Coming from the other side of the table- there are two processes. Design-bid-build, where the engineering is paid up front, and the installation bid separately off the completed (stamped) drawings. And design-build, where you're bidding for the design and construction as a package deal. In both cases, you would get a contract before doing any design work beyond mere scoping estimates- engineering is as expensive as production work, and generally not done for free. Any basic sketches to base your estimate off would be considered scoping estimates, part of the bid process, and not charged for; though you may charge for the more detailed design that will come before you start work.
Are they asking you for a design, or simply to install power?
