BOS Worksheets and Instructions

These worksheets are used to specify the components and equipment required to finish your PV system. They were first developed by experienced system designers and published in "Stand-Alone Photovoltaic Systems - A Handbook of Recommended Design Practices". Fill in the other worksheets to determine the size of your load and the PV array and battery needed to provide the required power. The blank worksheets in each section may be copied and saved.

Battery Controller

Inverter

Switches, etc.

DC & AC Wiring

The next five sheets help you specify hardware for stand-alone PV systems. It is recommended that data from several manufacturers be obtained and studied to complete these sheets. These manufacturers' data are required to make intelligent design tradeoffs. Also, appropriate articles in the NEC should be studied. These electrical code requirements have been developed to ensure safe, durable system installations.

Use Worksheet #6 to specify a battery charge controller for your system.

Select a controller that operates at the nominal dc system voltage.

A1 Array Short Circuit Current (A): Enter the value from Block 62 Worksheet #4.

A2 Minimum Controller Current (A): Calculate the minimum controller current. The multiplier of 1.25 oversizes the controller by 25 percent to allow for current production at highest solar irradiance conditions.

A3 Rated Controller Current (A): If a single controller cannot be found that will handle the current calculated in A2, parallel controllers may be used. Enter the manufacturer's rated value of the selected controller.

A4 Controllers in Parallel: Calculate the number of controllers in parallel.

A5 Controller: Use the form to describe the controller characteristics and features desired or available from the manufacturer. If any of the controller settings are adjustable, indicate the desired setting.

Use Worksheet #7 to specify parameters for the Power Conditioning Equipment (if required)

B1 Waveform: Specify the wave form desired. Inverter characteristics are discussed here.

B2 DC System Voltage (V): Enter the dc system voltage from Block 9, Worksheet #1. This value might have to be changed depending on the availability, performance, and cost of inverters.

B3 AC System Voltage (V): Enter the ac voltage desired.

B4 Surge Capacity (W): Enter the highest power that might be required for a short period. Starting motors may require up to six times the rated operating current.

B5 Total AC Watts (W): Enter the total ac load from Block 11B Worksheet #1.

B6 Maximum Single AC Load (W): Enter the maximum single ac load from those listed on Worksheet #1.

B7 Maximum Simultaneous AC Load (W): Enter the maximum expected simultaneous ac load. This is determined by summing the loads that could possibly operate at the same time. See Worksheet #1.

B8 Inverter Run Time at Maximum Simultaneous Load (Min): Estimate and enter the length of time in minutes that the inverter will have to support the maximum simultaneous ac loads.

B9 Inverter Continuous Duty Rating (W): Estimate the average power required from the inverter. Study the loads listed on Worksheet #1.

B10 Required Inverter Efficiency at Load (%): Enter the desired inverter efficiency at the average load. (Note: Manufacturers usually quote inverter efficiencies under ideal test conditions.)

B11 Inverter Specifications: Enter data from the manufacturer's data sheet for the selected unit.

The parameters listed should be considered when specifying a dc to dc converter. List the desired system specifications in the spaces provided and review manufacturer's literature to identify converters that meet the requirements.

C1 Input DC Voltage (V): Enter the input dc voltage from Block 9, Worksheet #1.

C2 Output DC Voltage (V): Enter the required output dc voltage of the converter.

C3 Output Power (W): Enter the power requirements of the specific loads that the converter must supply.

C4 Operating Temperature (C°): Enter the operating temperature range that the converter will be subjected to.

C5 Converter: Select an available converter and list its specifications.

Use Worksheet #8 to specify the switches, fuses, and other protection devices required operate a safe system.

Switches, circuit breakers, fuses, and diodes are used for safe operation and maintenance of a stand-alone PV system and are necessary to protect people and equipment. Review applicable electrical codes for guidance. Any switch must be capable of interrupting the current, ac or dc, flowing in the circuit. Ac rated switches and fuses will fail in dc circuits and should not be used. A fuse must be rated for dc current if used in a dc circuit. Fuses and switches are often included in a single package. A blocking diode may be used to prevent current flowing from the battery toward the array. Some controllers provide this protection internally. All protection components should be installed in weather resistant enclosures.

D1-D14 Protected Circuit: List the circuit where the protection device is to be installed. For example, the PV array output, the inverter to ac load, etc. Then check the device to be installed and its current and voltage ratings.

Use Worksheet #9 and #10 to determine the wire size (DC or AC) for your system

Review applicable codes and regulations before selecting the wire to be used in a stand-alone PV system. Determine whether single or multiple conductor wire should be used. Select a wire with a sheath (covering) that will withstand existing conditions. Be sure to specify sunlight resistant wire for locations where the wire will be exposed. Consider using metal conduit to protect the wires. Allow for temperature derate on ampacity if the wire will be exposed to temperatures exceeding 30°C. Grounding should be done according to local regulations. More about wire and wire sizes can be found here.

The following instructions apply to both ac or dc wire sizing.

E1 or F1 System Voltage (V): Enter the system voltage for each circuit.

E2 or F2 Maximum Current (A): Enter the maximum current for each circuit.

E3 or F3 One Way Length (m): Measure or estimate the length of wire runs in the system. This is the distance between components in the system--such as array to controller or controller to battery.

E4 or F4 Allowed Voltage Drop (%): Specify the maximum voltage drop for each of the circuits. If local regulations do not specify a maximum, use a default value of 3 percent voltage drop in any branch circuit and a maximum of 5 percent voltage drop from voltage source to load.

E5 or F5 Allowance for Temperature Derate: If current carrying conductors are exposed to temperatures greater than 30°C (84°F) their ampacity will be reduced. Consult Table 310-16 in the NEC or ask wire manufacturers for amount of derate.

E6 or F6 Wire Size: Determine the size of wire for each of the wire runs. Use the following tables to determine the maximum one-way wire distance for copper conductors used in 12-, 24-, 48-, and 120-volt circuits. Always be conservative in selecting wire sizes.

E7 or F7 Wire Type: Note the wire type including insulation material or coatings.

E8 or F8 Equipment Ground: Wires are typically No. 8 bare copper or larger. Refer to applicable codes.

E9 or F9 System Ground: Ground wires should be equal to or larger than the largest current carrying conductor. Locate the ground as close to the battery as possible. Refer to local regulations for grounding requirements. Note: All grounding for your system should be made at a single point.

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