Most newer treadmill models are rated with CHP, or continuous horsepower, instead of HP, or horsepower. HP measures the energy output of the treadmill at its highest which tells you only what it is capable of when pushed to its limit and only for a short time. A CHP rating tells you how much horsepower the treadmill can put out continuously, without exceeding the limits of normal use.
If you only plan on using the folding treadmill for walking, or perhaps a slow jog, 2.0 CPH is enough. Faster jogging or slow runs require a CPH of 2.5. For those that run, or plan to in the future, you need to look for a treadmill with a CHP of at least 3.0.