Add battery back-up power option to existing grid-tied PV and solar systems: Page 11 of 14

August 05, 2013 //By Mark Mays
Add battery back-up power option to existing grid-tied PV and solar systems
Mark Mays of OutBack Power explains how to add a battery back-up power option to existing grid-tied PV and solar systems.
kWh to 30 kWh are the working numbers to use in determining what to expect in sunny weather.
 
Assuming all loads in the previous example are on at once, they will be drawing an average of 1.4 kW per hour of available power from the PV array through the GT inverter. That leaves 4.6 kW for battery charging if the PV array is achieving its nameplate power production. If the three-string battery bank starts the day at 80% depth of discharge (DOD), then it would take about 4.2 hours (19.5 kWh ÷ 4.6 kW = 4.2 hrs) to charge the battery while simultaneously providing the 1.4 kW demanded by the loads. At the point of GT inverter shutdown when the batteries are full, load demand can be sustained for nearly 14 hours before reaching 80% DOD on the battery bank—likely enough to last from sundown until bedtime with enough power left in the batteries to provide a power source to the GT inverter the next morning so it can deliver power from the PV array and start the cycle over again.
 
However, on cloudy days or when daily sun hours are below average due to winter conditions, adding a generator can assure adequate backup power under all conditions. A 5 kW generator would take 5.4 hours to charge the battery bank for another 14 hours of operation while also maintaining the 1.4 kW load demand (19.5 kWh ÷ (5kW1.4kW=3.6kW) = 5.4 hrs). As stated before, a generator running at 85 to 90 percent of maximum output can increase hourly operational fuel use significantly, compared to running the generator at low load conditions. Avoiding noise, lack of fuel availability and high maintenance cycles are additional benefits to higher fuel efficiency when using a generator with a battery-backed up inverter system.
 
A more detailed look of OutBack’s GSLC175-AC-120/240 AC Coupling solution (including some drawings) follows; here is a list
Design category: 

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