How is Zpowe/Virtual Power Calculated

Trying to get a 10 year old set up to use Zwift with rollers using Zpower. The Zwift setup guide recommends a cadence sensor for “a better experience” but it’s not actually clear if it helps with power calculations at all or if they are purely calculated from the speed sensor? Limited budget (pocket money) so the less he needs to buy the better.

not sure a cadence sensor would help with power calculations, but I assume that wheel size x cadence will give a rough speed which then can be equated to a form of power?

For Zwift, Cadence does nothing. Theoretically if Zwift knew the gearing and wheel size…Cadence would be helpful but for a multi-speed bike outputting gearing…just get a smart trainer.

You will need a Speed sensor for each of the following except for the power meter(s):

For Zpower the “dumb” trainer needs to have a profile within Zwift. Not sure you can do that with rollers…some have resistance add-ons and they might have a known curve.

Easier just to get a Power Meter on the bike…I have seen some really inexpensive G3 hubbed wheels (used…just reread…these wheels will be 700C so maybe too large for a 10-yr old), find some used power pedals or a crankarm PM is relatively inexpensive (single-side).

The other option is to use a known dumb trainer. A Saris or KK fluid trainer will have a profile in Zwift and can be found cheap on the used market.

Given I’m here asking about a £20 cadence sensor, I don’t think his pocket money is stretching to a power meter, even an inexpensive one😂


If I could link, I would send you to below (fix the broken https word):
ht tps://

Basically you have to use a supported wheel on trainer, and have something that measures speed on the wheel.

Here’s supposedly where to go to find if your trainer is supported (another broken link):
ht tps://

Just a guess, as I don’t think a Cadence sensor improves accuracy, I think they’re just saying that cadence is ‘better’ as it can be used as a training tool for feedback. Some training rides/workouts also provide cadence targets that they want you to hit during some segments.

I think it’s just badly worded in the Trainer list when it says “ZPower with Speed and**/or** Cadence Sensor”. specifically the word “or”

Yes this is the bit that is causing me confusion.

Rollers have historically been an issue for Zpower users, most rollers aren’t on the supported list, meaning Zwift doesn’t know at the current speed input from your speed sensor what the correlating watts would be resulting in extremely low speeds in game. You might be better off finding a used wheel on trainer for cheap, and a speed sensor. Most likely even a used trainer will be more than the 10 yr old can afford… maybe you can ask friends if they have any old trainers lying around they could donate?

Thanks, but he’s not interested in riding a trainer (apparently it’s not like “real riding”), if he was he’d just be using mine. He very specifically wants to use rollers, which he’s embarrassingly good at compared to his dad, but hook up to Zwift if possible. I’m well aware of the issues with Zpower generally, the only part which is unclear is what purpose a cadence sensor is, as the Zwift guide is confusing.

It displays the cadence on screen, that is all it is used for.

The “and/or” is wrong. It means use a Speed Sensor, or a speed sensor and cadence sensor. Someone has truncated that to speed sensor and/or cadence sensor, which is wrong.

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Yes seems to be the case, very confusing wording but having never used Zpower before it was not clear if it was somehow necessary (or useful) for the virtual power calculation.

All brands are about the same. WM may have a cheap one.
Usually, they give you some bonus watts, which is handy for a 10 year old.

Ask Chad for advice :slight_smile:

Who’s Chad?

Have you not seen Chad’s posts of him on his rollers on Zwift Riders?

I feel that my witty comment has been lost in translation :joy:

Blocked Zwift Riders ages ago, doesn’t look like I’ve missed much😂

Will the speed sensor work by itself? The rollersresistance device will need a profile in Zwift to work is my understanding.

It will work in the sense that it will generate a power number. But if the trainer is unknown to the game, the number might be way off, either high or low. One way to approach that would be to do an outdoor ride, use Strava to estimate the power, and then in Zwift try to adjust the wheel size up or down to get a similar power estimate at the same level of effort. Telling Zwift that the wheel is larger will increase estimated power (more distance per revolution). You have limited options for wheel settings but it might help a little.

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