Page 152 - Alternative Energy Systems in Building Design

128 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

power requirements by the air-conditioning, laundry equipment, and kitchen appliances;
hence the norm used for sizing the power requirement for a residential unit boils down
to a fraction of the previously calculated power. As a rule of thumb, an average power
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demand for a residential unit is established by equating 1000–1500 W/1000 ft of
living space. Of course, this figure must be augmented by considering the geographic
location of the residence, the number of habitants, occupancy time of the population
within the dwelling unit, and so forth. As a rule, residential dwellings in hot climates
and desert locations must take the air-conditioning load into consideration.
As a side note when calculating power demand for large residential areas, major
power distribution companies only estimate 1000–1500 W of power per household,
and this is how they determine their mean bulk electric power purchase blocks.
When using a battery backup, a 30 percent derating must be applied to the overall
solar power-generation output efficiency, which will augment the solar power system
requirement by 2500 W.
In order to size the battery bank, one must decide how many hours the overall power
demand must be sustained during the absence of sun or insulation. To figure out the
ampere-hour capacity of the battery storage system, the aggregate wattage worked out
earlier must be divided by the voltage and then multiplied by the backup supply hours.
For example, at 120 V ac, the amperes produced by the solar system, which are stored
in the battery bank, will be approximately 20 A. To maintain power backup for 6
hours, the battery system must be sized at about 160 Ah.

EXAMPLE OF TYPICAL SOLAR POWER SYSTEM DESIGN AND
INSTALLATION PLANS FOR A SINGLE RESIDENTIAL UNIT
The following project represents a complete design and estimating procedure for a small
single-family residential solar power system. In order to establish the requirements of a
solar power system, the design engineer must establish the residential power demand
based on NEC design guidelines, as shown in the following.

Project design criteria The residential power demand for a single-family dwelling
involves specific limits of energy-use allocations for area lighting, kitchen appliances,
laundry, and air-conditioning systems. For example, the allowed maximum lighting
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power consumption is 3 W/ft of habitable area. The laundry load allowed is 1500 W
for the washer and dryer.
The first 3000 W of the total combined lighting and laundry loads are accounted at
100 percent, and the balance is applied at 35 percent. The total appliance loads, when
there are more than five appliances, are also derated by 25 percent. Air-conditioning and
other loads such as pools, saunas, and Jacuzzis are applied at their 100 percent value.
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The demand load calculation for this 1400-ft residential dwelling indicates a con-
tinuous demand load of about 3000 W/h. If it is assumed that the residence is fully
occupied and is in use for 12 hours a day, the total daily demand load translates into
36,000 W/day.
Since the average daily insolation in southern California is about 5.5 hours, the
approximate solar power system required to satisfy the daily demand load should be