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Sizing Your Solar System
The steps that you should follow to design the right solar system for your home or
business include the following:
1. Determine Your Power Consumption Demands - Make a list of the appliances
and/or loads that you are going to run from your solar power system. Start your list
with three columns, one for the appliance name, one for the wattage, and one for
current type (AC or DC). Find out how many watts each device uses and whether it
runs on AC or DC power. Most appliances have a label on the back that lists the
wattage. Specification sheets, local appliance dealers, and product manufacturers are
also good sources of this information. If you can not find the wattage but you do
know the volts and amps, simply multiply the volts by the amps to determine the
wattage (Volts X Amps = Watts).
Step 1 Example
Appliance Name
Watts
Current
Television
100
AC
Lights
60
AC
Laptop Computer
35
 DC
                  
2. Optimize Your Power System Demands - At this stage, it is important to
examine your power consumption and reduce your power needs as much as possible
(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. Also, replace incandescent light bulbs with fluorescent lights wherever
possible (they provide the same amount of illumination, but at lower wattage levels.
After you've completed this process, you have your optimized load list.
3. Size Your Inverter - We'll do an easier one first. Take a look at the AC appliances that are on your optimized load list. Add together the wattage of all of the appliances that must/will run at the same time. This is your rough inverter size. We suggest that you add 25% to this number to allow your system to grow. When you've completed this step, you should have your wattage total + 25%. Since manufacturers measure the size of their inverters in watts, your total tells you the size of the inverter required.
Step 3 Example
Appliance Name 
Watts 
Current
Television 
100 
AC
Lights 
60 
AC
Laptop Computer 
35 
DC
TOTAL AC 
160
 
(160 X .25 ) +160 = 200 WATTS (Inverter Size)
 
4. Size Your Battery Bank - Sizing your battery bank is a little harder. We'll give
you a relatively simple method to develop a rough estimate of your needs.
A. Add an additional column to your optimized load list that you developed in step 2 and call it Hours of Daily Usage. In this column you are going to estimate the number of hours you expect to use your appliance each day. For example, if I expect to watch my television for 4 hours a day, I place a 4 in the new column. If I expect to use a laptop computer for an hour and a half I would enter 1.5 into the new column.
B. After you've completed your estimates for all of the items on your list, add a second column called Daily Watt Hours Used. In this column, multiply the Watts by the Hours of Daily Usage for each row in your list. Finally, add the Daily Watt Hours Used column to determine the Total Daily Watt Hours Used.
C. Now, multiply the Total Daily Watt Hours Used by the anticipated number of consecutive cloudy days expected or the number of days of autonomy you would like your system to have (i.e. days between charging - usually between 1 - 5) to determine your Rough Battery Size in Watt Hours.
D. Multiply your Rough Battery Estimate by 2 to determine your Safe Battery Size in
Watt Hours. This allows for 50% maximum battery discharge during normal operations, and provides for an additional 50% for emergency situations.
E. Now we need to determine your Safe Battery Size in Amp Hours by converting from
Safe Battery Size in Watts. To do this you simply divide your Safe Battery Size in Watts by your Expected System Voltage (12, 24, or 48 volts DC).
F. Determine the Battery Amp Hour Rating and the Battery Voltage for the battery that you would like to use in your system.
G. Divide the Safe Battery Size (from step E) in Amp Hours by the Amp Hour Rating (step F) for your chosen battery. You now know the Number of Batteries in Parallel.
H. Divide the Expected System Voltage by the Battery Voltage (step F). You now know the Number of Batteries Serial.
I. Multiply the number of batteries that you will need to wire in parallel by the number of batteries in series to determine Total Number of Batteries in Your Battery Bank.
Step 4 Example
Appliance Name
Watts 
Current 
Hours of
Daily Usage
 
Daily Watt
Hours Used
 
Television 
100 
AC 
4.0
400
Lights 
60 
AC 
4.0
240
Laptop Computer
35 
DC 
1.5
52.5
 
Total Daily Watt Hours Used
692.5
 
C. Number of Days of Autonomy = 3
C. Rough Battery Estimate = 3 X 692.5 = 2,077.5
D. Safe Battery Size in Watt Hours = 2 X 2,077.5 = 4,155
E. Safe Battery Size in Amp Hours = 4,155/12 Volts = 346.25 AH
F. Selected Battery Characteristics = 12V, 60AH
G. Number of Batteries in Parallel = 346.25/60 = 6
H. Number of Batteries in Serial = 12/12 = 1
I. Total Number of Batteries in Bank (supports the load for three days without
charging) = 1 X 6 = 6
5. Determine the Sun Hours Available Per Day - Another easy one. For India,
you can take 5-1/2 hours.
6. Size Your Array -
A. Take your Total Daily Watt Hours Used in step 4-B and multiply by seven. This is your Total Weekly Watt Hours Used.
B. Multiply this number by 1.2 to correct for inverter loss.
C. Divide the result by 12 or 24 (12 for 12 volt systems). You now have your Total
Amp Hours Per Week.
D. Multiply this number by 1.2 to compensate for loss from battery
charge/discharge.
E. Divide this number by the average Sun Hours Available Per Day (from step 5).
This is your total solar amps required to support your electrical load.
F. Divide this number by the optimum or peak amps of the solar module you intend
to use. Round-off to the next highest whole number. This is the number of modules
you need wired in parallel.
G. If your system is 12V you don't need any modules wired in series, 24V you need 2
modules wired in series, 48V you need 4 modules wired in series, assuming the
nominal panel voltage of 12 V.
H. If your system is 2V4 or 48V, multiply the number of modules in step F by the
number of modules in step G. This is the total number of modules required in your
system. If your system is 12V use the number from step F to determine your total
number of modules.
7. Size your Charge Controller - Your controller must be able to handle the
maximum currrent produced by the PV modules; as well as the maximum load
current, if the controller includes load control. Clouds and reflected sunlight from
snow or building surfaces can increase the amount of PV output current by as much
as 25% above the modules short circuit current rating. The controller must be sized
to handle this current rating. Underwriters Laboratories (UL) and the National
Electrical Code (NEC) require that the controller is oversized by an additional 25% so
that the controller does not constantly operate at 100% of its rating. Therefore, the
controller needs to be able to handle at least 156% of the module's (or modules', if
wired in parallel) short circuit current rating.
 
For Example:
12Volt DC System
Array Size: 2 - 75 watt solar panels (short circuit current - 4.8 amps each)
Modules wired in parallel:
4.8 amps x 2 = 9.6 amps
9.6 amps x 1.56 = 14.97 amps
So, a 15 amp controller or larger will do the job. (Don't forget to allow for expansion
of the system - it is cheaper in the long run to buy a bigger controller that can
accommodate an extra PV module or two.)
8. Size your System Interconnects - Choose the appropriate size wires/cables to
connect your solar modules, batteries, and inverters. Consult our wire sizing page for
details.
9. Buy products from Sadhana Energy Devices (This step is not mandatory, but it
does helps us maintain and expand our information pages!)
10. Install Your System Safely - Make sure you have the knowledge and skills
necessary to install your system without hurting yourself or damaging your property.
Be sure to comply with all applicable codes in your area.

 

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