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@GrahamLaming
Depends on what electrical things you need to power and how long you will be away from power.
Will you be riding at night?
If you are only riding during daylight hours, your power needs are much, much less.
A backlight on any LCD screen like a phone or Garmin will really clobber battery life, so make sure all backlighting is off when you don't need it, and kept to the bare minimum brightness when you do.
If you are going to be in an area of poor phone reception, and don't mind missing incoming calls, switch your phone off when you aren't using it, as it will chew great lumps of power trying to keep connected to a network if the signal levels are low.
As a lot of things can now be charged up via a USB port these days, like Garmin, phones, etc I use LiPo battery packs with in-built USB ports. They have high energy density (lots of capacity in small weight and size). I use a fairly small one from Maplin, if I'm away from power for a couple of days. - http://www.maplin.co.uk/p/lithium-ion-5000mah-portable-power-bank-n98kq
You can keep a Garmin 800 going for a week of daytime-only use on one of those, if you keep the backlight off.
See also http://www.pcadvisor.co.uk/test-centre/mobile-phone/3534490/26-best-power-banks-2014-2015-uk/
You can periodically recharge the Battery pack if you have occasional access to mains power and remember to take the mains adapter with you, with the correct power prong adapter if you are going outside UK.
Most important - do a trial run well before you go, to make sure you know the time limitations for your particulat collection of kit, and take more fully charged battery packs if you need more. And keep everything dry, or your plans will fall apart.
I would go for dynamo lights if you will be doing extended night time riding, as your lights will likely be the biggest drain of power and carrying a lot of pre-stored power in batts will be a pain on a long trip.
Solar is not really practical on the bike IMO.
HTH