Cash-strapped SolarCity Corp. is trying to cut marketing costs amid competition from smaller upstarts using cheaper sales tactics—such as paying Uber drivers to pitch solar panels to passengers.

Big solar companies such as SolarCity and Vivint Solar Inc. have been engaged in a fierce battle to amass market share, analysts say, and have spent heavily on old-fashioned advertising, telephone and door-to-door canvassing, and booths at retail outlets to acquire new customers. But the companies are spending more than they are taking...