Just buy a cheap Cateye Astrale computer and take about 5 minutes to run the sensor to the back wheel and crank to get speed and cadence. It takes about that long to take it off in the spring if you don't like the wires. If you have a Kurt Kinetic trainer that has an accurate correlation between speed and power (you can download the power to speed chart on their web site), you will have an accurate estimate of power output (it is correlates pretty well with my Powertap).

It is clearly more effective to train with some data on the trainer. In fact, it is pretty hard to do interval training in the winter without it.