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
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
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
(160 X .25 ) +160 = 200 WATTS
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
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
C. Number of Days of Autonomy = 3
C. Rough Battery Estimate = 3 X 692.5 =
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 =
G. Number of Batteries in Parallel =
346.25/60 = 6
H. Number of Batteries in Serial = 12/12
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.
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
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
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
9. Buy products from Sadhana Energy
Devices (This step is not mandatory, but it
does helps us maintain and expand our
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.
Total Daily Watt Hours Used