Using the 17,200 watt hours (17.2 kWh) in table 1, and a battery depth of discharge (DOD) of 80%, we would need a battery bank with a capacity of 21.5 kWh to back up the loads for 24 hours. Quite often, a DOD of 50% is used in battery-based off-grid systems since a shallower discharge extends the life of the battery. However, 80% DOD can be used for backup applications since the battery bank will only be cycled a few days per year, or a week or two at the most, and going with a deeper DOD means a smaller battery bank. In the example above, 50% DOD for 17.2kWh energy demand would equate to a 34.4 kWh battery.
The next step is to translate energy demand in kWh into battery amp hours (AhUsing the load profile above and a 48Vdc nominal battery bank, divide 21,500 Wh by 48V and 448 Ah is the minimum size battery bank for this application. Since the energy demand is based on a 24 hour rate, then the battery Ah for the same discharge rate of 24 hours should be used as the battery capacity (Ah) will vary depending on how fast it is discharged (see table below). Using the OutBack batteries listed in the table below, two strings (four 12V batteries in series for each string) of the 220GH batteries could be used for a total of 432 Ah and be slightly under our estimate. If we wanted to be more conservative, then we could choose to use three strings of the 170RE batteries for a total of 471 Ah.
The first step in sizing a backup power application is to start with the electrical loads. The basic backup system for most homes would include power for a refrigerator, lighting and some receptacles for powering small appliances. Table 1 shows a typical load profile and the amount of energy in kilowatt hours (kWh) required to run them over a 24 hour period.
Using the 17,200 watt hours (17.2 kWh) in table 1, and a battery depth of discharge (DOD) of 80%, we would need a battery bank with a capacity of 21.5 kWh to back up the loads for 24 hours. Quite often, a DOD of 50% is used in battery-based off-grid systems since a shallower discharge extends the life of the battery. However, 80% DOD can be used for backup applications since the battery bank will only be cycled a few days per year, or a week or two at the most, and going with a deeper DOD means a smaller battery bank. In the example above, 50% DOD for 17.2kWh energy demand would equate to a 34.4 kWh battery.
The next step is to translate energy demand in kWh into battery amp hours (AhUsing the load profile above and a 48Vdc nominal battery bank, divide 21,500 Wh by 48V and 448 Ah is the minimum size battery bank for this application. Since the energy demand is based on a 24 hour rate, then the battery Ah for the same discharge rate of 24 hours should be used as the battery capacity (Ah) will vary depending on how fast it is discharged (click here to see table). Using the OutBack batteries listed in the table below, two strings (four 12V batteries in series for each string) of the 220GH batteries could be used for a total of 432 Ah and be slightly under our estimate. If we wanted to be more conservative, then we could choose to use three strings of the 170RE batteries for a total of 471 Ah.
Energy Storage eFeature |January 2015
