Power output on rollers doesn't seem to match perceived effort
Perceived effort isn't determined only by power output. That is, power output is certainly one of the things that affects perceived effort but there are others. There are (at least) two other possibilities: 

First, you may be overheating during indoor riding, so the perceived exertion is high compared to riding outdoors. In that case, a large fan can help you maintain convective cooling and reduce your perceived exertion. 

Second, crank inertial load on rollers is quite different to that on the road. You can think of crank inertial load (CIL) as related to how much momentum is maintained with each pedal stroke. Outdoors on real roads CIL is high while on a trainer or on rollers CIL is relatively low: that means when you're on real roads you can coast or pedal softly and maintain speed while when on rollers as soon as you let up the wheel speed drops. Some riders are more sensitive to low CIL than others; some riders are initially sensitive but with training get used to it. If you're relatively new to roller riding, your muscles may not be attuned to having to spin at high cadence and low torque, so your heart rate and perceived exertion may be higher than when riding on the road. Some trainers have a large flywheel, so although the CIL may be less than on the road, they will have higher CIL than rollers.