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Calculating the Costs of a PV System

Six Steps To Sizing A PV System

by Steven Shepard
sbtdesigns@earthlink.net

We have provided you with an easy to follow, step-by-step guide for sizing your photovoltaic (PV) system. Follow these six steps to determine your requirements and specify the components you will need.

1. Determine your power consumption demands

Make a list of the appliances and/or loads you are going to run from your PV system. Find out how much power each item consumes while operating. Most appliances have a label on the back which lists the wattage. If an appliance is rated in amps, multiply amps by operating voltage (120/240) to determine watts. Specification sheets, local appliance dealers, and the product manufacturers are other sources of information. We have provided a chart that lists typical power consumption demands of common devices which you can use as a guide. Once you have the wattage ratings, fill out the load sizing worksheet.

Power Consumption Chart

Estimated ratings for common appliances

Appliance Watts Appliance Watts Appliance Watts
Coffee Pot 200 Shaver 15 CF Lights - 40W  (Ican Equiv) 11
Coffee Maker 800 Computer - Laptop 20-50 CF Lights - 60W  (Ican Equiv) 16
Toaster 800-1500 Computer - Desktop 80-150 CF Lights - 75W  (Ican Equiv) 20
Blender 300 Printer 100 CF Lights - 100W  (Ican Equiv) 30
Microwave 600-1500 Typewriter - Electric 80-200 1/4" Drill 250
Hot Plate 1200 TV - 25" Color 150 1/2" Drill 750
Washing Machine - Automatic 500 TV - 19" Color 70 1" Drill 1000
Washing Machine - Manual 300 TV - 12" B&W 20 9" Disc Sander 1200
Vacuum Cleaner - Upright 200-700 VCR 40 3" Belt Sander 1000
Vacuum Cleaner - Hand 100 CD Player 35 12" Chain Saw 1100
Sewing Machine 100 Stereo 10-30 14" Band Saw 1100
Iron 1000 Clock Radio 1 7-1/4" Circular saw 900
Clothes Dryer - Electric 400 Satellite Dish 30 8-1/4" Circular saw 1400
Clothes Dryer - Gas 300 CB Radio 5 Frig/Freezer 20cf (15 hours) 540
Water Pump 250-500 Electric Clock 3 Frig/Freezer 16cf (13 hours) 475
Ceiling Fan 10-50 Lights - 100W Incandescent  100 SunFrost 16cf DC (7 hours) 112
Table Fan 10-25 Lights - 25W Compact Fluorescent  28 SunFrost 12cf DC (7 hours) 70
Electric Blanket 200 Lights - 50W DC Incandescent  50 Freezer 14cf DC (15 hours) 440
Blow Dryer 1000 Lights - 40W DC Halogen  40 Freezer 14cf DC (14 hours) 350
Lights - 20W Compact Fluorescent 22 SunFrost Freezer 19cf DC (10 hours) 112

AC and DC Load Sizing Work Sheets

Use this worksheet to determine the total Amp Hours per Day used by all the AC and DC loads in your system.

Step 1: Calculate your AC loads. If there are no AC loads, skip to Step 2, "Calculate your DC loads".

1. List all AC loads, wattage and hours of use per week (Hrs/Wk) in the spaces below. Multiply watts by Hrs/Wk to get watt-hours per week (WH/Wk). Add all the watt hours per week to determine AC Watt Hours Per Week.

Description of AC Loads Run by an Inverter Watts X Hrs/Wk = Wh/Wk
    X   =  
    X   =  
    X   =  
    X   =  
    X   =  
    X   =  
    X   =  
  Total Wh/Wk AC  

2. DC watt hours per week. Multiply total of step 1 by 1.2 to correct for inverter loss.

3. Inverter DC input voltage; usually 12 or 24 volts

4. Divide line 2 by line 3. This is total amp hours per week used by AC loads.

Step 2: Calculate your DC loads

5. List all DC loads in the spaces below:

Description of DC Load Watts X Hrs/Wk = Wh/Wk
    X   =  
    X   =  
    X   =  
    X   =  
    X   =  
    X   =  
    X   =  
  Total Wh/Wk DC  

