Xantrex Technology 120 VAC/60 Battery Sizing, Estimating Battery Requirements, Page 101, Watts

TECHNICAL INFORMATION

BATTERY SIZING

Batteries are the inverter’s fuel tank. The larger the batteries, the longer the inverter can operate before recharging is necessary. An undersized battery bank results in reduced battery life and disappointing system performance.

Batteries should not be discharged more than 50% of their capacity on a regular basis. Under extreme conditions (such as a severe storm or a long utility outage), cycling to a discharge level of 80% is acceptable. Totally discharging a battery may result in permanent damage and reduced life.

For stand-alone applications, it is common to size a battery to provide between 3 and 5 days worth of storage before the battery requires recharging. The power contribution from other charging sources is not included in this calculation to duplicate the conditions present during a cloudy or windless period. This is often referred to as the “number of days of autonomy”. If the system is a hybrid system with daily generator run periods, then the battery size may be smaller. During cloudy periods the generator would be expected to run longer.

Utility connected applications often have very small batteries. If the system does not provide utility backup function, the minimum battery capacity recommended is 320 amp-hours @ 12 vdc, 160 amp-hours @ 24 vdc, and 80 amp- hours @ 48 vdc. If utility back up is required, larger batteries will be needed.

ESTIMATING BATTERY REQUIREMENTS

In order to determine the proper battery bank size, it is necessary to compute the number of amp hours that will be used between charging cycles. When the required amp hours are known, size the batteries at approximately twice this amount. Doubling the expected amp hour usage ensures that the batteries will not be overly discharged and extends battery life. To compute total amp hours usage, the amp hour requirements of each appliance that is to be used are determined and then added together.

You can compute your battery requirements using the nameplate rating of your appliances. The critical formula is Watts = Volts X Amps. Divide the wattage of your load by the battery voltage to determine the amperage the load will draw from the batteries.

If the AC current is known, then the battery amperage will be:

(AC current) X (AC voltage) = DC amps (battery voltage)

Multiply the amperage by the number of hours the load will operate and you have, reasonably enough, amp-hours.

Motors are normally marked with their running current rather than their starting current. Starting current may be 3 to 6 times running current. Manufacturer literature may provide more accurate information compared to the motor nameplate. If large motors will be started, increase the battery size to allow for the high demand start-ups require.

Follow this procedure for each item you want to use with the inverter. Add the resulting amp hour requirements for each load to arrive at a total requirement. The minimum properly sized battery bank will be approximately double this amount. This will allow the battery to be cycled only 50% on a regular basis.

Table 5, Typical Wattage Of Common Appliances

Appliance

One FL Light

B&W TV (12”)

Color TV (19”)

Computer

Stereo or VCR

Hair Dryer or Iron

Vacuum or coffee maker

3/8” Drill

TYPICAL APPLIANCE WATTS

Watts	Appliance
10	Microwave (compact)
100 - 500	Microwave (full size)
150	Toaster
200 - 350	Hot Plate
50	Washer/Dryer
1000	Blender
1200	*Refrigerator (3 cu ft)
500	*Refrigerator (12 cu ft)

Watts

600 - 800

1500

1000

1800

375 - 1000

400

180

480

*Refrigerators and icemakers typically only run about 1/3 of the time, therefore the running wattage is 1/3 of the total wattage of the appliance.

 2001 Xantrex Technology, Inc.	Telephone: 360/435-8826	SW Series Inverter/Charger
5916 - 195th Street N. E.	Fax: 360/435-2229	Part No. 2031-5
Arlington, WA 98223	www.traceengineering.com	Rev. C: February 2001

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