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Does anybody know how many Watt/hours are held in a fully charged pair of dual batteries?
I want to calculate how long I can run a pair of 18W lights without turning on the engine. Or my 300W lightbar. 
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I'll add a tip so you can do it easier yourself in the future.
For some reason I don't know, many sites use units of Watts/hour when doing these kinds of estimates. That is an incorrect unit of measure and leads to confusion. It is not watts "per" hour. The correct unit of measure in this case is watt multiplied by hour, not divided. Obviously if you divide instead of multiply everything will be wrong, or the numbers won't make sense. Or the units won't make sense. As far as energy goes, 1000 watts for 2 hours is the same amount of energy as 2000 watts for 1 hour, or 1 watt for 2000 hours. They are each 2000 watthours. Or 2 kilowatthours. I'll also add that the three examples above, while equal in amount of energy, do not have the same effect on batteries. That's because the faster batteries are discharged, the lower amount of energy we can get out of them. The standard battery rating is based on discharging over a 20 hour period. So the 75 Amphour capacity only applies at that 20hour rate. And that's around 4 Amps, or roughly 50 watts for these particular batteries (I'm rounding off numbers). When we start pulling higher loads like 300 watts we will get less out of the same battery. 
Chance, good explanation. If you want to take it further and know the Peukert's constant (k) for your battery, you can use the formula to calculate the actual time a particular load can last (t) and the AH rating at that load (It). That might be good for the final analysis before setting a system in stone.
First calculate t = H (C/IH)^k then It = C (C/IH)^k1 H is the rated discharge time, in (hours). C is the rated capacity at that discharge rate, in (Amperehours). I is the actual discharge current, in (Amps). k is the Peukert constant, (dimensionless). t is the actual time to discharge the battery, in (hours). No, I am not that smart The full explanation is here > http://www.batterystuff.com/kb/tools...capacity.html 

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