6. DC system voltage. Usually 12 or 24 volts. (Same as line 3)

7. To determine total amp hours per week used by DC loads, divide line 5 by line 6.

8. To determine total amp hours per week used by AC loads enter line 4

9. Add lines 7 and 8.  This is total amp hours per week used by all loads.

10 Divide line 9 by 7 days. This is total average amp hours per day.

2. Optimize your power system demands

At this point, it is important to examine your power consumption and reduce your power needs as much as possible. (This is true for any system, but it is especially important for home and cabin systems because the cost savings can be substantial.) First identify large and/or variable loads (such as water pumps, outdoor lights, electric ranges, AC refrigerators, clothes washers, etc.) and try to eliminate them or examine alternatives such as propane or DC models. The initial cost of DC appliances tends to be higher than AC, but you avoid losing energy in the DC to AC conversion process, and typically, DC appliances are more efficient and last longer. Replace incandescent fixtures with fluorescent lights wherever possible. Fluorescent lamps provide the same level of illumination at lower wattage levels. If there is a large load that you cannot eliminate, consider using it only during peak sun hours, or only during the summer. (In other words, be creative!) Revise your Load Sizing Worksheet now with your optimized results.

3. Size your battery bank (if necessary)

Choose the battery you want to use (See "Characteristics of Batteries"). Fill out the Battery Sizing Worksheet. Other types of storage are available depending on the type of system you are considering. (Example: water storage tanks for pumping applications.)

Battery Sizing Worksheet

The first decision you have to make is how much storage you would like your battery bank to provide. Often this is expressed as "days of autonomy" because it is based on the number of days you expect your system to provide power without receiving an input charge from the solar array. In addition to the days of autonomy, you should also consider your usage pattern and the critical nature of your application. If you are installing a system for a weekend home, you might want to consider a larger battery bank because your system will have all week to charge and store energy. Alternatively, if you are adding a PV array as a supplement to a generator based system, your battery bank can be slightly undersized since the generator can be operated if needed for recharging. Once you have determined your required storage capacity, you are ready to consider the following key parameters.

1. Enter your daily amp-hour requirement. (From the Load Sizing Worksheet, line 10)

2. Enter the maximum number of consecutive cloudy weather days expected in your area, or the number of days of autonomy you would like your system to support.

3. Multiply the amp-hour requirement by the number of days. This is the amount of amp-hours your system will need to store.

4. Enter the depth of discharge for the battery you have chosen. This provides a safety factor so that you can avoid over-draining your battery bank (Example: If the discharge limit is 20%, use 0.2) This number should not exceed 0.8

5. Divide the amp-hours (line 3) of storage needed by the depth of discharge (line 4) limit.

6. Select the multiplier below that corresponds to the average wintertime ambient temperature your battery bank will experience.

Ambient Temperature Multiplier    

80F     26.7C 1.00
70F 21.2C 1.04
60F 15.6C 1.11
50F 10.0C 1.19
40F 4.4C 1.30
30F -1.1C 1.40
20F -6.7C 1.59

7. Multiply the amp-hours by line 6. This calculation ensures that your battery bank will have enough capacity to overcome cold weather effects. This number represents the total battery capacity you will need.

8. Enter the amp-hour rating for the battery you have chosen.

9. Divide the total battery capacity by the battery amp-hour rating and round off to the next highest number. This is the number of batteries wired in parallel required.

10. Divide the nominal system voltage (12 or 24V) by the battery voltage and round off to the next highest number. This is the number of batteries wired in series.

11. Multiply the number of batteries in parallel by the number of batteries in series. This is the number of batteries required.

4. Determine the sun hours available per day

Several factors influence how much sun power your modules will be exposed to:

  • When will you be using your system? Summer? Winter? Year-round?

  • Typical local weather conditions

  • Fixed mountings vs. trackers

  • Location and angle of PV array

We have provided the following charts which show ratings that reflect the number of hours of full sunlight available to generate electricity. Your solar array's power generation capacity is dependent on the angle of the rays as they hit the modules. Peak power occurs when the rays are at right angles or perpendicular to the modules. As the rays deviate from perpendicular, more and more of the energy is reflected rather than absorbed by the modules. Depending on your application, sun tracking mounts can be used to enhance your power output by automatically positioning your array.

