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Dynamo charging calculator

Will your dynamo hub keep your phone and GPS charged on a hilly day, with or without the light on? Or do you need to find somewhere to plug in?

Any axle or brake type. SON make two generators: the SON 28 gives more at low speed, the SONdelux is lighter and needs a little more speed.

Smaller wheels spin the hub faster at the same speed, so they make a bit more.

Light and charger

One or the other. With the light on, nothing goes to USB.

Your riding day

HoursSpeed (km/h)
Slow climbing
Flat
Descents

Moving time only. Loaded bikepacking speeds: walking pace to 10 km/h on climbs, 15 to 20 on the flat. 8 hours in all.

Riding in the dark, or the light on in daytime to be seen.

What you're charging

mAh is printed on the battery or in the maker's specs. Daily use is how much of the battery you'd run down in a day of riding.

Your day on the dynamo

You'll keep your phone and GPS topped up, just. There is not much to spare, so a small power bank is a good idea for slow days.

Charging stops below about 8 km/h with the light off. With the light on, this set-up doesn't charge at all.

km/hTo USB
Slow climbing, 2 h70 Wh
Flat, 5 h1713.3 Wh
Descents, 1 h283.5 Wh
The day16.7 Wh

That's about 0.9 full charges of a 4500 mAh phone. Your devices use about 13.3 Wh a day.

What the hub makes at each speed
km/hHubUSB, light off
50.6 Wnone
81.3 W1.0 W
101.8 W1.4 W
122.3 W1.8 W
153.0 W2.4 W
203.8 W3.0 W
254.2 W3.4 W
304.4 W3.5 W

Hub figures from SON's own published curves. USB figures allow about 20% for the charger. All are "about".

I build SON dynamo wheels here in Rochford and sell the kLite lights and chargers this is worked out for.

Dynamo wheel buildskLite kit builderBuy this loom

Why I made this

On a long bikepacking trip the question is always the same. Will the dynamo keep up, or do I need a café with a plug? On a hilly route you spend hours crawling uphill, where a hub makes very little, and minutes flying down the other side. This works out what that does to your charging.

Where the numbers come from

The hub output comes from SON's own published curves. They make two generators: the SON 28 makes its 3 watts at about 15 km/h, and the SONdelux at about 20 km/h, both on a 28 or 29 inch wheel. The charger side uses kLite's published maximum for the Micro, with an allowance for the charger's losses. I haven't used anyone else's calculator figures.

Getting the most from a dynamo

  • Charge a power bank during the day and your phone from the bank at night.
  • Turn the light off in daylight if you can. It's the biggest drain.
  • Put the phone in flight mode and use offline maps. The battery lasts far longer.
  • On a really slow, hilly route, plan a café stop every couple of days anyway.

Want a dynamo set-up?

I build SON dynamo wheels to order here in Rochford, and I'm the European home of kLite lights and chargers. My dynamo lighting guide goes through the whole system.

Dynamo charging questions

How fast do I need to ride for a dynamo to charge my phone?

With a SON 28 on a 700c or 29er wheel and the light off, charging starts at about 8 km/h (5 mph) and gets properly useful from about 12 km/h. A SONdelux needs a couple of km/h more. Below that the phone just keeps connecting and disconnecting, which is why I charge a power bank and charge the phone from that.

Can I charge and run the light at the same time?

Only with a loom that keeps USB on with the light, like the kLite Race loom, and only when you're going fast enough for both. The light takes its 3 watts first, so with a SON 28 you need about 25 km/h (15 mph) before anything is left for USB. In practice that's descents. With the lights-or-charge switch it's one or the other.

SON 28 or SONdelux for bikepacking?

The SON 28 if charging matters to you. SON build it to give more at low speed, and that's where you spend most of a loaded day. The SONdelux is lighter and spins a touch easier, and suits faster riding where you mostly want the light.

Does wheel size change the output?

Yes. The hub makes power from how fast it turns, not how fast you're going. A smaller wheel turns more times per kilometre, so a 650b or 26 inch wheel makes a bit more than a 29er at the same speed. SON's figures are for 700c, and the calculator adjusts for yours.

Should I charge my phone straight from the dynamo or use a power bank?

Use a power bank. Charge the bank from the dynamo all day, then charge the phone and GPS from the bank in the tent. Phones don't like the power coming and going on climbs, and a bank soaks up the spare on the fast bits.

How accurate is this?

It's a good guide, not a promise. The hub figures come from SON's own published curves. kLite publish the Micro's top output (1400 mA) but not a full curve, so I allow about 20% for the charger. Your phone's own charging, the cold and how much you stop all change it.

What about Shutter Precision and other hubs?

I've only put in hubs where the maker publishes output figures. Most 6 volt 3 watt hubs land somewhere near the SON curves, so pick the SON 28 as a rough guide.

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