System sizing depends on the power consumption of the devices you want to power up and for how long, in terms of hours or even days. It is important that you check the power ratings indicated on your devices, normally given in Watts or Amps. This will determine the inverter or inverter/charger size. Then identify how long your power outages are or in case of solar system, simply how long you intend to run your electrical devices on the system. This should determine the number of batteries