Green Mountain Solar

How to Use Solar Backup Battery in Vermont

Plus, we explain Green Mountain Power’s battery rebates and programs.

Tesla Powerwall installed by Green Mountain SolarMost people think about electricity one way: Turn on a light, pay the utility for it. (Grumble about how expensive it is and turn the light off.) But when you go solar with a battery backup system, you get a whole host of ways to use—and optimize—your power.

“Home energy storage systems have developed a lot over the last few years,” says Robert Dunn, Project Manager at Green Mountain Solar. “These new systems are really user friendly and have a lot of added functionality compared to their predecessors.” Here, he explains some of the ways to use a solar battery backup.

Read On: Battery Backup for Solar Panels: What to Know

Battery Backup Mode

Whether you’re facing summer thunderstorms or the ice and snow of a Vermont winter, you know that you want your electricity to stay on year-round to power things like your fridge and freezer (and all your food!), the lights, internet, maybe a basement pump that keeps your home dry. Enter battery backup mode.

“Pure battery backup mode is where your solar system keeps the battery at 100% charge so whenever the grid goes down you have access to the full capacity of that battery,” says Robert.

Solar during power outages

But wait, can I use solar during a power outage? There’s a lot of confusion when it comes to solar and power outages. But here’s the truth: Solar panels on their own don’t work during a blackout. It’s a safety issue.

When you have solar, any extra power your panels produce gets sent back to the grid (and you get compensated for this thanks to net metering). During an outage, however, this puts linemen at risk of being electrocuted if they have to go out and work on power lines. So, as a safety measure, the solar panels automatically turn themselves off.

Except when you have a battery. Regardless of what usage mode you use, having a battery means your panels will continue to produce during a power outage. The battery disconnects your home from the grid during a blackout and safely stores the power your panels produce.

How does it compare: battery backup vs. a generator?

Batteries offer some very real upsides over generators, explains Robert. “Battery backup often times can check all of the boxes of what you’d want from a generator while being no maintenance, totally silent, and totally automatic.” In the event of an outage, your home seamlessly switches over to the battery.

Cost-wise, batteries are more expensive than generators, but the sticker price is not the full picture. When you pair a battery with solar, you can take advantage of incentives like the investment tax credit and net metering. Plus, you’re likely protecting more of your home than you would with a small, portable generator.

Best of all, the fuel is free, clean sunlight, not gasoline. And as long as there’s sun, your battery will continue to recharge itself. “If it’s sized correctly and you can be careful with your electricity use during the grid-down events, you can have an indefinite power source,” he says. “Even if you fully discharge the system, it will wake itself up to check if there’s sun available and it immediately starts recharging and powering your house off of that system.”

Self-consumption Mode

Another way to use your battery is to employ more home-grown power at home instead of sending it to the grid. “Through self-consumption mode, you are effectively reducing your reliance on the grid and acting as a small grid in your home itself,” explains Robert. “In self-consumption mode, as soon as there’s not enough energy coming from your solar panels to compensate for the loads in the house, the battery starts discharging.”

“A lot of people think self-consumption mode is just a time thing, so at night when there is no sun, the battery just discharges,” he adds. “It’s actually a lot smarter than that.” For example, if you’re using a total of 4,000 watts, the battery can see 1,000 is coming from the solar and you’re importing 3,000 from the grid. The battery will then discharge 3,000 to zero out any import from the grid.

For many, this is a great way to seriously lessen reliance on the grid, especially since going completely off-grid isn’t feasible for most budgets. “Doing a 100% off-grid energy storage system would require a very large battery,” explains Robert. “But this can get someone to an 80-90% grid agnostic condition, without completely breaking the bank.”

Vermont utility company Green Mountain Power (GMP) offers a self-consumption mode credit to the tune of $850 per system.

Robert also mentions that many batteries have built-in features to ensure that you’re not depleting your battery when a storm is brewing. “For instance, Tesla has a storm watch feature: if your zip code has a storm coming, self-consumption mode will be overridden, and it will stay at a full state of charge until that watch is no longer in effect.”

Hybrid mode

As the name suggests, hybrid mode is a combo of self-consumption and backup mode. “It’s a self-consumption mode with a high reserve,” says Robert. “Homeowners know that for short term durations they can last at least a full day on just that buffer alone, and they have the top 20-30% to use for self-consumption.”

Green Mountain Power Battery Programs

GMP offers two more ways to use your battery through the Tesla Powerwall lease program and the bring your own device (BYOD) program.

For the lease program, customers can lease a Tesla Powerwall for about 20% of the actual cost. “The utility incurs about 4/5ths of the cost, which means that the utility owns and uses your battery when the grid is on. When the grid is down, it’s all yours,” says Robert.

The BYOD program works in the opposite direction—the customer buys the battery and then grants GMP permission to use it. The difference between the two programs is that, with BYOD, the homeowner owns the power and dictates how much GMP can use. “If you want to just give them a little bit and get a little bit of the incentive or if you want to max it out, that’s up to you.” (Oh, and maxing it out can mean $10,500!) GMP has a BYOD program for businesses as well. Even our Vermont statehouse is participating! (Read about it here on VT Digger.)

The point of these programs is to help provide the utility with an extra boost of power when it's needed most. “You have the base load for the grid – which is what the utility typically gets from hydro Quebec or other resources that they have that are always on or always available. This runs most needs in the state,” explains Robert. But, some events require extra power, called peak events—for example, a really hot day when everyone turns on their air conditioners. “During these peak events, we need more energy than we normally do for just a moment.”

Typically, the utility meets this extra demand on the grid with peaker plants. “Some of these peaker plants are old, large diesel generators,” he says. “For GMP, they’re expensive to run, really inefficient, and really dirty for the environment.”

That’s where the lease program and BYOD program come in. “Instead of one large diesel generator kicking, now your batteries and all of your neighbors’ batteries will discharge a little bit to make sure they don’t need to turn on those peaker plants as frequently.” It’s a win all around. You get the security of having a backup power source. It’s head and shoulders better for the environment. And GMP gets a less expensive source of energy—and can pass along those cost savings to all of its customers.

Not a GMP customer? Other utilities are looking to create their own battery programs like these as well. Green Mountain Solar tracks these developments, so if you’d like to get updates on this, reach out to a solar advisor or subscribe to our newsletter.

By Julia Westbrook

#Energy Storage

Energy Storage

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