An off-gird photovoltaic system has the same principals as a grid-tied system with a few additional components like the charge controller, DC load center and a battery bank. Instead of the excess electricity produced going into the electricity utility’s grid it goes into the battery bank, where the energy is stored until there is no electricity from the utility’s grid to service the appliances on the DC load center. When designing an off-grid system you would have to consider which appliance will be serviced on the DC load center, what the voltage, wattage and amperages are of those appliances and how long you would want your battery bank to run these appliances. See diagram below: