Use the following worksheet and steps to determine the dualfunction inverter size and the battery capacity that your solar gridtie PV system will require.
First, use the Solar PV system worksheet on Useful Solar Info to calculate the PV array size that you'll use for your gridtie PV system.
Once you know the size of your PV array, this will be described in Power (Watts), you can size your inverter and battery bank:
Calculate your solar pv power system by defining your electrical loads both in terms of Power demand (the power pulled by the load when everything is turned on), and the Energy required  which is the hours per day you need to run that load. If you load is 1,000 watts, and you need that load for 12 hours per day, then your energy demand is 12 kWh of energy.
Step 1  Find the power requirements (watts) for the appliances you need to power during a blackout.
Make a list of the loads and appliances that you absolutely need to power during an outage. Only list the essential items since the system size (and cost) will vary widely with the power needed.
The Wattage of the individual appliances are usually found on the back of the appliance. List all of your loads that you need during a blackout and add up all of the power ratings (watts).
This will determine the size of the inverter (power rating), that you'll use ____________________ (watts)
Step 2  Decide the blackout duration you want to be prepared for.
Power outages last from a portion of an hour to a day or more. Again, this decision will greatly affect the system size and cost, so it is more costeffective to stay on the conservative side. ____________________________ (hours)
Step 3  Find the amount of stored power required.
Multiply the power requirements (Step 1), by duration in hours (Step 2). The result will be the energy you need during the blackout. This result will be in watthours. For example, if you need to power 1000 watts for 2 hours then you need to have 2000 watthours, or 2 KWH of stored energy. ____________________ (watthours)
Step 4  Calculate the power storage needed.
Multiply the figure arrived at in Step 3 by 1.7. In the example, 2 KWH x 1.7 = 3.4 KWH of stored energy. ___ (Kwh)
Step 5  Calculate battery capacity needed.
Divide the power storage requirement needed from Step 4, by the DC voltage of the system (usually 48 VDC) to determine the Battery Capacity in AmpHours (Ah). In our example, 3.4 KWH of energy using 48 VDC battery bank will have a Capacity (Ah) rating of 3,400/48 VDC = 70 Ah. _____________ (Ah)
