Our electric bill is less than $100 every month, and we have two Kindle Fires, two iPhones, an iPod Touch, and a Android. Plus my MBP. A pack of 8 AA batteries is like $5-10 depending on brand. It has to be a big savings really if we can power our entire house for less than $100 a month.
And yeah, it DOES save money. Especially when you're talking about digital cameras, which can go through 8 of those in one day. Our rechargeables don't add much to our bill.