Originally Posted by
rm -rf
Another fairly good estimate is to see your speed on a known grade, since wind resistance is fairly minor at slow speeds and most of the power is used to lift the rider and bike against gravity.
This. It's the most accurate you can do without a power meter, and much better than any HR based formula. But use it on a relatively consistent grade and calculate the grade percent using the bottom and top elevation and distance, i.e. the average for the entire grade. If you use one that takes about 20 minutes then you can also use it to estimate your FTP (95% of your 20 min wattage). The gps based mapping tools usually do a poor job of indicating grade on smaller sections. Get the elevation from a map, not your GPS based computer. Those are also not that accurate.
Be sure to include the weight of your bike and equipment (shoes, clothes, water).