Training Peaks can be used without power. Training Peaks primarily revolves around TSS (Training Stress Score) and, by default, TSS is calculated using pace for running and, if power is not available, HR for cycling. The usefulness of the TSS score is therefore dependent on having accurate threshold pace for running and FTHR for cycling. Even then, TP acknowledge that, when using HR in cycling, the TSS score becomes less accurate / valuable when the activity includes varying efforts, rather than being steady / continuous because of HR lag and drift (whereas as power is instant and a watt is always a watt etc). Similarly,the rTSS score (calculated using running pace) is going to be less accurate when running off-road, trails, fells etc if the threshold pace was based on road / track running.