Oh ok. I don't think there is a cheap way to do it. You can get programmable charge controllers that can supply power to load once the batteries are charged but they are quite expensive. It would make an interesting Arduino project if you're into that kind of thing.
Otherwise it would be a...
Hi, I'm not an engineer but I have a train load of solar experience. Can I ask why you think you need to use this extra power?
Do you have your panels directly connected to your batteries or are you using a regulator?
If you're using a regulator, there is no need for anything else. The regulator...