install a residential battery.
However I anticipate as more home owners fit solar arrays to their houses the utility supply charge will increase to compensate for the reduction in demand and loss of revenue. Moreover residential battery prices are likely to slowly reduce, which means we will need to monitor the situation with the view of further reducing the cost of our electrical consumption.
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