Hi Dingbat,
On average your body needs only around 50g of carb per day to provide you with all the energy you need for normal activities. If you limit yourself to 50g and then need more energy than this provides, then your body will burn some of the stored fat for energy. Also in the absence of carbohydrate, your body will convert some of your dietary protein into glucose. So a low-carb diet (i.e. lower than 50g per day) doesn't reduce your energy levels - it simply allows your body to get it's energy by using fat (and hence you lose weight) or by converting protein.
If you need more energy, for example because you exercise a great deal, then you will either need a higher carb intake or alternatively expect to lose more weight.