execise machine wattage vs real bike wattage?

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i use a kettler ergometer on training, is those watt mentioned accurate? Or is it an estimate? Imean when i see the readings it feels really accurate it could increase 5 watt a time and it cost me 500 - 1000 euros as well to get the thing. It feels a bit different then on a normal bike though.Would it be normal to be able to put out the same watt on a road bike as on a kettler then(if i trained on both)????

I know it might be weird that i train a lot on an indoor bike but i love it. I love outdoor biking to but those are the easy sessions where i can enjoy. (they are short though at the moment).