Small motors like lawnmowers and other 2-stroke engines produce a large amount (pdf) of the air pollution in the world. “One hour on an average riding mower emits roughly the same amount of NOx as driving a very clean new car for 75,000 miles. (via)”

To start with I needed to design a mower that would fit my needs. I have a yard that takes about forty-five minutes to mow so it was important to size the battery accordingly. To do this I had to find out what kind of amperage the motor would be pulling for the forty-five minutes it takes to mow the grass. This is dependent on the type of motor used. Motors are rated in HP (horse power) and I wanted around the same HP as a standard internal combustion mower. The newer gas powered lawnmowers are about 4 to 6 HP and the older mowers are around 3 HP. According to the article in “Home Power” magazine 1 HP of an electric motor is equivalent to about 4 HP of an internal combustion engine. I was hoping to purchase a 1 HP motor but after pricing them I decided I needed to go with the least expensive motor that would get the job done. This turned out to be a 12 VDC, 3/4 HP, Dayton electric motor. (See link below for details on parts and cost.)

The 3/4 HP Dayton motor is specified to pull 58A with a full load. I used this figure to help determine the size battery I needed. I knew I wanted a 12VDC battery, they are easy to find, use, and come in a variety of sizes. The next step was to figure out the amp-hours I needed to run this particular motor for 45 minutes. Amp-hours are a measurement of the length of time it takes to discharge a battery at a certain amperage. For example; a 35 amp-hour battery should give 35 amps for an hour before being discharged. I know I want to run my mower for 45 minutes but I’ll just round it to an hour to be safe. Although the motor is rated for 58A, that amperage is with a full load and typically while mowing I will not be running a full load. While just spinning the blade (not cutting grass) the amps will only be about half of the full load or 30A. So a 40 Amp-hour battery should allow me to mow moderately tall grass for about 45 minutes to an hour. It just so happens that I had a 40 amp-hour deep-cycle battery from a past project.

With this in mind create your own DIY Bad-Ass Solar Lawnmower. Find the rest of the instructions here.