Depends on how long the lights will be out and how low the temps drop and what kind of chameleon.
Down to 50 degrees is safe for most all chameleons, including panthers. If you don't think your temps would drop lower than that, you are OK. Veileds and jacksons can go even lower- low 40s.
They will also be fine without lights for a few days- it is no different than being shipped or having some cold cloudy days in nature.
But if you expect lengthy outages, you should probably get creative and give yourself some options.
Chemical heat packs used for warming hands or feet are sold in hunting stores and walmart's hunting section. These last anywhere from 8 to 24 hours or so depending on brand. They are also available online in even longer durations. The walmart ones are very cheap and can be swapped out as needed. If you put your chameleon in a cooler with one of these, it will keep temps inside safe and warm- do not put it in contact with the chameleon's skin- tape it to the lid or something to keep it away from the lizard.
Another option- most homes have south facing windows that get sunshine during the day. After the storm has passed and sun is returned, you can use the area in front of such a window to provide heat and light for your chameleon. You can even trap some of it in a bay window using a bit of plastic and creativity during the day- just be careful not to trap too much heat and cook your lizard (vent the top and bottom and adjust the vent size).
You can also get a power converter for your car cigarette lighter for about $30-$40 at walmart. You can then use the car battery and an extension cord to run power to your basking light.
If you've got a wood stove or kerosene heater and can keep the room warm, I'd probably just move the cage near a window for sunshine if possible, or not stress about it too much if not possible, unless you expect to be out of power for longer than a few days.