How to store power Off-The-Grid ?

VickyRC​  There is no object or way to store OTG generated power actively.  What you have to do is install enough solar panels or wind generators to generate enough power to carry you through the night.  So, add either a panel or turbine and see if that gets you through the night.  If not, add another and keep adding until you get through the night.  Another way to do that is add up the consumption of all the power using objects in the house and then then add enough panels and/turbines to cover the total.  Also, lighting can be a lot of your electrical load so where you can, switch out electric light fixtures for candles. Also, watch what you have sims doing, especially in larger households, and have the doing things that don't involve using electricity at night like using the piano, guitar, violin, pipe organ, painting, etc.  You can also use the generators when the power dies during the night.  But to do that you need to use the bug boxes and raise the insects to provide fuel.  

Hope this helps.