The Solar Lawnmower?

(C)1995 Institute For Ordinary Research

Rob Peterson, Director

The Institute has been asked by yard-of-the-month wannabes to evaluate the usefulness of what appears to be a solar-power lawnmower advertised lately on the cable channels. In addition to qualifying one for the inner circle of New Age, environmental awareness groups, can all the gasoline, oil, plugs and filters actually be done away with and still get your lawn mowed without dragging an electrical power cord? The idea is appealing and one wonders if those little solar cells shown hanging on all sides of the mower deck can provide enough power to cut the wispy blades of grass seen in the commercial. The Institute staff decided that this subject is indeed Ordinary (not Advanced) and offers the following observations:

Power from the sunís rays peak (at local noon) around 1000 watts per square meter of absorbing surface area. The mower seen on TV has perhaps 0.16 sq. meters of solar cell area (approx. 16" x 16") and thus around 160 watts strike its surface (ignore the vertical panels which absorb almost nothing). The most expensive crystalline silicon solar cells convert around 15% of this light into electrical power, yielding 24 watts. (The mower in question undoubtedly uses the cheaper, less efficient amorphous cells but the Institute is a conservative organization and will assume the higher number.) So, is 24 watts enough power to cut your grass?

The Institute For Ordinary Research has a modest campus and maintains even more modest grounds-keeping gear. The equipment lineup is headed by a 20" wide push-mower powered by a 4.5 horsepower Briggs and Stratton four-cycle gasoline engine ($200 at Kmart in July 1995). Operators here report that it satisfactorily handles Saint Augustine grass but one must slow down in overgrown areas to keep the engine from stalling. Therefore, the Institute assumes that a minimum of 4 horsepower must be delivered to the cutter shaft in order to provide acceptable performance in the mid to lower latitudes of the United States. Current DC motor efficiency stands at 60% and thus the electric motor must be provided with 4967 watts ( 4HP* 745w/HP/0.6 ) in order to produce 4 horsepower at the blade. It seems that the 24 watts available from the solar cell array (assuming one is mowing at high noon in the summer) may fall a bit short of the mark.

Speculation at the Institute is that the advertised mower produces more like 0.4 horsepower and can indeed cut the very thin grass shown in the commercial. In this case, the 24 watts from the cell array is still woefully inadequate to provide the 497 watts needed. This power must come from a battery near the electric motor, under the solar cells. Assuming a 12VDC motor running at 41 amps (12 x 41 = 492 watts), mowing a small yard for 20 minutes (0.33 hour) requires 13.5 amp-hours (AH) which could be adequately supplied by a 25 AH battery (the typical automobile battery holds 50 AH). Since the solar cells can provide less than 5% of the power needed while mowing, the unit must be hooked to a charger (included in the $850 price?) or left out in the sun for at least 10 near-noontime hours to be ready for the next mowing. Battery management (and replacement) could be as obnoxious as providing the necessary care and feeding for a small gasoline engine. Care for normal lawns could be contracted to an outside vendor.

{IFOR-102, 95/07/20}