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Load Analysis
The first step in the process of designing your
renewable energy home power system is to determine the amount
of electrical power that you need to generate. This process
explanation will take you through the steps required to add
up the power used by each appliance in your house to determine
your total power needs.
It is important to be as accurate as possible
when calculating the electrical load. If your renewable
energy system is too small for your power needs, then equipment
damage and possibly an electrical fire will occur. It is important
not to underestimate any appliances power consumption and
to account for every power draw in you home.
Step 1 –
Download Load Calculation Spreadsheet
A load
calculation spreadsheet has been designed for you
to add up your home power loads. Open this page
on another browser to switch back and forth easily. If you
do not have Excel, then print out a copy of the form and fill
in the numbers by hand. The following steps explain how to
fill out the load calculation spreadsheet. A sample
spreadsheet can be viewed to help understand how the form
should be used.
Step 2 – List Household
Appliances (Column A)
Systematically go through your house and list every appliance
that requires electricity. These are usually obvious appliances
such as refrigerators, lights, TVs and microwaves. However,
do not forget appliances used less often such as hair dryers,
power tools, irons and vacuum cleaners.
*Look for appliances that might have a continuous
electrical draw even when it is turned off. An example is
a VCR with a clock/timer. List these items twice, once for
the continuous load and once for the amount of power it uses
when it is operating.
Step 3 –
List Quantity of Each Item (Column B)
Simply record the number of each appliance that you recorded
in step 1.
Step 4 –
Determine Voltage (Column C & D)
Find the voltage and type of current (DC or AC) of each appliance.
Most household appliances are 110 V (117 rms.) AC. Some items
may be 12 volts DC. Don’t forget that most power tools
are 220 volts AC. Read each item's label or call the manufacturer
if you are not certain.
*Remember that the solar power system produces
DC (Direct Current) voltage. The more appliances in your home
that run on DC the better since an inverter is required to
convert DC power to AC power to run your AC appliances. An
inverter is less efficient then using straight DC power. However,
most homeowners find it impractical to change all of their
appliances to DC models, so inverters are commonly used.
Step 5 – Determine Power
for Each Appliance (Column E)
Determine the power (watts) required to run each item. This
can also be found on the label or from contacting the manufacturer.
Remember that power (watts) can also be calculated by multiplying
the voltage and current [P=V x I]. For an explanation of the
relationship between power, voltage and current click on the
key terms link.
Step 6 –
Usage (Column F & G)
Estimate the amount of time that each item is “on”
during a typical day. Next estimate the number of days each
week that the item is used for that amount of time. Be as
accurate as possible without underestimating the amount of
usage.
*Remember that a refrigerator, although plugged
in all the time, only runs about half of the time. So for
this example you would record 12 hours per day and 7 days
per week.
Step 7 –
Surge Watts (Column H)
Each appliance has maximum surge wattage that it is capable
of drawing. The surge wattage should also be on the manufacturers
label of each appliance.
*The surge wattage is especially important on
power tools where the surge can be greater than twice the
running wattage.
Step 8 –
Average Watt-hours per day (Column I)
The spreadsheet provided will calculate the average watt-hours
per day automatically when you have entered your data in Excel.
If you are recording your data by hand use the following formula
for each item:
Avg. Watt-hrs = (Qty.) x (Run Watts)
x (Hrs/Day) x (Days/Wk)/7
Step 9 –
Percent of Total (Column J)
Once again the spreadsheet has a built in formula for finding
the percent of the total power that each item uses. To calculate
this by hand, you must first add up the total average watt-hours
per day for every appliance. Then you can find the percent
of that total that each appliance uses with the following
formula:
% Total = (Watt-hrs per day) x 100 / (Total
Watt-hrs per day)
*It is good to see what appliances use the most
power in your home. This will help you find ways to decrease
your overall power consumption which lowers your electricity
bill and helps the environment. For example a Sun Frost DC
refrigerator uses about one-third the power that a regular
refrigerator of the same size uses. Also note the amount of
power consumed by your lighting. Fluorescent lights use about
one-quarter the amount of power that incandescent lights require.
Click to view a list
of common household appliances and the amount of power
that is required to operate each of them.
Step 10 –
Totals (Bottom of Spreadsheet)
When you have finished recording the power needs of each appliance
in your home, scroll to the bottom of the spreadsheet to view
the total watt-hours consumed by your home per day. If you
are calculating by hand, then simply add up each number in
column I. This is your total daily power consumption.
Step 11 – Surge
Wattage
Next look at each appliance’s surge wattage and record
the largest single surge wattage at the bottom of the spreadsheet.
This value is important for determining the size of your inverter.
Step 12 –
Simultaneous Items
Finally you need to determine the largest continuous power
draw from items running simultaneously. The idea is to determine
the maximum continuous draw that is possible for an extended
period of time (15 minutes or more). Record only the single
largest simultaneous wattage (not all of them added together).
To check maximum continuous wattage, follow these steps:
Open another copy of the load
calculation spreadsheet
Record only items that run simultaneously.
Look at the total run watts required.
Repeat using a different set of simultaneously
running appliances.
Record the largest set of simultaneously running
appliances.
Step 13 - Record Final Loads
To account for any minor inaccuracies:
Multiply all final values by 1.15 - This is
your required load data!
You now should have three important numbers:
Average daily watt-hrs consumed
Largest continuous wattage
Largest surge wattage
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