Retro Future Electrics

A page about retro electronics, from the introduction of electricity until 1970-something. We refit old gadgets with new tech, and wonder at the makers who came before us.

45 days with Tesla Solar and Powerwall 2 in Hawaii (Oahu)

Aloha Retro Future Friends. This is maybe more just future future. We recently installed 10 kilowatts (kW) of photovoltaic solar panels on our house on the island of Oahu, in Hawaii. Hawaii is ideally suited for solar power, being in the tropics we get strong sunlight all year. Our house is in a dry area (leeward side) of the island, so most days are sunny with only 14 inches of rain a year on average. Hawaii also has some of the highest electric rates in America, right now on Oahu electricity is 33 cents a kilowatt hour (kWhr, or kWh). Our moderate house had an electric bill from $250-$450 a month (including a plug-in hybrid and depending largely on Air Con usage). In this post I’m going to talk about the process or ordering and installing, the cost savings, and some cool tidbits about the Powerwall 2.

What’s a Powerwall 2?

Tesla is famous for cars and batteries, the Powerwall 2 is an Alternating Current (AC) to AC battery system. This means that it has a built in inverter and rectifier. In layman’s term it works with the same kind of power as the plugs in your house, vs DC (of course the batteries themselves are DC). The powerwalls are surprisingly small (remember Tesla makes batteries that have to fit in cars). Here is a picture mid-install, and fully installed. The Powerwalls are the white boxes on the right with the Tesla logo sitting on the ground.

A Powerwall 2 stores 13.5 kWhr of useable power, so two of them store 27kWhr of power. This is a little less than an average day’s use for us.

Why use batteries as part of solar?

Hawaii has a lot of solar power, so much that the utility has limited Net Metering. So for new installs like ours, it means that we buy power at 33 cents a kWhr, but sell power to the utility at 11 cents a kWhr. So if you want to have “zero” power bill, then you have to either build 3 times the capacity into your system (we don’t have enough roof area), or get batteries. There is a significant additional bonus to batteries.

Tesla App showing a grid outage

The batteries act as a backup power source to your house. So anytime of day if the power goes out, the batteries will support your house up to 5kW per battery (so 10kW for us). In fact the only way we knew about a grid outage was the phone notification. We have lived thru multi-day (up to 4) outages with no power as part of typhoons in Okinawa. So the idea of having enough power to run our fridge and coffee maker is comforting. On a sunny day our system makes more power than we consume, so we can live “off-gird” indefinitely. Of course a recent record-breaking storm made us happy for the grid as well.

Storm Watch

So Hawaii had a record breaking winter storm in Dec 2021. It was 59 degrees F in Honolulu, a record low. We got almost 8 inches of rain in a day. During significant storm events Tesla will activate “storm watch”, which is an opt-in program that will fully charge your powerwall off the grid and keep it at 100% for the duration.

Storm watch mode showing the Powerwall 2 charging at 6.7 kW (for 2 powerwalls) from the grid during storm watch mode.

A couple of things to notice from the app screen shot. It’s 3:42 in the afternoon and the solar panels are making nothing, and that it had only made 2.8 kWhr all day! (average around 40 kWhr). You can set the percentage of battery to reserve for backup, and I normally have it set at 20%. As Tesla didn’t activate storm watch until it was in the storm, in the future I will set this higher while the sun is out! (In normal operation you can only charge the batteries via solar, this is due to the wording of the federal tax credit).

Panel look

So something that is important to some is how do these panels look on the roof. Our HOA requires flush mounting (not angled up at the sun, as Hawaii is tropical this doesn’t matter as much as it might in the lower 48(The sun is to the north of us for 2 months in the summer)) The current install has a “skirt” all the way around the panels that make them blend well with the roof. Also they are all black, you can see in one of the photos below our neighbors older style with shiny metal grids. I like the all black better, but this is personal preference.

Show me the Money!

So one of the main reasons to install solar panels is to save money. In Hawaii this happens very quickly, the payoff is about 5 years after tax credits, and it would only be about 8 years with no credits. Given that the system should work for 35+ years, it’s money in the bank. (The batteries might give out at 25 years, and the warranty is only 10 years, but I’m also assuming that the next 20 years of battery technology will be even better than the last 10). Let take a look at our last couple of months of power bills:

April was the month we bought the house, and we did a lot of work on it before moving in. So the June bill (june 30th) is the first real bill. We were on vacation for a week in July. Now lets see the first full month with solar (previous month in the image had 8 days of solar, so I excluded it above) :

So bills went from mid $358 to $26. A savings of around $4k a year. That is not chump change! Now the system wasn’t cheap. Our price was exactly what Tesla said it would be online (I know there have been some reports of them upping prices, but ours never changed). The price was $39,267:

So at full price it would have a payoff of 8-10 years (we now make more power than we used previously, allowing us to lower the A/C temp, and live more comfortably without worrying about the bill). Luckily for us there are tax credits. I can be sure about the amount in 2022 after the refund check clears, but we expect a federal tax credit of 26% ($10,209) and a Hawaii state tax credit of $5,000 (35%, capped at $5k). So a net cost of $24,058.

That’s still a lot of money. Tesla offers a loan program, but we did it as part of our mortgage, which comes out to roughly $100 a month for power.

45 day results

So far we are very happy with the system. We have a net surplus of power, the battery backup has kept our power on during some (so far brief) power outages, and of course now our carbon footprint is reduced. Keep in mind that we have a plug in hybrid vehicle, so this also powers 30 miles a day or so of moving the car. In fact I’ve not put gas in the car in about 3 months, today I hit 3/4 of a tank.

(Update after 11 hour power outage)

So shortly after writing this article there was a flashover in an underground transformer near me. It caused an eleven hour power outage in clear weather starting about 3am. We were really happy with how the powerwall performed. We didn’t even know the power was out, as I was walking down the stairs I thought the sky looked funny, it was really clear how the city lights were lighting up the clouds. I soon realized that was because all of our streetlights were out, and the app had sent a notification the power was out. We were able to shower and get ready for work as normal, in particular I had a busy work from home day that would have been ruined with no power. Below you can see a screen shot at 7:44am, with the sun just starting to come up and 57% battery left after the night. Later at 12:35pm you can see the battery has charged to 90% (it eventually made it to 100% and turned the solar panels on and off to maintain).

I also couldn’t help but do a quick video flex (I then turned off the Christmas lights as I didn’t know how cloudy it would be during the day)

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