I'm commuting 3 days a week 20 km one way (40 km daily) on an e-bike in the UK. It takes around 50 minutes. Days are getting darker and I am about to buy a better light. I'm tempted to buy a 1600 lumen light, but not sure whether that's worth the money as it would be £50-£60 which is quite expensive for a light. Also I'm contemplating a 100 lumen back light. The route is part city, part outside urban area, but all the time on public roads.
https://bikelightdatabase.com/ has a lot of good information.
There is much more to a light than output. Optics is far more important (and in parts of the world, its illegal to use bike lights that do not meet specified standards).
The problem with Lumen as a measure is it is the entire output of the light in all directions. If the light has a super bright center spot, and no wide-angle light, you get blinded to the side by the brightness of the spot. If the light is evenly spread over too wide area, you get to see the periphery, cannot see far enough ahead to ride safely at speed. Lumen values are usually overstated in cheap lights, when it comes down to it, "Mines Bigger than Yours" is used by marketing geeks more than jocks. For comparison, a halogen car headlight on high beam has around 1200 lumen of actual output (Theoretical maximum of around 1600, but you only get about 75% of that in real life). You are probably not riding around at 60mph - 80mph, so you probably don't need the same light output and range as a car.
For the rear light the same applies -no point having a bright rear light if it throws a narrow beam that most motorists will not see. It needs to throw a wide enough beam motorists off at an angle will see you - for example a four lane road and a car in the left lane (right lane for those that drive on the left) lane changes to the right lane you happen to be in.
Suggest you look at reviews and focus on quality of optics as far more important than light output.
For illuminating the road ahead at speed where you'll get motor assistance on a UK-legal e-bike, 300 lumens in a well-shaped beam is plenty. You only need the sort of output you mention for off-road, and those super-bright lights tend to have very wide and high beams (useful for seeing overhanging branches). If I ride home from the trails with my 1200 lumen light I refit the top cutoff I made, and use it on low (about 1/4) power. That's adequate even with its rather wide beam.
At the back I strongly suggest running 2 lights, of which at least one is steady. That's easier for someone to locate in their field of view than a slow flash, and far easier than a strobe. A bright flashing light on a helmet or high on a backpack can be seen from far away and won't dazzle a cyclist coming up behind like one on the seatpost or lower would. Two lights are far better than one with both functions - if one fails, it's a very good idea to have a second already on.
You must see more, not less with the powerful light. What is the point of the bright light if you actually see less? This may be contra-intuitive, but:
- A light so bright that you see colors of the flowers in the grass will cause your eyes to adapt to brighter light conditions. You may see less where this super bright beam does not reach well.
- And if the light has "good optics", of course it will not. The beam will be properly directed to the ground and not too wide, so while the shoes of the human in front of you may be perfectly illuminated, the rest can stay in the darkness. Same for the side, especially if you are making the turn.
- Finally it is really annoying when another bicycle faces you with the light such that you need to brake because of not seeing where are you going. A weaker or moderate light is bearable also with bad optics or wrongly adjusted, but not the really bright light.
As no optics is perfect and it will often be some light outside the shaped beam anyway, less extreme brightness level may actually give more overall situation awareness.
As a result, brighter is not always better, unless you use a wide angle "wilderness light" shining like a high beam of the car without any care about optics. My light has a separate "high beam" button for this mode that I find much safer when alone on the bad gravel forest road. But, of course, I need to be as careful with this beam as the car drivers are.
A tricky topic, and one that is ever evolving as technology changes. 15-20 years ago when LED lights were new we'd have told you 200lm was excellent, these days some will tell you 2000lm is not enough.
For me, the answer depends on fitness, terrain and riding style.
If your roads are flat and you never find yourself going over ~15mph then 200-300lm with a well shaped beam is probably enough. At 20mph this requirement raises to more like 600lm.
If you are riding in rolling/hilly terrain and are frequently reaching speeds of 30mph things become more tricky. At this point you might want 2x1000lm. One with a broad and flat beam and one that punches further ahead (consider this like low and high beam on a car).
Note that currently in the UK the vast majority of high powered lights sold have very wide floody beam patterns that are not particularly well suited to road riding and can dazzle other road/path users. Please be wary of this when setting the position of your light and turn down power levels when appropriate to do so.
I had a similar commute. 1600 lumens would be far too much (if it really is 1600 - keep in mind that some manufacturers will claim a light to be brighter than it really is).
