As others have said, the low-discharge type (eg Eneloops) seem to give the best of both worlds. You can stock-pile them fully-charged and they'll retain a usable charge for years (they lose about 15% in the first year, less there-after). And if the power cut extends indefinitely, you can recharge them by whatever means you can. Eg solar panels or car charger. Where-as with primary cells when you've used your stockpile, you're stuck.