Powerwall Batteries

Here in Alameda our local utility is city owned and has been very active in getting people to be aware of renewable electricity instead of being dependent on fossil fuel (mainly natural gas) generated power. Although recently that has been harder to keep track of. Oddly, the recent power labels for their main service, while stating it is <1% from natural gas, has a crazily large 60% tagged as unspecified (compared to 15% for the statewide average). Prior to them introducing an option to switch to 100% renewable power they had a much more specific power content label, and it was mostly renewable sources.

The net effect of the city being very aware of renewable options, as well as having relatively good weather for solar generation, has meant that the city has already reached the cap on net metering and solar rebates. In place of the net metering plan, Alameda now has a new program, Eligible Renewable Generation or ERG, which essentially pays a market rate for power fed into the grid. That is a little over 6¢ per kWh, which is far less than the amount I’m paying for electricity from them (and a great example of why net metering was such a good deal).

Daytime vs Nighttime

Most of the day our electricity usage is low. It is the evening & night time when we use electricity. Unfortunately, solar generates its power during the day. With the EV in the picture, which is not at home during the day most days of the week, we would not be able to take advantage of the solar energy to charge the car. That is, unless we can store that energy. Enter the Powerwall: a Tesla battery pack for the house.

Now some of that excess solar energy can be stored in a battery, then used later in the day when the panels are no longer generating.

Alameda is not yet allowing these batteries; they are working on getting the interconnection agreements, permits etc sorted out though, so it won’t be long. In the UK, they are already allowed & being fitted. Fully Charged did a segment about the Powerwall 2 which explains nicely how it works:

With 13.5 kWh of capacity, the battery could easily run the house in the evening, and have capacity to spare to top up the car battery. And, if needed, a second one (or up to 9 more, for a total of 10 batteries) can be added to boost that capacity even further.

Given we no longer have the option for net metering, having an option to save more of the solar energy we generate in the day for our own use in the evening makes the most of the solar installation.