Going Green: Batteries for Off-Grid Homes

People keep asking about Tesla’s Powerwall, its new lithium-ion battery for homes. Now that the price of photovoltaic modules has dropped to $1 per watt or less, the missing piece for going off-grid, or even just for having a reliable backup power system, is an affordable battery. This is what the Powerwall is trumpeted to provide. It is a great product, except for three problems: It is undersized, overpriced, and unavailable.

The typical American home uses 31 kWh (kilowatt hours) per day. To provide backup power for three days, usually considered the minimum (the sun doesn’t shine every day), would necessitate at least 14 Powerwalls, each rated at 7 kWh. This assumes that each Powerwall can be fully discharged without damaging the battery — still a question at this point. The cost would be about $49,000 — each Powerwall costs $3,500 — not counting installation. Because of this high price, it would be wise for any homeowner to buy new energy-efficient appliances and light bulbs before considering the purchase of a huge battery array.

After reducing the electrical load as much as possible, lead-acid batteries are still a better option, costing less than half that of a lithium-ion battery array of the same capacity. Lead-acid battery technology is mature and well understood. Cost, however, is not the only consideration: Lead-acid batteries require frequent maintenance (mainly adding distilled water to the cells and cleaning the terminals); they can be easily ruined, either by undercharging or overcharging; and at best, they will last eight years but, in most cases, get damaged from improper care or usage and need replacing in five years or sooner. Not enough is known about the Powerwall to know what its issues will be. Compared to the 30-50 year life of a solar-electric array, a battery bank — whether lithium ion or lead acid — will need to be replaced multiple times. Furthermore, the fabrication and disposal of all those batteries have negative impacts on the environment.

Considerable research and capital are focusing on storage systems for renewable energy, both distributed and large-scale. This focus makes it likely that prices will drop for existing types of batteries and that entirely new technologies will be developed. As these breakthroughs are realized, the initial employment by homeowners will probably be not to go off-grid but rather to load-shift — that is, reduce the use of grid electricity during peak hours, especially in areas that offer time-of-use billing. Homeowners will be able to make a little money by charging their batteries at night, when grid power is cheap, and selling electricity back to the utility during peak hours. (Utilities also benefit from this strategy by being able to get along with less generating capacity.) Homeowners with electric vehicles may be able to use their car batteries for the same purpose.


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