California’s three biggest utilities are sparring with their own customers about systems that store energy from the sun, opening another front in the battle that’s redefining the mission of electricity generators.

Edison International (EIX), PG&E Corp. and Sempra Energy (SRE) said they’re putting up hurdles to some battery backups wired to solar panels because they can’t be certain the power flowing back to the grid from the units is actually clean energy.

The dispute threatens the state’s $2 billion rooftop solar industry and indicates the depth of utilities’ concerns about consumers producing their own power. People with rooftop panels are already buying less electricity, and adding batteries takes them closer to the day they won’t need to buy from the local grid at all, said Ben Peters, a government affairs analyst at Mainstream Energy Corp., which installs solar systems.

“The utilities clearly see rooftop solar as the next threat,” Peters said from his office in Sunnyvale, California. “They’re trying to limit the growth.”