Six Steps To Sizing A PV System
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 |
|
= |
|
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 |
|
= |
|
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.
| 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