Hot water bottles, that's what I use. I raised a batch of 30+ chicks last year and a batch of 11 ducklings using only hot water bottles.
During the first couple weeks I keep a hot bottle in the brooder all the time, then depending on the weather I gradually decrease the amount of time I replenish the hot water till I am only using it at night. If the brooder is pretty big, I place a Tupperware tote on its side and put the hot water bottle(wrapped in a towel) in there and hang a towel of the most of the front of the tote to retain the heat inside. The chicks learn pretty quickly to go in the box for warmth.
If you have good size PV system, try looking at the Brinsea Eco-glow, it's not a terrible energy hog. Brinsea also makes a incubater that pretty energy efficient as far as those things go.
Also, consider ducks...they are much more hardy and feather out faster, they can go outside after 3 weeks or so. Much sooner then chicks. The draw back to ducks is that they are MESSY, and a bit harder to market if you are doing it for profit.
I wish I had some broodies, would love to let a mamma(or a couple) do all the work:)