Sizing Examples for
Residential PV Systems
DC Cabin
AC/DC Residence
In this section you will find filled-in worksheets to illustrate the calculations required in determining the size of power system needed to meet the load. Blank worksheets and instructions may be accessed and copied by going here. To see an example of a residential application that uses a PV/engine hybrid system go here.
This system is designed to power lights, a stereo, and a refrigerator for a vacation cabin occupied three days per week, May through September, in the mountains of Vermont. The cabin is far from utility service and located in a scenic area where utility lines would not be welcome. A 4 cubic-foot refrigerator, typical of those used in the RV industry, is a major part of the load. The refrigerator will require power seven days per week during the summer period but on four of those days the unit will not be opened.
Site
Vermont
Location/Elevation
44°N - 73° W - 540 meters
Environment
Mountains
Temperature Range (°C)
-25 to 35
Maximum Wind Speed (m/s)
20
Availability Required
90 percent (not critical)
Days of Storage
3
Load Profile
Variable when cabin is used - refrigerator only in summer
Installation
A two-story south-facing wall was used as the mounting surface for the array. This allowed much better solar access than a pitched roof or a ground mount and leakage was not a problem with the wall mount. The upper portion of the array was fastened directly to the wall, while the lower portion stood out from the wall on mounting legs to achieve the proper tilt. A weatherproof junction box was used to prevent moisture from following the array conductors into the building. The meters (array current, load current, and system voltage) were located in the kitchen area where they could be observed easily. The load center was located adjacent to the meters. The system batteries were located in an upstairs storage loft and vented to the outside. Because of the cabin construction (milled tongue & groove logs), all wiring was enclosed in metal raceways to prevent physical damage from exposure and rodents. Plugs and receptacles with a special dc configuration were used in place of standard receptacles. These units were approved for this application by the local electrical inspector. The use of special plugs and receptacles prevents an unfamiliar user from plugging the low voltage dc equipment into the standard ac receptacles that are also available in the cabin. (A portable ac generator is used for an occasional maintenance job or to run the vacuum cleaner.) The homeowners were supplied with a complete manual for their system. The manual describes expected system performance, what items require maintenance at what intervals, and some simple troubleshooting steps to be taken in case of system malfunction.
Worksheets
The worksheets below, numbered 1 to 4 were filled in to calculate the residential load, design current required, battery size, and PV array size. For blank worksheets and instructions click here. Worksheet #5, Hybrid determinator, was not used for this small power requirement. Some explanatory notes follow the worksheets. For more information contact PVSAC.
The total load power that would be required if all loads were operated at the same time is 204 Watts. Given the estimated time each load would be operated gave a total of 55.4 Amp-hours per day. When losses for wiring and battery efficiency are included this total becomes 62.8 Amp-hours per day.
The controller is mounted in the kitchen where meters can be monitored by the homeowner.
The wiring diagram for this system is shown in the figure below.
AC/DC Residential
A homeowner living full time in a remote location has been using a portable generator to provide ac power to his home. The family is not happy with the generator noise and need for fuel. They already have a battery bank that is charged with the generator and dc power is used for some lighting. The major loads such as washing machines, pumps, etc., use ac power. They designed this example PV system to supply their power needs. They made provision to manually switch the generator on and charge the batteries in an emergency.
Colorado
41°N - 105° W - 2000 meters
-30 to 33
15
4
Variable
The array was ground mounted on a series of concrete poles about 100 feet from the home. This configuration allowed the array to be oriented at true south while avoiding the high wind hazard associated with a roof mount. Because of the long wire run, aluminum conductors were used to minimize cost. The conductors, installed underground in metallic conduit, were terminated using the appropriate lugs for aluminum wire. The central electrical distribution system was located near the laundry room in the house. This location was for ready access to existing distribution equipment in the house. The house circuits were already segregated into two service panels, one panel containing the dc circuits and the other the ac circuits. The dc panel was served directly by the battery bank through the controller. A current limiting fuse was used in the positive battery lead. A manual transfer switch was used to allow the generator to charge the batteries. All equipment was grounded according to Article 250 of the NEC. The inverter was protected by a fused safety switch, so it could be easily isolated from the batteries for maintenance. Fused safety switches also isolated the controller from the battery and array. The negative conductor of the dc system and neutral conductor of the (240) ac system were connected to ground. All ungrounded conductors were protected by either circuit breakers or fuses. Surge arresters were installed in both the ungrounded dc and ac system conductors to suppress transients induced by lightning. A low voltage alarm (visual indicator) was placed in the kitchen to alert the homeowner of a low battery state-of-charge.
Two worksheets are required to list all the loads. The total loads are summed on the next sheet.
Since the ac loads are larger the system voltage is selected as 24 volts to be compatible with the inverter input voltage.
The winter time load is expected to be the largest. The system is sized to meet this load.
For this non-critical application the number of modules was "rounded down" to save money.
When the water pump is operating, the washing machine and toaster oven are automatically isolated from the inverter to prevent overloading.
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