Xantrex Technology 120 VAC/60 - Manual (Page 99)

OPERATION
 2001 Xantrex Technology, Inc.
5916 - 195th Street N. E.
Arlington, WA 98223
Telephone: 360/435-8826
Fax: 360/435-2229
www.traceengineering.com
SW Series Inverter/Charger
Part No. 2031-5
Rev. C: February 2001
Page
91
The system should be designed so that the battery is able to operate the loads for the entire peak rate
period without reaching the low battery transfer voltage. This may require that heavy loads be operated
only during the non-peak rate period.
This mode may be advantageous when the utility offers time-of-day metering that allows you to buy power
at a variable rate during different time periods. For example, the rate may be only $0.04 per kilowatt-hour
from midnight to 6 AM, but $0.16 during the peak period from noon to 6 PM. The in-between periods might
cost $0.08 per kilowatt-hour. The idea is to not use utility power during the peak period and to charge the
battery at night, during the off-peak period. A solar array is not required but, since it produces the most
power during the peak period, it will reduce the amount of power required from the battery.
The greater the difference between the peak and off-peak rates, the greater the value of this operating
mode. When combined with a solar array, this mode may be more economic than operating the system in
low battery transfer mode.
If the battery is full and power is available from the solar array, it will be used to directly power the AC
loads connected to the inverter output, even though the utility grid is also connected to the loads. If the
amount of power exceeds the AC loads connected, the battery voltage will increase. An external solar
array controller is required to limit the battery voltage to a safe level and to prevent overcharging and
possible damage to the battery when only light loads are being powered.
Note: The round-trip net energy efficiency of charging and discharging a Deep Cycle battery rarely
exceeds 50%. In order for this technique to be cost effective, the off-peak rate foe electricity should be no
more than half of the on-peak rate. The cost of battery replacement should also be calculated and
included when evaluating this technique.
To program the inverter for the example above:
•
Set the SET GRID USAGE menu item, under the INVERTER SETUP (9) menu heading, to FLT.
•
Set the START CHARGE TIME menu item, under the GRID USAGE TIMER (18) menu heading, to 6
PM (18:00).
•
Set the END CHARGE TIME menu item, under the GRID USAGE TIMER (18) menu heading, to
NOON (12:00).
•
Set the SET START BULK TIME menu item, under the BULK CHARGE TRIGGER TIMER (15) menu
heading, to a time after MIDNIGHT (00:30) to delay most of the recharging for the cheapest period.
•
Setting the SET FLOAT VOLTS DC menu item, under the BATTERY CHARGING (10) menu
heading, to the nominal full battery voltage level will reduce the amount of battery charging that occurs
between time periods, yet will allow limited charging to prevent sulfation of the battery while sitting
partially discharged.