As our reliance on solar and wind energy grows, so does the challenge of reliability: The wind and sun can’t be turned on and off whenever people need electricity. One part of the solution is energy storage.

That’s why Vermont’s largest utility, Green Mountain Power, is piloting a new project: It’s sold 500 Tesla home batteries to customers, both for the homeowner’s private use, and for the utility to draw on as a source of electricity.

Green Mountain Power is the first utility in the country to pilot the Tesla Powerwall battery in this way.

Both the utility and Tesla are betting that this could be the way of the future, as we increasingly rely on disparate power sources in fields and homes — and not on massive centralized power plants.

On a recent sunny morning, homeowner Miguel Orantes had a sleek gleaming white Powerwall installed in his rustic basement in Bellows Falls, Vermont.

“It’s going to be a great back-up system,” Orantes says. “So when power goes out, obviously there are some key things I’d like to keep going, like the alarm system, medical alert, the hot water heater if possible, obviously the food, just in order to to bridge the gap between the outage and whenever the power gets set back again.”

The battery itself stores about 6.4 kilowatt hours of energy, which could power the bare-bones needs of an average home for about six hours. It’s not meant to be relied on as a back-up generator that could power a home for days, but for Orantes, who lives downtown, it should be plenty to meet his needs.

He had thought about getting a generator before, but didn’t want to keep diesel or gasoline around and deal with maintenance in the winter.

The battery and the inverter combined cost $6,500, not including installation. But Orantes has opted to pay monthly installments of $37.50 for 10 years — a total of $4,500. The price is reduced for homeowners who to allow Green Mountain Power to pull power from the battery.

For Orantes, it’s not just a luxury item. Orantes is disabled and walks with a cane. His doctors recommended he have some kind of power backup for potential outages so his medical alert system would still work.

“My fear has been, what if I fall and there’s no one around?” he says.

The installation of the lithium-ion battery – the same technology that’s in cell phones and laptops — takes all day. It involves hanging the 350-pound battery and an inverter, which is critical because it converts AC electricity from the grid into DC to be stored in the battery, and vice versa.

A virtual power plant

Right now, Orantes is charging his battery from the grid, but others who are buying Tesla Powerwall batteries are charging them from their rooftop solar arrays.

For Green Mountain Power that potential is the crux of this project — and the future of the grid.

When these 500 batteries have been installed in basements scattered across Vermont, they will act as individual sources of power, sort of like a virtual power plant made up of lots of tiny energy generators all connected to the grid.

"You're essentially taking what used to be a couple big plants and power flowing down to homes to now, thousands and thousands of points out on grid that all need to be choreographed together." — Josh Castonguay, Green Mountain Power chief innovation executive

“It is a system: You’re essentially taking what used to be a couple [of] big plants and power flowing down to homes — to now, thousands and thousands of points out on grid that all need to be choreographed together,” says Josh Castonguay, the chief innovation executive at Green Mountain Power.

That choreography needs to take into account many factors, including whether the sun is shining or the wind is blowing — or how much electricity consumers are using at peak times, like when everyone gets home from work and starts turning on appliances.

Evolving to a grassroots power system

All this information needs to be calculated every minute to determine if the batteries should be charging from the sun or the grid — or sending out energy to meet peak consumer demand.

Castonguay says this pilot project will be a way to test and refine that complex algorithm.

“If the customer chooses the option where they’re sharing access, when a peak time comes we can have it scheduled to say, ‘OK, start discharging between this hour and this hour,’” says Castonguay. “And then refill it right back up and have it available for the customer in case of an outage.”

The goal is to test the system for some future day when there could be enough storage devices on the grid that they could be relied on to cover peak electricity demand.

Handling more unpredictable electricity demand

Battery storage is vital for utilities looking to rely on solar energy even when the sun isn’t shining.

But storage is also increasingly important because as the amount of solar being built in Vermont has skyrocketed, customers’ energy use has become more variable and unpredictable.

For example, people might not need to draw electricity from the grid as much during sunny times, but if a cloud passes over, demand could shoot back up.

“You have solar and wind being installed, and in many cases these are a new type of power plants, but they’re not controllable, in the sense that you can’t ramp up and down solar or wind as you want,” says Mads Almassalkhi, an assistant professor of electrical engineering at the University of Vermont.

“So they’re not as well-behaved as our old-school traditional power plants.”

Almassalkhi says when you couple that with the fact that today’s electricity demands are more variable and change faster from minute-to-minute, “what that means is you have these slow generators that have to track this much faster moving [demand.] So this means … you have to have power plants sitting by, waiting basically to ramp up and down very quickly.”

That’s where utilities traditionally rely on natural gas plants. But batteries are also quite capable of ramping up and down quickly.

“One of the biggest challenges is intelligently coordinating hundreds, thousands [someday] of Powerwalls,” he says.“If we can regulate this in smart way, we can cancel out the variability that’s introduced from renewable energy.”

So far, Green Mountain Power has only installed about 10 Tesla Powerwalls in customers’ homes. But once all 500 are hooked up to the electrical grid, the utility will have a new source of power generation: The basement batteries will add up to about 1 megawatt of power.

That doesn’t seem like much when compared to the fact that in peak use times the whole state of Vermont demands over 700 megawatts. But every power source counts.

And there’s already nearly 150 megawatts of solar cranking in Vermont — enough to power more than 24,000 homes. So if utilities had more battery storage, all those basement battery packs could be a significant source of power.

This report comes from the New England News Collaborative. Eight public media companies coming together to tell the story of a changing region, with support from the Corporation for Public Broadcasting.