The drive to reduce power use is an admirable one. By using manual versions of items, you conserve electricity and fuel and generally avoid contributing a lot to pollution. However, sometimes motors and fuel use are necessary. If you live in a home with a big yard, you'll need a mower with a motor if you have a lawn and want to keep the lawn in place. A manual push mower just can't cover that distance in a reasonable amount of time, so a riding mower is needed. But you can use the mower wisely so that your motor use doesn't end up being excessive.
If You're Concerned About Being Eco-Friendly, Go Electric
Mowers, whether they're push mowers or riding mowers, can have gas or electric engines. Gas engines are more time-efficient as one tank will allow you to go farther before having to stop to refuel. However, electric engines (running on a rechargeable battery) don't contribute to pollution in the way gas engines can, and they run off a semi-renewable resource, depending on how the electricity used to charge them is produced. If you have a moderately sized lawn, don't mind stopping to recharge, or tend to care for sections of the lawn on separate days (giving you time to recharge the mower), an electric mower is perfect.
You Can Have Two Differently Powered Mowers
Depending on your storage space, you can have two different mowers that run off different fuel sources. Maybe you have an electric riding mower for most of your lawn and a gas-powered push mower to handle the perimeter of the lawn, which could be too close to a wall for the riding mower to comfortably handle. You could have a manual push mower if you really didn't want to use anything other than your own power for smaller areas of the lawn. Some push mowers are compact, so you don't need to set aside too much extra space.
Learn Your Lawn's Height Range
Ideal grass lengths for different types of lawn grass usually give you enough time to go a couple of weeks without mowing. If you mow the grass to the minimum acceptable length and don't mow until it reaches the maximum acceptable length, instead of mowing weekly or trying to keep the grass at one consistent length throughout the summer, you use the mower less and thus use less power. Just remember to check the lawn frequently because letting the lawn get a little too long can make it look unkempt and prompt complaints from neighbours if the lawn is in the front yard.
It's understandable that you'd want to conserve energy and fuel, but don't trade your entire day for the sake of not using power. Look for energy-efficient lawn mowers for sale that let you mow as quickly as possible.