Once the domain of a few pros and well-heeled amateur nerds, power meters are now common in cycling and triathlon. When used properly, they're great not only for helping you and your cycling coach optimize your training, but for helping you pace yourself during your target events.
If you have been training with power, then you know what your FTP (Functional Threshold Power) is as you approach your event. In theory, if you tapered properly and don't have any extenuating problems on the day, you ought to be able to hold this power for roughly one hour. So, with this in mind, set a target that's achievable. If your event is a short prologue TT, you'll be able to hold a power number higher than your FTP – perhaps up to 105% of FTP. If it's a 50.3 Triathlon, you'll be on the bike for more than 2 hours following a swim and preceding a run, and should plan accordingly. For an event like that, you'll ideally keep your power in the high Tempo range – about 80% of your FTP.
The golden rule of timed solo events is: don't start too hard. It's far too easy to do, and the cost is very high, as your power data will clearly show. Take your time and work into your effort. Plan on starting at no higher than Tempo (86%-90% of FTP) for the first 5 minutes; then, work your way up to full effort over the next 3 to 5 minutes after that. It sounds easy to do, but in reality the thought of cruising along at L3 (or below if your event length is measured in hours) for more than a couple of minutes is enough to leave even the most patient racers feeling like they are throwing away valuable time. In fact, by not starting too hard, you're ensuring that you don't waste time riding below your FTP after you've started too hard and blasted your legs full of lactate. Think of it as unrolling a carpet – at first it is slow and heavy. As it unrolls, it goes progressively faster until it lies flat.
How does one best monitor this sort of effort? Set your power meter's head unit to show Normalized Power (NP), and hit the lap button as you start. This accounts for undulations in the course, corners, etc. Remember: ideally, it will start low and gradually increase. Focus on your effort, keep an eye on the numbers and with careful pacing you'll ride your way to your best bike split ever.