Don’t let the sun go down on me

Like most people, the biggest change in your life comes when you have a child.


I’d expected day to day changes but I didn’t realise how much it would change my thinking and my choices. In the years following my daughter’s birth, I realised that as well as planning for her upbringing, her education and her future financial stability, my wife and I can also make choices to influence the environment she grows up in.


So, a couple of years ago when we were buying a house, we had a couple of extra priorities. We needed off street parking to charge a future electric car and a roof suitable for a solar array.


We installed solar panels less than a month after moving into our new home, opting for a 6.27kW array, the biggest we could afford. But as there is no-one at home most of the day, most of our generated power went back to the grid, on many days in the summer, we were using less than 20% of the power we generated. Conversely, on other days when we were charging our electric car (a Zoe) and we were pulling more power than we could generate so were having to take power from the grid.


In an attempt to use as much energy as we could, we were carefully scheduling as much energy usage to be during daylight hours. Our lives seemed to be run by timers - timers for the washing machine, the tumble-dryer, the dishwasher. We even scheduled the slow-cooker to come on mid afternoon. We definitely improved our power-usage profile, but it was often inconvenient and required endless forethought and planning. I also developed an uncanny knack of scheduling the dishwasher for the exact moment the clouds would appear.


Getting a house battery had always been the plan, and was part of the reason why we had invested in such a large solar array. However finding a suitable battery, that would power us through the night, and help smooth out our pull from the grid to charge the car, had been fruitless - after a year of talking to multiple providers and looking at multiple solutions, we had yet to find a battery which had the capacity or functionality we thought we needed.

When the Tesla Powerwall 2 was announced with its 13.5kWh and built in 5kW inverter it looked promising. With its advertised price of £5,400 for the battery and about £1,000 to install it looked great. Hours after the announcement we had a deposit down!


We did have to wait a while for the Powerwall 2 to be released and, with our install being more complicated than a standard install, the final cost was more than we originally expected. However, with energy prices continuing to rise and with a better understanding of our energy usage within the house, we were still confident that the Powerwall 2 still made financial sense.


The Powerwall has really delighted us so far. We still try to do things when the sun is shining but now, as we are charging the battery from solar during the day, we don’t stress about setting timers for everything as we know we can run our appliances from the battery overnight. Most nights we only use about 40% (around 5 kWh) of the Powerwalls capacity, however the larger size battery gives some extra contingency, meaning we don’t have to think about our power usage.

We can also manage charging the car pulling from both the battery and solar in parallel, so as to reduce (or remove) the pull of energy from the grid. During the summer, we can fully charge our car from empty to full for under £1, but if we just top up the battery every couple of days, it costs nothing.

One area which we are particularly impressed by is the app and the monitoring provided as part of the Powerwall. Previously, we had had to invest in a 3rd party device (which was hard to configure and view) to give a graphical depiction of our historical energy usage. Now, from our phones and other devices, we can see clear graphics showing the flow of energy between the battery, our solar panels, the grid and our house. There are also graphs covering the last day, week and month which illustrate the peaks and troughs of battery charging and discharging alongside our solar power generation and flows in and out of the grid.

In August, our first full month with the Powerwall, we only drew 55kWh from the grid including charging the car. Nearly 90% of all the power we needed was provided by the Solar and Powerwall. There was even a 10 day period in August where we only used 2 units of electricity, about 30p.

Through winter, our solar panels generate about 25% of our energy need, but we can only use half of the energy generated directly. Previously we would have sold the rest back to the grid, but now with the Powerwall, we make full use of what we generate. We had our first “fully self-sufficient” day on 13th February.

With our install being more complicated than most (the battery is installed in the garage, which is not directly adjacent to our house) the final cost came in at a little over £7000 installed. Buying the Powerwall was never about making a quick return on investment, although we had an aspiration that it would “pay for itself” within its lifetime. As the software improves (time-based control) and we learn how to use the battery properly, we are very optimistic about hitting that goal.

I really believe the more people who invest in this technology now, the faster it will evolve and become more widely affordable. I know that home batteries and Vehicle to Grid will change our national power requirements and help our move to renewable energy sources. More importantly attitudes will change and society will become more aware of the things we can do to improve our environment.

For us, the Powerwall has been a great investment, never letting the sun go down on our solar energy usage.

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