The final summation you're after is really a calculation of more than one equation.
First--Anticipated draw in AH of the devices you want to power (lights, furnace, fridge, etc.)
Downloadable calculator Second--Type and size of battery storage capacity. The amount of capacity you have will effect how, how much, and when you need to charge the bank back to as close to 100% as you can for best battery longevity.
Third---Type and output of the battery charging devices. Solar is the most efficient, IMO but the capitalization is higher than grid based chargers. The sun also has to shine. If using just grid based charging, amount of output from the charger defines how long it needs to run to get to an acceptable SOC. Using both solar and grid places a variable into this part of the efficiency outlook, as will use of a generator which, unto itself has it's own efficiency (output for amount of gas used).
If you dance with these three wolves, you'll get an approximation of costs involved.
