My '84 craftsman is still running well, but keeping an eye on electric mowers for when it dies. I only have to mow a very small patch by hand, so an electric seems to make sense. It's just the replacement of the battery every few years (even if you maintain it properly) that makes me hesitate. One battery replacement would probably cover the cost of all the replacement parts in the gas mower for the past 20 years.
I wouldn't worry about battery lifespan if I were you.
I've had my set of electric lawn tools (described above) for at least two, or I think three, years, with no noticeable reduction in battery life so far. With four batteries, and randomly cycling through them, no one battery is getting used all that often. And lithium-ion batteries have a typical life of 600-1000 cycles. At maybe 25 lawn mowings per season, that's >30-year life IF I used the same battery each week, which I don't.
I don't actually expect 30 years - but still it's likely to be much longer than a gas mower will last.
I used to use at least a half gallon of gas each week, depending on whether I also used the weed eater and blower, so over 12 gallons during a season. Even at today's low gas prices, that's at least $25 gas per summer. That would pay for replacing the battery every four years or so assuming battery prices don't drop (which they do).
That would be offset VERY slightly by the electricity used to charge the battery - but BARELY. If I fully discharge a 4AH battery once each week (which I don't), that's about 0.48 kWH at about $0.15/kWh, so it's costing me just a dollar or two per summer of electricity.
Caveats and recommendations:
- I don't really count on the ability to find a direct replacement for the battery in several years, since technology changes so quickly, so I would suggest having several batteries at the start, and rotating them periodically.
- Don't buy anything with NiCad battery packs. Just don't.
- Be sure to use each battery every so often; they have built-in charging-monitor circuits that need a small charge to stay alive, and if the battery goes completely dead you can have trouble charging it again. But that "shelf life" is several years under average conditions.