Although wind and solar energy are growing in popularity, storing that clean energy costs a lot of money. If consumers wanted to keep their green energy in Tesla's Powerwall home battery, for example, they would have to shell out 3,500. That may be why Tesla discontinued sales of its largest Powerwall a couple weeks ago. Instead of expensive battery storage, utilities are turning to a low-cost alternative: For a just few hundred dollars, your electric water heater can be made to work like a battery, a fraction of the cost of a Tesla Powerwall. If you're a homeowner, you may have noticed a man-sized cylinder in your cellar.