We actually have two lawn mowers: a Ryobi 40v for the front yard and a riding mower for the back. We bought the Ryobi first and it just couldn't tackle the 1/2 an acre in the backyard. The front yard is an average size, and the Ryobi does great. We can get through the entire front yard with one battery charge. It's also surprisingly lightweight and easy to fold up and put away.
Conclusion: for a yard over about 1/3 an acre, I'd go with a gas-powered mower. Anything under 1/3 an acre could easily be tackled by the Ryobi 40v. It's quiet, and you'll make up for the small cost increase by not having to purchase gas for the machine.
