We make 2 products for this exact purpose so if someone already has a traditional birdbath that they love they can turn it into a solar fountain without buying a whole new birdbath. The product is called an Aquanura Solar Fountain Kit and it comes in 2 sizes. The small Aquanura (model# 21510001) is 6 inches wide and only works when in direct sun. The large Aquanura (model# 21310024) is 9 inches wide and can store power using its removeable battery pack and has a remote control you can use to turn it on and off.
Both sizes of Aquanura come with water sensors so they will stop running when dry to protect the pump and they both have removeable foam to make them float so if you want to use them in a pond or pool that's an option too.
The model that customers choose depends on various factors the size, depth and shape of their birdbath bowl, or how much direct sun their birdbath receives. Minimum water depth required is 1.5 inches but 2 inches is ideal.