What closed
The AES coal plant at Barbers Point on Oahu ran for exactly 30 years, from September 1, 1992 to September 1, 2022. It was 180 megawatts, it sat on 8.5 acres, and at its peak it made up to a fifth of the electricity on the island. It also put out 1.5 million metric tons of carbon dioxide equivalent a year, by the count of the utility itself. It closed because a state law, Act 23 of 2020, banned extending its contract and barred coal for power generation after the end of 2022.
What actually replaced it
These are contract prices for 2 different kinds of plant, not a full system cost. The 9 cent figure is 1 solar plus storage project named by the utility, not the island average.
Show the numbers
| Coal plant | 12 |
| Solar plus storage | 9 |
Not the battery. The battery was not finished. Hawaiian Electric ran its old oil fired plants harder instead, and it did that in the months when oil prices were spiking after the invasion of Ukraine. The utility warned before the closure that a typical Oahu household would pay about 7% more, roughly 15 dollars a month. Its own chief executive said the timing was unfortunate.
Unfortunately, the timing has converged with global events that are currently increasing the cost of electricity.
Shelee Kimura, chief executive of Hawaiian Electric, in August 2022. Source 1.
9 renewable projects were meant to arrive on Oahu by 2024. A year after the closure only 2 were finished. The coal plant sold its power at 12 cents per kilowatt hour. One of the replacements, a 39 megawatt solar farm with 156 megawatt hours of batteries, is contracted at 9 cents. Cheaper, when it finally runs. The lights did stay on.
There has not been a single blackout related to the coal plant closure.
Nicole Lowen, Hawaii state representative, a year after the closure. Source 5.
What the battery actually is
Like for like on power, but not on staying power. The coal plant ran as long as it was fed. The battery holds about 3 hours at this output, which is 565 megawatt hours divided by 185 megawatts.
Show the numbers
| Coal plant | 180 |
| Battery | 185 |
Kapolei Energy Storage is 158 Tesla battery units on about 8 acres, rated at 185 megawatts and holding 565 megawatt hours. It came online on December 19, 2023, 15 months after the coal plant stopped. The megawatts and the megawatt hours matter in different ways. The first says how hard it can push, and it beats the coal plant. Divide the second by the first and you get 3 hours, which is how long it can push that hard before it is empty.
That is the honest limit of the comparison. A coal plant makes electricity. A battery only moves it, from the middle of the day when Hawaii has more solar than it can use, to the evening when it does not.
What it does that coal could not
The battery reacts in 250 milliseconds, a quarter of a second, where a coal plant needs minutes. That speed lets it do jobs the grid used to need spinning machinery for. It can steady the frequency the moment a generator trips, imitate the physical inertia of a heavy spinning turbine, and restart a dead grid from nothing. Hawaiian Electric modelling says it should cut the amount of renewable power thrown away by 69%, and make room for 10% more large scale renewables within 5 years.