Bottom right button on Sense launcher that looks like a paintbrush -> widget -> Get more HTC widgets -> then click on and install "battery"
When that's done click and hold on your main screen (or any screen you want), select widget, then select the 'battery" widget you just downloaded
Also, there are other alternatives in the market that accomplish this same feature. I personally use one called "current widget" since it can be toggled back and forth to show either battery percentage and charge/discharge rates too (which I often use)