After talking to a customer in Sengal, West Africa today (7/01/2012), we thought we would post the following to help all our customers calculate their load and battery solution correctly when considering Power Inverter applications:

Selecting a battery or system of batteries that will provide your Inverter/Charger with proper DC voltage and an adequate amp-hour capacity to power your application is straight forward. Even though Tripp Lite Inverter/Chargers are highly-efficient at DC-to-AC inversion, their rated output capacities are limited by the total amp-hour capacity of connected batteries and the support of your vehicle’s alternator if the engine is kept running.

• STEP 1: Determine Total Wattage Required
Add the wattage ratings of all equipment you will connect to your Inverter/Charger. Wattage ratings are usually listed in equipment manuals or on nameplates. If your equipment is rated in amps, multiply that number times AC utility voltage to determine watts. (Example: a ¼ in. drill requires 2½ amps. 2½ amps × 120 volts = 300 watts .)

Note: Your Inverter/Charger will operate at higher efficiencies at about 75% - 80% of nameplate rating.

• STEP 2: Determine DC Battery Amps Required
Divide the total wattage required (from step 1, above) by the battery voltage (12) to determine the DC amps required.

540 watts ÷ 12V = 45 DC Amps

• STEP 3: Estimate Battery Amp-Hours Required (for operation unsupported by the alternator)
Multiply the DC amps required (from step 2, above) by the number of hours you estimate you will operate your equipment exclusively from battery power before you have to recharge your batteries with utility- or generator-supplied AC power. Compensate for inefficiency by multiplying this number by 1.2. This will give you a rough estimate of how many amp-hours of battery power (from one or several batteries) you should connect to your Inverter/Charger.

45 DC Amps × 5 Hrs. Runtime
× 1.2 Inefficiency Rating = 270 Amp-Hours

NOTE! Battery amp-hour ratings are usually given for a 20-hour discharge rate. Actual amp-hour capacities are less when batteries are discharged at faster rates. For example, batteries discharged in 55 minutes provide only 50% of their listed amp-hour ratings, while batteries discharged in 9 minutes provide as little as 30% of their amp-hour ratings.

• STEP 4: Estimate Battery Recharge Required, Given Your Application
You must allow your batteries to recharge long enough to replace the charge lost during inverter operation or else you will eventually run down your batteries. To estimate the minimum amount of time you need to recharge your batteries given your application, divide your required battery amp-hours (from step 3, above) by your Inverter/Charger’s rated charging amps.

270 Amp-Hours ÷ 55 Amps
Inverter/Charger Rating = 5 Hours Recharge

NOTE! For Tripp Lite Inverter/Chargers providing 1250 watts or less of continuous AC power, a full-size battery will normally allow sufficient power for many applications before recharging is necessary. For mobile applications, if a single battery is continuously fed by an alternator at high idle or faster, then recharging from utility or generator power may not be necessary. For Tripp Lite Inverter/Chargers over 1250 watts used in mobile applications, Tripp Lite recommends you use at least two batteries, if possible fed by a heavy-duty alternator anytime the vehicle is running. Tripp Lite Inverter/Chargers will provide adequate power for ordinary usage within limited times without the assistance of utility or generator power. However, when operating extremely heavy electrical loads at their peak in the absence of utility power, you may wish to “assist your batteries” by running an auxiliary generator or vehicle engine, and doing so at faster than normal idling.

If you need additional information or assistance with sizing your battery please call us on 0845 519 3638 or contact us via e-mail.