I had a front light that could be switched between 800/400/200 lumens (Cateye Volt 800). 200 lumens was sufficient for cycling in the city, on cyclepaths, and on roads with street lighting. At 200 lumens, I would still occasionally get complaints from other cyclepath cyclists that it was too bright, even though I took care to follow the instructions and mount the light so that it was angled slightly downwards towards the ground. Test your lights after mounting - prop the bike up, walk away a short distance, and then look back - if blinded, reduce the lumens or angle further down. Using a lower power than the maximum also has the advantage that the battery will last longer (8 hours versus 2 hours in my case).
I'd recommend also getting two rear lights (as suggested by others). Partly this is so that you can have one constant and one flashing, but also it provides a bit of redundancy in case one of the batteries runs out. I found this happens more often to rear lights, which tend to have smaller batteries.
Another recommendation is to get a cheap hi-viz vest from ebay. The reflection they provide is far superior to almost all regular jackets, and you can get a oversized XXXL that covers both yourself and a backpack if needed.
For some specific recommendations, and a visual comparison tool, see road.cc's 2021/22 front lights comparison engine.
Personal experience: between me and my wife we have a Magicshine Allty 1000 and a Cateye AMPP 1100 which each do about 1000 lumens. And either of those is absolutely enough and then some to bike at night on completely unlit paths. We usually have them turned somewhat downwards so as not to blind other people.
Two considerations are 1) how well lit the roads travelled on are and 2) regulations. You mention your route is public roads in city/urban UK, but are there regular lights from buildings or street lights? The adjustable bike lights may be a good option here, but also True Commuter recommends 600 Lumens tops. I personally feel blinking lights work better for being seen, regardless. Regulations in the UK stipulate that both a front white and rear amber lights must be fastened to the bike and not the helmet, and that at least one red reflector be attached to the back of the bike. This Bike Radar article sums it up nicely.
1600 lumens is overkill. Car H4 headlights have 1650 lumens per bulb at high beam, and 1000 lumens per bulb at low beam. Note that most cars with auto high/low beam these days switch the high beam on only at about 45 km/h, and high beams are generally used at speeds up to 100 km/h.
The amount of light needed is proportional to speed squared, because illuminating an area at certain distance requires luminosity proportional to square of that distance.
So if car high beams have 3300 lumens total at 100 km/h and if we can assume they are good enough, and with bikes you won't exceed 40 km/h if riding in complete darkness, this means 528 lumens is enough for a bike headlight (square law).
Actually the luminosity you need may be even smaller, for example most cyclists ride in areas with street lights and those areas require very little light. If your route has a small area with no street lights, you can perfectly well restrict your speed to 30 km/h only in these non-street-light areas, which means 297 lumens is acceptable according to the square law and comparing bike lights to car high beams.
Consider also how you are going to power the light. If it's a lithium ion light, most likely the battery is built-in and not designed to be changed. In this case, you can expect a life of five years. If you buy a NiMH AA light, one Eneloop low self discharge battery with 27 grams can produce 2.4 watt-hours (Eneloops have a useful life of at least 15 years, I just measured a pair of 13 year old Eneloops and they had lost less than 10% of their capacity). You should probably have 3 hours of light to allow for battery degradation and unexpected circumstances. 1600 lumens at 130 lumens/watt (many of these high brightness lights have only 1-2 emitters that are driven with such high power levels the lumens/watt figure suffers) is 12.3 watts. So to have three hours of light, you'd need 37 watt-hours. That's 16 batteries or 432 grams for the batteries alone. Are you really willing to carry that many batteries? Do you have the possibility to charge 16 batteries every day -- for example Maha MH-C9000 charger takes 4 batteries and costs maybe $60, so you need 4 chargers and $240 in charger costs if you want to charge all of them at the same time.
37 watt-hours is more reasonable with lithium ion lights, but then you need to factor in the expense of replacing that light maybe every 5 years due to battery degradation, whereas one NiMH light and a set of Eneloops lasts at least 15 years.
Dynamo headlights are restricted to about 3 watts by the dynamo. So if you have a dynamo powered light, the best you can do is maybe 400-450 lumens by the limited power you have.
Also, if you find a 1600 lumen light for £50-£60, I'm 99% sure it's either not really 1600 true lumens (just marketing lumens), or has a very poor beam pattern that makes it worse than a 200 lumen light with an optimized beam pattern, or has a very poor battery life, or a beam pattern that blinds all oncoming traffic. Most likely all of these four are true, plus maybe the battery li-ion cells are cheap Chinese cells that won't last even the 5 years (been there, done that, my lawnmower cells last about 2 years -- today, I use a gasoline powered lawnmower to avoid having to replace the cells every 2 years).
To find a light that doesn't blind oncoming traffic, your light should be StVZO-compliant (German law). StVZO lights have an optimized beam pattern which means they are infinitely better than a non-StVZO light having a circularly symmetric beam, even if that non-StVZO light has 5 times the lumens.