DIY Manual

3 | P a g e 10-543-1 REV A
DIY Manual
If the wattage of an appliance is not known or marked on the appliance, use amps. Watts = Volts
x Amps. For a 120VAC hair dryer drawing 10A, that will be 1200W (1200W = 120VAC x 10A).
Don’t forget the time element: If you use the 1200W hair dryer for 10 minutes, your watt-hours
for this appliance will be 200Wh (1200W x 1/6 hour = 200Wh).
Refrigerators, freezers, well pumps, et cetera are cyclical loads, turning on and off at random
(based on usage). Charts online show the average 24-hour consumption of such cyclical
appliances. Be mindful that refrigeration energy usage varies with room temperature. To over-
design a system, just in case, you can use the wattage rating of the appliance and multiply by 6
hours for a refrigerator or freezer. For an AC blower motor on your woodstove or furnace, for
example, you need to determine realistically how long it will be on. Even though it may have a
thermostat which turns it off and on, if you set the thermostat up high, then it will be on longer.
So, use common sense and adjust the watt-hour calculations accordingly. If to be cautious you
err on the high side, that is OK, but that means higher costs in terms of extra panels and batteries.
Use of a Watts Up or Kill-a-Watt meter to determine actual energy consumption per appliance is
a great way to help determine your real power needs. Most are available for $20 or less.
The above load analysis example gave us a planned 24-hour consumption of 4500Wh. This
number will now be used to determine the size of the battery bank and will be used to determine
the size of the solar panel, or photovoltaic (PV), array.
Step 2 BATTERY BANK SIZING
Calculate Total Watt-hour Capacity
Most solar systems are designed with a planned depth-of-discharge of
50% for flooded or sealed batteries; Lithium batts are normally
discharged to 80% DOD, and some allow up to 100% DOD. That means
you will use 50% - 100% of the bank’s capacity each day (24 hours), then
recharge the bank the next day. For a longer battery bank lifespan, use
a lower depth of discharge (DOD), such as 20%, which means you will
only consume 20% of the total capacity, not half. The trade-off being
more batteries are required to meet your energy needs.
State of Charge (SOC) refers to “How full is the battery bank?” and Depth
of Discharge (DOD) refers to “What percentage of the total capacity of
energy have I consumed?” The terms/concepts are inversely related.
Depth of Discharge
Vs
State of Charge
DOD SOC
0% 100%
20% 80%
50% 50%
90% 10%
Consider increasing the total watt-hour figure by 20% to account for missed appliances,
unknown load usage, and just to give yourself a bit of a buffer, just in case!