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Determining Your Battery Capacity Needs
Another very important determinant of the success of your installation is the battery capacity allocated to your power
inverter system. It is only by providing the necessary 12 Volt input power that you can achieve the maximum rated
output of the power inverter.
There are many different ratings used to value the energy stored in a battery. Most starting batteries rate their power
using "Cranking Amps" or "Cold Cranking Amps". While these ratings are a very good way to determine a battery's
viability in a specific starting application, they are of little use when determining if a specific battery is suitable for a
power inverter system application.
Deep Cycle batteries are usually rated in either Reserve Capacity (expressed in minutes) or Amp Hours. Both of
these measures provide a rating of how long the battery can provide power, either expressed as how long a battery
can provide a certain power output (Reserve Capacity Minutes) or how much power can be delivered over a certain
timespan (Amp Hours).
To determine your power requirement, and therefore your battery capacity requirement, you will need to combine the
power needs of all of your typical daily applications, or uses, of the power inverter system.
First, determine the wattage requirement of each tool, electronic item or appliance that you will use, which is usually
listed on the item. To determine the power draw in watts of an item that is listed only in amps, multiply the amp
draw times 120. Then, for each item, determine the length of time each item will be powered. Multiply the wattage
requirement times the hours for each item, which yields the watt-hours required for each.
Add the watt-hours for all items to get a total watt-hour requirement. Then, multiply the total watt-hour requirement
(for all items) times the number of days between charging your battery(ies). (Note: This step is unnecessary if
you plan to recharge your battery(ies) nightly.) Divide your total watt-hour days by 10 to derive your amp hour
requirement.
Here is an example of the calculation that you might perform:
Item to be Powered
Computer
3/8" Drill
Recip Saw
Shop Vac
Total Watt-Hour Requirement: 3240
Battery Amp Hour Requirement (total Watt-Hours Divided by 10): 325
Note: If a battery you are considering is rated in Reserve Capacity, use the following formula to convert reserve
capacity to amp hours: (Reserve Capacity/2) + 15.5 = Amp-Hour Rating
We recommend that you create an installation with a total capacity at 1.75-2.5 times your derived amp hour
requirement. This will allow you to perform unanticipated applications or delay your normal charging cycle without
detriment to the system (by over-discharging the system's batteries) or your planned applications. In the above
example, you should plan to provide between 700 and 1100 total Amp Hours in battery capacity, depending on the
variability of your application needs or the sureness of your recharging schedule.
Batteries Connected to a Vehicle System
We recommend that, when using batteries that are connected to a vehicle's charging system, you use auxiliary
batteries to supply power to your inverter, rather than the vehicle's starting battery. In addition, we recommend that
you utilize a battery isolator in order to avoid draining your vehicle's starting battery while powering your inverter.
A battery isolator allows you to recharge your auxiliary power batteries from the vehicle's charging system, while
protecting the vehicle's starting battery. Battery isolators can be found at most auto parts stores. Please see
Charging System Requirements for more details.
SOLAR Power Inverter USER'S MANUAL
Draw
Expected Run Time
180 Watts
840 Watts
1440 Watts
960 Watts
Watt-Hour Requirement
4 hours
1 hour
.5 hours
1 hour
7
720
840
720
960
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