- Excess power generated by a rooftop solar system to be stored in home batteries may actually not be the perfect solution, according to a study by Stanford University
- Home batteries are made using fossil fuel generated power, and these also discharge 8% less power than the amount required to charge them
- The better idea is to have this excess power generation to be fed into the grid for other consumers to use it, the study claims
Adding a home battery to store excess energy generated by the rooftop solar panels may not be the best idea for the return on invest as it may lower the dividends that one can expect from the power generated by such systems over their lifetime. In a new study funded by Stanford University’s Global Climate & Energy Project, the authors found adding batteries to a home PV system reduces the energy payback of the entire system by 21% on an average for 2 main reasons.
Firstly, batteries are made using fossil fuel generated power (which is a poor argument as this depends on the manufacturer and is quickly changing toward renewables), and secondly the amount of electricity a battery discharges is 8% less than the amount required to charge it. This lost power will be better utilized when it can be fed into the grid for other consumers to use it.
At the same time, with no grid connection and no battery system, all excess energy generated by a rooftop solar system just goes waste.
“The overall high returns show the energy benefits of photovoltaics across a wide range of US geographical regions and, presumably, elsewhere,” said Sally M. Benson, co-director of Stanford’s Precourt Institute for Energy and professor of energy resources engineering, and co-researcher for the study. “But they also demonstrate that if a state encourages homeowners to invest in rooftop systems to reach clean energy goals, then letting excess power flow to the grid makes the most of those investments.”
Various US states have their own policies of reimbursing customers who generate their own electricity through solar panels. So there isn’t a uniform system where all get paid same or they sell the same amount. Because of these reasons, researcher Simon Davidsson Kurland, postdoctoral scholar at Stanford’s Global Climate & Energy Project says it is difficult to justify adding batteries to residential PV systems as long as excess generation can be made available for other users on the grid.
Nevertheless, home batteries can still be useful for certain climates like that of Washington at the higher end of the range where there is little use for residential electricity, but the sun shines bright during early afternoons. At the lower end, the researchers place Florida where air conditioners run full blast because of hot and humid climate.
Utilities complain that when consumers feed excess power generation into the grid, they are being used as free battery, which can put a financial strain on them. “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,” added Benson.
However, the learning is that it’s all a question of policy frameworks that need to be adapted to support flexible solar and wind. Self consumption is key – in a net metering environment adding batteries makes little economical sense – and a feed-in premium as well, which is why nearly every second household in Germany, for example, is now installing solar and batteries together.
The study appears in the journal Sustainable Energy & Fuels available on the website of the Royal Society of Chemistry that has published the study.
