Assuming you are sitting up tall, coat a flapping, and weigh between 180 and 200, no wind, and no slope.... BTW no slope is really rare, but mild rolling hills turn out about same as flat. You get that free ride down the next roll.
And, pedaling some. hence the range in wattage depending on your effort, that is likely 100w or less.
15 mph can be as low as 200w, 250 more likely. But at this speed if you pedal 100w or more, you might see 100w on a watt meter.
20 mph can be as low as 400w, 500w more likely
25 mph can be as low as 600w, 800w more likely
30 mph, usually at least 1000w. By 30 mph, your not able to pedal at all unless you have special gears.
If significantly heavier, then add about 100w for slower speeds, 200w above 20 mph. In general, a lot heavier means a lot less aero, so a wider body, or a set of gigantic panniers affects wattage more and more the faster you go. 15mph or less, aero stops mattering so much, if no wind.
Once hills are involved, very hard to pin it down. So dependent on every pound of weight for one. But with minimal effort, you can count on pulling 800-900w up hills, if you have a 1000w bike. Even mild hills. The wattage will remain about the same as seen on a watt meter, but with varying grade your speed will be slower and slower as the grade increases. With hard pedaling, so much variation of throttle position, pedaling effort, gear you are in. It just gets impossible to generalize.
