Renewable energy
Panels
Start with old house panels if you can. When someone upgrades their rooftop system, the replaced panels often still have plenty of life in them.
Salvage your panels
Ask around, watch local listings, and you can frequently pick up replaced panels for free. They are not showroom pretty, but they work, and free beats new every time.
If you want to buy new: flexible panels
Salvaged rigid panels are my first choice, but flexible panels are also worth a look if you are buying new. They are light, easy to mount on a shed roof or caravan, and handy when you do not want the weight and rigidity of a glass panel.