Actually, watts is the fundamental unit of power and watt-hours is the energy stored. The key is to use the watts you know to calculate the amps at the battery voltage.
- For example, say you want to run a 250 watt 110VAC light bulb from an inverter for 5 hours.
- Watt-hours = watts * hours = 250 watts * 5 hours = 1250 watt hours
- Account for the efficiency of the inverter, say 85%
- Watt-hours = watts * hours / efficiency = 1250 / 0.85 = 1470 watt-hours
- Since watts = amps * volts divide the watt hours by the voltage of the battery to get amp-hours of battery storage
- Amp-hours (at 12 volts) = watt-hours / 12 volts = 1470 / 12 = 122.5 amp-hours.
- If you are using a different voltage battery the amp-hours will change by dividing it by the battery voltage you are using.
What if you don't have a constant load? The obvious thing to do is the thing to do. Figure out an average power drawn. Consider a repetitive cycle where each cycle is 1 hour. It consists of 20 amps for 1 second followed by 0.1 amps for the rest of the hour. The average current would be calculated as follows.
- 20*1/3600 + 0.1(3559)/3600 = 0.1044 amps average current.
- (3600 is the number of seconds in an hour).
- In other words, figure out how many amps is drawn on average and use steps 1 and 2. Step 3 is very difficult to predict in the case where you have small periods of high current. The news is good, a steady draw of 1C will lower the capacity much more than short 1C pulses followed by a rest period. So if the average current drawn is about a 20 hour rate, then you will get closer to the capacity predicted by a 20 hour rate, even though you are drawing it in high current pulses. Actual test data is hard to come by without doing the test yourself.
Popular UPS Systems
APC BR1500LCD
