Putting solar panels on your roof is probably a good investment, no matter where you live. But adding a home battery may do more harm than good. //

The energy produced over the lifetime of rooftop solar panels more than makes up for the energy it takes to make, mount, and eventually recycle them. But adding a home battery can lower those dividends, new research finds. //

Previous studies estimated the energy output at about nine times the energy invested in solar panels. The new study, which appears in Sustainable Energy & Fuels, however, examined the output from a typical rooftop system installed in five diverse American states. Researchers found that the energy payout ratio ranges from a low of 14 in Alaska to a high of 27 in sunny Arizona—but only when homeowners are able to send surplus power to the grid.

When homeowners install a battery and charge it with excess electricity before sending leftovers to the grid, however, the energy return on investment for the entire system is 21 percent less than solar panels alone, researchers found.

When homeowners have no battery and no grid outlet, it just wastes extra electricity. Then, the system’s return on invested energy falls to seven in Alaska and a high of 14 in Florida—on par with earlier studies. Because homeowners in that scenario need to purchase electricity during the evening, adding a standard lithium-ion home battery improves the energy payback. //

Adding batteries to a home photovoltaic system reduces the energy payback of the entire system by 21 percent on average due to two factors. First, adding batteries means more energy in the form of fossil fuels invested in making the entire system. Second, a battery discharges 8 percent less electricity than the amount of electricity required to charge it—a loss compared to sending electricity directly to a larger electricity system with customers who can use the power immediately.

The current study acknowledges the dilemma. “As rooftop solar and large, photovoltaic power stations grow, electricity grids will not be able to accept more afternoon power, unless new uses of cheap afternoon electrons grow,” Benson says.