Energy storage for the residential solar market has always been something of a holy grail for advanced energy companies. If storage becomes cheap enough, it could allow a rooftop solar system to provide all of the energy a homeowner needs, potentially making it possible to go off-grid. It could also be the energy hub for the home, deciding how to use energy most efficiently and connecting the smart devices that are beginning to become more common. 

In 2017, the commercial and utility energy storage markets started to thrive and grow, and in 2018, it looks like the residential energy storage market will start to show the same promise. Here's why that is and why SunPower (SPWR 0.63%), SolarEdge (SEDG -3.69%), and Sunrun (RUN -6.89%) -- and not Tesla (TSLA -2.76%) -- are the three to watch next year. 

Home with a rooftop solar system.

Image source: SunPower.

Energy storage systems finally make financial sense

The reason energy storage hasn't been common in the home is that there was no financial reason to have it. Net metering allowed customers with solar systems to sell excess electricity to the grid at the same price they paid for electricity, effectively making the grid their storage location. 

As net metering has come under pressure across the country the economics of residential energy storage systems have changed. In some cases, like Hawaii, utilities are paying lower rates for rooftop solar exported to the grid, allowing a storage system to perform arbitrage. In others, there are demand changes based on the peak energy use of a home during a month, and if a storage system can lower those charges, they can be economical. Another popular structure is time-of-use rates, which adjust the cost of electricity throughout the day, something California has begun implementing. If a storage system can shift when a consumer uses grid electricity from an expensive time to a cheap one, it can make the storage system economical. 

These rate structure changes have only become widespread in the last year, driven by rate changes in California, which also happens to be the biggest solar market in the U.S. And those changes are what will make residential energy storage a booming business in 2018.  

Installers need an energy storage solution

We might now know that energy storage will start to make sense for customers, but the products still need to get to installers, who will be the sales engine. And there are a few companies and models to watch. 

One model is the installer compiling and controlling the energy storage system. This is what Sunrun does with BrightBox and BrightSave. It wants to be your contact for home solar and energy storage and will compile the components and control systems in the background. The battery it now uses is from LG and solar panels are also made by third parties. 

SunPower's model moves up the value chain to manufacturing components and controls in-house. This gives it full control over what the system looks like and how it works together. But SunPower isn't the installer, instead of pushing that responsibility to third-party contractors, who are also its sales conduit. 

SolarEdge is a component supplier that is trying to cast a thinner but wider net than either Sunrun or SunPower. It currently has a large market in residential power optimizers and inverters, which then make it a go-to for solar system monitoring as well. It's leveraging that position to sell energy storage products to installers. These would be similar to the installers SunPower works with, but SolarEdge isn't selling a full suite of solar panels, racking, wiring, and other components like SunPower is. 

Who wins in residential energy storage? 

What's not clear is which model will win in residential energy storage -- and the model will be important. In the early days of residential solar, the installer model was where the value was (Sunrun), and in recent years, components and technology have taken market share and value (SunPower and SolarEdge). It's not yet clear which model will be better for energy storage and which is complementary to residential solar. 

What I'm watching in 2018 is what SunPower is able to do because it has a model that's in the middle of other companies. If its complete solutions package is successful in gaining market share in residential energy storage systems, it will tell a lot. But maybe customers want to work with an installer, like Sunrun, that does all of the component optimization and just sells a solution at a fair price.

Tesla has its eyes elsewhere

One interesting trend is that Tesla doesn't look like it's going to be an aggressive player in residential energy storage next year. It may have already given up its No. 1 position in U.S. solar installations and is rapidly shrinking its solar business. Powerwall, for all of its strengths, also doesn't seem to be gaining traction as an energy storage unit of choice for installers, partly because installers don't want to support a competitor by buying Tesla's energy storage unit. 

That's why SunPower, Sunrun, and SolarEdge are the three to watch in residential energy storage systems in 2018. If they can kick-start the industry and start to build market share, it could catapult them to a very large and sustainable business long term.