For a number of reasons, with equal saddle time, any indoor workout on home trainers is considered more intense and demanding than outdoor ones.
One of the first things to consider is the actual power output while working out, which is the main thing you look for when assessing workout effectiveness.
For instance, if you look at the power output data with an ergometer on the bicycle when outdoors, you'll see many intervals with very little to no pedaling. This is true for amateurs because of traffic, traffic lights, roundabouts and all those events that force cyclists to halt their physical effort. Conversely, indoor workouts are more effective in terms of performances as recovery phases aren't subject to the gravity you'd find on turns and corners, to changes in the wind or caused by the movement of cars or trucks.
Indoor workouts do not have down times and any recovery phase is always effort-driven. There's also more accuracy when calculating performances as you can exclusively concentrate on workout parameters, disregarding any variable you'd have on the road.
Another reason indoor workouts are more effective in terms of performances is the increased metabolic effort required during sessions, as it takes longer for the body to cool down. Outdoors the body cools thanks to the constant air flow caused by movement, which is proportionate to the cyclist speed. Indoors, the body generates more heat, which takes longer to dissipate, thus increasing heart rate.
As it generates more heat, the body also generates more sweat, so liquids drain faster. This is why it's fundamental to hydrate while working out as you would outdoors. Post-workout hydration is also fundamental - an abundant intake of water and isotonic solutions of mineral salts is recommended.
Another reason indoor workouts are more effective in terms of performances is tied to the pedalling movement’s neuromuscular and mechanical differences between indoor and outdoor workouts.
While working out indoors, the athlete's position is usually maintained for longer, subsequently the muscular engagement always concentrates in the same areas, thus with increased effort.
For all these reasons, any indoor workout on home trainers is at least 1.5 times more intense than the same workout done outdoors. This also determines longer recovery times from the session, which in endurance sports requires 24 to 36 hours. This is why if you workout indoors every day it's advised to alternate days with intense sessions to days with recovery sessions.
The minimum duration for an effective workout is at least 40 to 45 minutes, while an ideal session would require about 60 to 70 minutes, but it should never take more than 80 to 90 minutes.
Ultimately, for an adequate indoor workout session, the quality and intensity of the workout should be prioritized, benefitting the athlete's performance and physical shape.
If you want to repeat the calories consumption and athletic effort of an intense outdoor session it is advisable to conduct 2 daily sessions rather than a single longer one, for instance with a 40 to 45 minutes agility-based session in the morning, even before breakfast, to a 60 minutes session in the afternoon with specific targeted workouts.