A fun app is "Bike Calculator". There are a few different ones, but they are all similar.
Bike Calculator
Ok, so for some sample data:
220 lb rider (100 kilos)
20 lb bicycle
tubular wheels
Aerobars
70 degrees F
500 ft
0 grade
0 headwind
20 MPH --> 143 Watts (1.43 watts / kilo)
25 MPH --> 252 Watts (2.52 watts / kilo)
30 MPH --> 408 Watts (4.08 watts / kilo)
35 MPH --> 623 Watts (6.23 watts / kilo)
40 MPH --> 905 Watts (9.05 watts / kilo)
45 MPH --> 1265 Watts (12.65 watts / kilo)
50 MPH --> 1711 Watts (17.11 watts / kilo)
Now, let's try the same with the same parameters, Clinchers/Drops.
20 MPH --> 183 Watts (1.83 watts / kilo)
25 MPH --> 322 Watts (3.22 watts / kilo)
30 MPH --> 523 Watts (5.23 watts / kilo)
35 MPH --> 799 Watts (7.99 watts / kilo)
40 MPH --> 1162 Watts (11.62 watts / kilo)
45 MPH --> 1625 Watts (16.25 watts / kilo)
50 MPH --> 2200 Watts (22.00 watts / kilo)
WHEW!!!
Note the huge power differences when comparing speed with aerobars/tubulars vs drops/clinchers.