Julieta and Andy's adventures in solar power - part 2

In our last episode ...

In 2019 we installed 19 solar panels and a single Tesla Powerwall in our suburban Chicago bungalow. This is documented here.


2024 - a New House and new panels

In 2024 we moved from Illinois to the Big Island of Hawai'i.

Power costs in Hawai'i are about 4 times as much as Illinois, given that 70% of power generation is from oil and that oil needs to be shipped in from the mainland. Since we have two electric cars, the electricity costs can add up quickly. $550 monthly electricity bills were common before installing solar panels.

Unlike Illinois, natural gas lines are not common in Hawai'i, but small propane tanks are common for cooking. The stove that came with our house had a propane connection, but we removed the propane tanks, and upgraded to an induction range after running a 240v line to the kitchen. This also increased our electricity usage.

The house came with solar water heating but no solar panels. We started getting quotes in January 2024 and were up and running in mid May 2024. We installed 38 east, west, and south facing solar panels and 3 Powerwalls.

South facing panels
Panels on the garage Overhead view of all the panels


Overview of our system

We installed 38 Hanwha Qcell solar panels on the roof (the same kind we had installed previously in Illinois) and 3 Powerwall 2s. The solar panels have a total max rating of 15.6 DC kw. Even though we installed twice as many panels as in Illinois, the technology is 5 years newer so we are getting 3X the maximum power from those panels. We paid a bit more to have Enphase micro-inverters installed at the panels so we could get the maximum power out of each individual panel, and if something unfortunate happens and a panel stops producing then it won't affect the other panels

As in Illinois, without the Powerwalls we would not be able to use our solar power if there is a power failure. Short power failures are not uncommon on the Big Island and there is a potential for longer term outages in case of fires or hurricanes passing by, so we added more Powerwalls this time. The 3 Powerwall 2s can store 42kWh with a max discharge rate of 15 kW. The Powerwalls also increase the maximum amount of instantaneous power they can generate, and with electric car charging, an induction stove, and an electric clothes dryer, our maximum power draw does occasionally get to 15 kW. On a typical night we will use up about one Powerwall worth of power through cooking, and the two dehumidifiers we run all night.

We typically use about half of the power that we produce, as we had sized our system on the assumption that we would be driving our electric cars more.

Cost and tax credits

The up front cost of the system (solar panels plus power walls plus installation) is $104,000. We should receive $31,000 back as a federal tax credit, and $16,000 as a state tax credit, so the total cost should be $56,000. This does cost almost double compared to the Illinois install on a per panel basis, but things cost more in Hawai'i. We should be saving about $500 per month, so at current rates it will take about 10 years to recoup the cost of the system.

As in Illinois we get credit for any excess power that we send back to the grid, but unlike Illinois we only get partial credit. We are credited for 11 cents per kWh produced but electricity costs 29 cents to 40 cents per kWh depending on the time of day it is consumed. We can not make money sending power back to the grid, but we can offset any days where we do need to use grid power, typically when we have people visiting and drive around a lot during the day and don't have time to recharge from solar. Also as in Illinois we need to pay a monthly fee (in this case $30) to stay connected to the grid.


Timeline in 2024


Production

Our system's estimated production was 23,400 kWh per year and so far we are on track to do slightly better than that estimate.

Compared to Illinois, sunlight is more regular in the tropics so our sunlight ranges from 13 hours in the summer to 10 hours in the winter, and our power generation ranges from a max of 100 kWh per day in July to 75 kWh per day in December. The lowest daily production (15 kWh) was when a hurricane was passing by the island. Each panel in Hawai'i is producing about twice as much power per year as their cousins in Illinois (due to both newer technology and sunnier weather).


Scatterplot showing power generation and hours of sunlight by
      day of the year

Similarly, on a monthly basis power generation is much more regular than in Illinois, ranging from 2 to 2.5 megawatt hours of power being generated per month.


Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
2024




1020*
2510 2580 2490 2400 2255 2010 2050
2025













Monitoring

We use the tesla app for monitoring the system and that continues to work well.


General Resources

- Google's Project Sunroof - https://www.google.com/get/sunroof
- Home Energy Tax Credits - https://www.irs.gov/credits-deductions/home-energy-tax-credits
- Homeowner’s Guide to the Federal Tax Credit for Solar Photovoltaics - https://www.energy.gov/eere/solar/homeowners-guide-federal-tax-credit-solar-photovoltaics
- Tesla Powerwall -
https://www.tesla.com/powerwall



last updated January 23, 2025

copyright 2024-2025 Andy Johnson