The charts reflect the difference in sunlight during spring, summer, autumn and winter. It is more difficult to produce energy during the winter because of shorter days, increased cloudiness and the sun's lower position in the sky. The charts list the sun hour ratings for several cities in North America for summer, winter, and year round average. If you use your system primarily in the summer, use the summer value, if you are using your system year-round, especially for a critical application, use the winter value. If you are using the system most of the year (spring, summer and fall) or the application is not critical, use the average value. Between the chart and the map, you should be able to determine a reasonable estimate of the sun's availability in your area.

Sun Hours / Day - Chart

State City Summer Winter Yr. Round Avg   State City Summer Winter Yr. Round Avg
AK Fairbanks 5.87 2.12 3.99   MO Columbia 5.50 3.97 4.73
AK Matanuska 5.24 1.74 3.55   MO St. Louis 4.87 3.24 4.38
AL Montgomery 4.69 3.37 4.23   MS Meridian 4.86 3.64 4.43
AR Bethel 6.29 2.37 3.81   MT Glasgow 5.97 4.09 5.15
AR Little Rock 5.29 3.88 4.69   MT Great Falls 5.70 3.66 4.93
AZ Tuscon 7.42 6.01 6.57   MT Summit 5.17 2.36 3.99
AZ Page 7.30 5.65 6.36   NM Albuquerque 7.16 6.21 6.77
AZ Pheonix 7.13 5.78 6.58   NB Lincoln 5.40 4.38 4.79
CA Santa Maria 6.52 5.42 5.94   NB N. Omaha 5.28 4.26 4.90
CA Riverside 6.35 5.35 5.87   NC Cape Hatteras 5.81 4.69 5.31
CA Davis 6.09 3.31 5.10   NC Greensboro 5.05 4.00 4.71
CA Fresno 6.19 3.42 5.38   ND Bismark 5.48 3.97 5.01
CA Los Angeles 6.14 5.03 5.62   NJ Sea Brook 4.76 3.20 4.21
CA Soda Springs 6.47 4.40 5.60   NV Las Vegas 7.13 5.84 6.41
CA La Jolla 5.24 4.29 4.77   NV Ely 6.48 5.49 5.98
CA Inyokern 8.70 6.87 7.66   NY Binghampton 3.93 1.62 3.16
CO Grandby 7.47 5.15 5.69   NY Ithica 4.57 2.29 3.79
CO Grand Lake 5.86 3.56 5.08   NY Schenetady 3.92 2.53 3.55
CO Grand Junction 6.34 5.23 5.85   NY Rochester 4.22 1.58 3.31
CO Boulder 5.72 4.44 4.87   NY New York City 4.97 3.03 4.08
DC Washington 4.69 3.37 4.23   OH Columbus 5.26 2.66 4.15
FL Apalachicola 5.98 4.92 5.49   OH Cleveland 4.79 2.69 3.94
FL Belie Is. 5.31 4.58 4.99   OK Stillwater 5.52 4.22 4.99
FL Miami 6.26 5.05 5.62   OK Oklahoma City 6.26 4.98 5.59
FL Gainsville 5.81 4.71 5.27   OR Astoria 4.76 1.99 3.72
FL Tampa 6.16 5.26 5.67   OR Corvallis 5.71 1.90 4.03
GA Atlanta 5.16 4.09 4.74   OR Medford 5.84 2.02 4.51
GA Griffin 5.41 4.26 4.99   PA Pittsburg 4.19 1.45 3.28
HI Honolulu 6.71 5.59 6.02   PA State College 4.44 2.79 3.91
IA Ames 4.80 3.73 4.40   RI Newport 4.69 3.58 4.23
ID Boise 5.83 3.33 4.92   SC Charleston 5.72 4.23 5.06
ID Twin Falls 5.42 3.42 4.70   SD Rapid City 5.91 4.56 5.23
IL Chicago 4.08 1.47 3.14   TN Nashville 5.20 3.14 4.45
IN Indianapolis 5.02 2.55 4.21   TN Oak Ridge 5.06 3.22 4.37
KN Manhattan 5.08 3.62 4.57   TX San Antonio 5.88 4.65 5.30
KN Dodge City 4.14 5.28 5.79   TX Brownsville 5.49 4.42 4.92
KY Lexington 5.97 3.60 4.94   TX El Paso 7.42 5.87 6.72
LA Lake Charles 5.73 4.29 4.93   TX Midland 6.33 5.23 5.83
LA New Orleans 5.71 3.63 4.92   TX Fort Worth 6.00 4.80 5.43
LA Shreveport 4.99 3.87 4.63   UT Salt Lake City 6.09 3.78 5.26
MA E. Wareham 4.48 3.06 3.99   UT Flaming Gorge 6.63 5.48 5.83
MA Boston 4.27 2.99 3.84   VA Richmond 4.50 3.37 4.13
MA Blue Hill 4.38 3.33 4.05   WA Seattle 4.83 1.60 3.57
MA Natick 4.62 3.09 4.10   WA Richland 6.13 2.01 4.44
MA Lynn 4.60 2.33 3.79   WA Pullman 6.07 2.90 4.73
MD Silver Hill 4.71 3.84 4.47   WA Spokane 5.53 1.16 4.48
ME Caribou 5.62 2.57 4.19   WA Prosser 6.21 3.06 5.03
ME Portland 5.23 3.56 4.51   WI Madison 4.85 3.28 4.29
MI Sault Ste. Marie 4.83 2.33 4.20   WV Charleston 4.12 2.47 3.65
MI E. Lansing 4.71 2.70 4.00   WY Lander 6.81 5.50 6.06
MN St. Cloud 5.43 3.53 4.53            

