# Power difference between power meters

I swapped my usual trainer bike for my good bike on my Wahoo snap. The good bike has a 4iiiis power meter. After swapping bikes I performed a calibration through Zwift, and a zero off-set for the 4iiiis power meter. I rode Watopia for an hour. With heart rate and cadence right where they normally are my power dropped from what I normally average (138W) to 100W. Not only that, but according to my 4iiiis app, I averaged 185W for that hour.
On a normal ride outside I average about 145W.
Has anyone else had a similar issue?

Sounds like Zwift was using the Snap for power ?? If so, donâ€™t! Use the 4iiii indoors and outdoors so that you are comparing apples to apples.

There are a number of explanations for this I can think of:

1. If your 4iiii is a left side single sided power meter and not dual sided, then it is taking what it sees for power and doubling it. If by chance you have a power imbalance between left and right, the 4iiii is not exactly correct. Itâ€™s hard to prove if you have an imbalance without a dual sided meter to show it so hard to know how to compensate. Most people have just a minimal imbalance if any so the error is irrelevant. For you, it could be playing into the equation if you left leg is stronger (reporting higher power as a result) or weaker (reporting lower power as a result).

2. The Snap is a wheel-on trainer. There is power loss occurring as the tire deforms on the roller. How much I donâ€™t know. As an engineer it seems logical that a 4iiii would report higher power than the Snap as the 4iiii sees what you are throwing into the pedal. The Snap only sees power downstream of that frictional loss. I started out with a Snap then traded it for a Kickr Core. I noticed an instantaneous 5-10% power increase as reported in Zwift (thus by the trainer) when I went to the Core. Iâ€™m presuming it is because of this loss that the Snap would not see. Iâ€™m assuming this loss is not adjusted for in the calibration. Someone more familiar with the Snap might be able to explain if they adapt for this frictional loss in the calibration but I donâ€™t think so.

3. Two power meters will never agree. The Snap is +/- 3%, I think the 4iiii is +/- 1%. Imagine if the Snap is reporting 3% low and the 4iiii is 1% high, thatâ€™s a fully explainable 4% difference to start with. Add that to the points above and you can have some notably different numbers.

In the end, while itâ€™s nice to know exactly what power you are putting out, thatâ€™s tough to do with either of these devices. They are there to give you a relatively close answer and day to day, week to week comparison. Stay with one platform solution. On Zwift look at your Snap. IRL use your 4iiii of course. I have a Core with a left side Stages. I ignore the Stages when that bike is on Zwift. I know the results I see IRL are going to be a little different than my Core and accept that.

Hi,
I am using Ronde powermeter ( which measures both sides and seems accurate to me) and wahoo kickr core trainer.
Zwift power indicates 40+ wats less then wahoo elmnt bolt head unit.

This screws my training analysis badly.
My well established power zones are wrong and only way to train is hr zones.

Seems that many cyclist have this problem.
Have not seen solution anywhere.

They are both calibrated ( kickr and ronde)

How did you pair the power meter. Do you have a picture

I will take a picture when I ride next time and will post it here.

Kickr core is paired with zwift via Bluetooth.
Ronde is not paired.
It is just for reference because zwift power seems to be off to me and I am confused.

Last but not least, which one is correct?

I have a similar issue. I started using my good bike with a Quarq power meter connected to my Wahoo head unit and checked the power it was displaying against the power showing in Zwift from my Wahoo Snapâ€¦ I wouldnâ€™t mind if the differences were small %, but this was out by a lot. Zwift was regularly showing 15% more then my power meter. I donâ€™t know which is outâ€¦ the trainerâ€¦ zwift or the power meter!

The Snap is a wheel-on trainer so will be a lot less accurate than your power meter. Your power meter is likely showing you the correct figures.

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