Even advocates of renewable power sources like wind and solar realize that these methods alone won't stop our dependence on fossil fuels like the hilariously overhyped "clean coal." "What happens when the sun goes down and the wind stops?" might be an annoying question meant to sideline any talk of a clean energy future, but the reality is, it's a valid one. Plus, there's another issue: Sometimes renewable resources produce too much energy.

The electrical grid is a heavily monitored system; as demand rises or falls power plants are brought online or turned off. Solar and wind stations are becoming a larger part of this incredibly complex system, but due to fluctuations in weather and the rotation of the planet, they are tougher to regulate. If a solar farm is generating more power than the grid requires at that moment, that energy is wasted.

Imagine a bucket with multiple hoses feeding off it. You want to fill the bucket at the same rate it's dispersing water so it doesn't overflow. If you don't keep up, the flow will sputter to some of those hoses; if you fill it too much, it will spill over the edge and the liquid will be wasted. If there were extra buckets to hold water created by some of the less reliable sources, that would mean there's an additional source to fill the bucket that's ready at any time.

Batteries that act as holding pens for electrons are playing an increasingly important role on the grid. A utility might decide to deploy all that stored power during peak hours instead of spinning up a "peaker" (a plant whose sole purpose is to deliver power to the grid when it's most needed). On Kauaʻi, instead of using fossil fuels to keep the lights on at night, a Tesla-built combination-solar-and-battery-power station is filling in the role of a nighttime peaker.

Unlike the diesel eyesore on the south side of the island, Kapaia project -- a partnership between Tesla and KIUC (Kauaʻi Island Utility Cooperative) -- is vast, 45 acres of solar panels situated among rolling green hills. Panels dip and rise with the contour of the land, feeding the energy of the sun to a 53MWh array of Tesla Powerpacks -- white boxes filled with batteries. That's more than double that of Tesla's battery Powerpack project in California. Those storage units then release that power onto the island's grid at night. The project isn't spewing anything into the air, and other than birds and gusts of wind, it's quiet. The acres of solar panels don't completely fit into the landscape, but they aren't as intrusive as a diesel-burning power plant.

For the residents of Kauaʻi, sites like this are not only ecologically friendly but are also saving money, according to KIUC and other utilities on the island. "Hawaii was the most dependent state in the country on imported fossil fuels," Hawaiian Governor David Ige told Engadget. "Even as recent as five years back we were still over 90 percent reliant on imported fossil fuel for our electricity generation. We had amongst the highest energy costs in the country. And when imported oil went to $150 a barrel, it really strained our economy, because so much of our money was going out of state just to purchase oil."

To move away from reliance on foreign oil, the state has an aggressive goal of 100 percent clean energy by 2045. Kaua'i has gone one step beyond that with its own plan to be 70 percent renewable by 2030. But the utility hit a wall in terms of solar generation, "We've been a victim of our own success on Kaua'i, and during the daylight hours, prime sunlight hours with all the solar that's going on we're hitting 97, as high as 99 percent renewable penetration," KIUC CEO David Bissell said.

Adding more solar panels wasn't going to yield more benefits, as they were hitting power limits when the sun was its most powerful. The utility's' next step was to add storage to large -- and relatively inexpensive -- solar arrays and push that electricity back to customers at night instead of firing up diesel turbines. It's cleaner and cheaper.

More than anywhere else in the United States, the island state needs energy-storage solutions like the Tesla station. It doesn't have access to a larger grid like the mainland. Each island is its own electrical ecosystem. But there's a lot the mainland can learn from the steps Hawaii is already taking to reduce its dependence on fossil fuels, and batteries are an important part of that long-term plan. Frank Wolak, director of the program on energy and sustainable development at Stanford, said that in order for renewable resources to get any real traction, there need to be storage solutions. "That's really how it will change the grid. It hopefully will enable increasing amounts of intermittent renewable resources to be added to the grid," Wolak said.

In California, storage stations have been deployed not only because of clean energy mandates (each utility must add 1,325MW of storage to their system by the end of 2020 and have it running by 2024) but also because of demand. The Mira Loma energy station that Southern California Edison spun up in January was built and deployed quickly to alleviate the loss of the Aliso Canyon natural gas storage facility. It not only was deployed quicker than a peaker but also cost less.

It turns out that those peaker plants that only run a few hours during high demand are pricey. "When you think about that, how much are you really paying for a resource that only runs two percent of the year, you know, per unit of energy it's producing? You're really just paying it to just sit around for emergency, and that is not cheap," said Matt Vespa, senior attorney for the Sierra Club.

The biggest disadvantage of battery stations is their inability to output electricity for more than a few hours. Meanwhile a peaker can run as long as it's being fed fuel. But like in the automotive world, there's a transitional technology: the GE Hybrid Electric Enhanced Gas Turbine.

GE's new battery packs are added to existing peakers and enable utilities to get instant power from the turbines. The battery delivers the electricity needed while the turbines spin up to speed. The hybrid system will then charge itself while the gas engine is running and during shutdown. It's not as clean as a pure battery station, but like a hybrid car, it belches fewer toxins into the air, and for utilities, it's a better value. "The business case is way more solid. The value is higher as compared to a standalone battery," said Vibhu Kaushik, Southern California Edison's principal manager, asset management and generation strategy.

As renewable energy sources transition from feel-good ecological dreams to fiscally responsible reality, large-scale battery packs are going to become more important to our nation's grid. The islands of Hawaii might be an extreme case, but what we can learn from places like the KIUC could benefit other states.

Tesla for its part is gearing up to meet energy-storage demand both on current sites in Hawaii and in California and futures ones. "Everything is integrated. It's very productize and modular. We can deploy it on a site know very quickly and we don't have extra systems needed all over the place and we're at the lowest cost," Tesla CTO JB Straubel said.

For decades utilities have been playing a real-time game of trying to keep up with our power demands. We've been walking a tightrope, making sure the lights stay on while making sure not to produce too much energy and waste valuable resources. That's why Kaua'i's combination solar farm and storage facility are are so important. They deliver on the promise of a renewable energy future. Batteries are making our transition to renewable energy sources a possibility. It's a huge evolution from how utilities have delivered electricity, and if done correctly, you'll never even notice.