5. Size your array

Choose the appropriate module size and fill out the Array Sizing Worksheet.

Array Sizing Worksheet

Use this worksheet to figure out the total number of solar modules required for your system.

To find average sun hours per day in your area (line 3), check the local weather data, or chart above for the city nearest your location. If you require year-round autonomy, use the winter numbers.

The peak amperage of the module you will be using can be found in the module specifications. You can also determine peak amperage if you divide the module's wattage by the peak power point voltage, usually 17 to 17.5.

1. Total average amp hours per day from the Load Sizing Worksheet, line 10.

2. Multiply line 1 by 1.2 to compensate for loss from battery charge/discharge.

3. Average sun hours per day in your area.

4. Divide line 2 by line 3,. This is the total solar array amps required.

5. Optimum or peak amps of solar module used. See module specifications.

6. To determine total number of solar modules in parallel required, divide line 4 by line 5.

7. Round off to the next highest whole number.

8. Number of modules needed to provide DC Battery voltage:

DC Battery Voltage # of Modules in each series string
12 1
24 2
48 4

9. To determine total number of solar modules required, multiply line 7 by line 8.

 

6. Interactive Spreadsheet (Excel)

We've put together a spreadsheet with sample values (from our RV) for you to play with. Follow the instruction on this page, and watch the number calculate for you. We made 3 assumptions on this sheet:

1. Battery - Trojan L16

2. PV Panel - Astropower AP1206

3. Location - NJ

Substitute your own values.

Excel Spreadsheet

7. Specify a kit or custom systems components

We have provided several kits tailored to meet your specific needs. Choosing a kit guarantees that you will get all of the components you need, often at a lower price than if the components were purchased separately. If you require a custom system, this catalog has all of the components you require. Depending on your application, you may need to specify the following components to complete your system:

  • Controller

  • DC/AC Inverter

  • Connecting wires and cables

  • Fuses, switches, and plugs

  • Meters

  • Mounting hardware

  • Back-up generator

You will find all of these items in our catalog. Call us and we will assist you in assembling a system that meets your power needs and financial requirements.

SBT Designs
25840 IH-West #1
Boerne, TX 78006
(210) 698-7109
http://www.sbtdesigns.com
sbtdesigns@earthlink.net


 

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