Redditor Tufflaw has been running a central air-conditioning system "24/7" during the New York heatwave. But the bills were offset by 26 home solar panels by Sharp that took three days to install and were subject to state and federal tax-credits, and will take 7-8 years to pay for themselves. Here is the most recent bill: $6.05. Tufflaw says that there are sometimes months that go by with no bill at all (and one year generated a $20 rebate from the power company!), and adds, "There's also an intangible benefit, feeling good about using a free renewable source of power."