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I'd also add it depends on your sleeping strategy.
If you're sleeping longer most night, you're riding less in the dark. And don't need a light for long.
If you like riding at night (and people do) then you'll want a dynamo.It really is a preference thing. But if you're not short for cash, I'd just take one. They're super useful.
It's 4w, or at least my son is. At even a poor 150W that's only 3%. It's nothing. You'll loose more forgetting to pump your tyres up to a nice psi every few days. Or carrying 1kg? of batteries does. Or the time spend sorting charging them.
skinny-
I'd also add it depends on your sleeping strategy.
If you're sleeping longer most night, you're riding less in the dark. And don't need a light for long.
If you like riding at night (and people do) then you'll want a dynamo.It really is a preference thing. But if you're not short for cash, I'd just take one. They're super useful.
It's 4w, or at least my son is. At even a poor 150W that's only 3%. It's nothing. You'll loose more forgetting to pump your tyres up to a nice psi every few days. Or carrying 1kg? of batteries does. Or the time spend sorting charging them.
Don't knock it until you've tried it!
There are good reasons to use dynamos but some of that is not right: battery lights are perfectly fine for riding all night and it only takes 5 hours max to recharge battery packs (if you have the right charger) so they don't compromise any sleep strategy that has more than one 5-hour hotel stop on the entire TCR (that is what Rimas actually did, using my batteries, last year!) And batteries don't weigh anything like a kilo.
The threshold power calc is based on an 'average' rider with a threshold of say 240W who is likely to be down to c.120W, 50% of threshold, after the first couple of days. Most dynos take about 6W when they're on which is 5% of 120W. With the dyno, he is down to 114W, making it equivalent of 50% of a 228W threshold.
I'm not trying to evanagelise about not using dynos, but if someone asks for the info it's only fair to give them it!
frank9755
Dynamos are essential on something like the tour divide where you are in remote areas and need high powered off road lights. The TCR goes through some of the most densely hotelled terrain on earth and, mostly, has better roads than here.
Lots of people use batteries, but as @hippy says, you don't hear about it because they don't need loads of advice on Facebook about how to set them up, how to waterproof chargers, etc.
I used batteries in 2016 TCR and also Indypac last year (and I lent my batteries to someone else for last year's TCR). If you charge at least every 4-5 days overnight in a hotel (5 hours to full charge), it works fine. If you are really careful about using power you could probably get all the way.
As well as power banks I also use AAs, for some of my lights and for a battery box. In worst case I could power everything off AAs. I normally stock up in the last shop I go to in each country to use up my change.
A dynamo is a good idea if:
You use loads of power - want to keep your phone on all the time for social media and want to use very high powered lights
You want to bivvy every night
You are going ultra minimalist so don't have bag space for a couple of extra phone-sized batteries
You're not in a rush so don't mind sacrificing the equivalent of c.10W threshold power.
You already have a dynamo and don't want to feel bad about having got it before you realised you didn't really need one.
Otherwise, batteries are a sensible option.