Can I offer a compromise?
Standard headlight bulbs are 55W and there are two of them, so 110W @ 12V = 9.1A and @ 14V = 7.8A.
Change the bulbs to 35W HID bulbs and your power usage goes down to 5.8A @ 12V and 5A @ 14V. You end up with much brighter (and safer) lights and still have a couple of extra amps to charge your battery. You can get a cheap and nasty Chinese rip off conversion kit for £30 (cheaper then posh H4 bulbs) on e-bay and an hour of your time to fit it. If you keep your old bulbs, you can refit them in a few minutes when your MOT is due. If you do like the benefits of HID, you can upgrade to a quality kit whenever it suits you, and if for any reason you don't like the results, you can rip it out and restore the bike to standard.
My personal opinion is that this is the best safety upgrade I have ever made on any bike I have ever owned. Never mind the dramatically improved night time and bad weather visibility, other road users seem to be much more aware of my presence in all conditions from bright sun to bad fog.