I find it difficult to locate C (coulombs) or J. I want to know how much energy or charge is in the store if I am going to get a rundown time. The manufacturers are very keen in advertising VA or W, not surprisingly because it determines how much kit you can plug in to the device, provided it doesn't die from energy starvation before the kit has had enough time to shut down cleanly, the whole point.
Batteries are always measured in ampere hour capacity. Never seen one measured in joules as joules are rarely used as a unit of measure in electrical design. The reason is that battery capacity varies as a function of discharge current (Peukert's law if you're interested) and temperature.
Working out capacity in joules (if you insist on doing so) is simple enough.
One amp at 1 V for one hour equals 3600 joules.
Lets take the UPS I have as an example :
The batteries are 12V and are rated at 18AH so that's 12V at 1A for 18 hours, so 12 x 18 = 216 then multiply by 3600 and you get a capacity of 777600J. Edit - there's two of them in series so bear that in mind in the example - ie there's double the capacity (and voltage) one battery has but the same peak current.
However that's a pretty useless measure in electrical terms.
In reality all you need to know is the current drawn by the load and the capacity of the batteries in ampere hours. You simply divide the capacity by the load and that gives you the time to completely deplete the battery.
However as its lead-acid then you're looking at probably 20% voltage drop between 100% charged (12V) and 30% charged (10V) so you've only got 70% usable capacity in all likelihood*. Now you also have to factor in the efficiency of the inverter/transformer etc. which we'll assume isn't junk so lets call that 85%.
So your 18AH battery becomes : 18 x 0.7 x 0.85 = 10.71 "usable" AH. Going back to your joules that'd mean a usable capacity (for this application) of 12 x 10.71 x 3600 = 462,672J
So I guess if you measure the
apparent power drawn by your load, multiply it by 3600 that'd give you joules, then you could divide the usable battery power in joules to give you the total runtime in seconds. However as said before, battery capacity varies as a function of discharge current so it won't be exact.
In terms of your request for a UPS to supply 300W (I'm assuming at least two hours runtime) for under £500 - very doubtful. Don't bother with second-hand as you've no idea how/how long its been used.
tl;dr you need to get an accurate figure for power consumption. If its more than 100W then things are likely to become expensive given the length of runtime you require. Edit - forgot, I tested mine last night and with a load of 150W I get 110 minutes on a deep discharge calibration. That's with two year old batteries.
*this is entirely dependent on how much money you spend on batteries, how old they are and how many times they have been discharged. Basically if you buy cheap then you're not getting deep cycle batteries (they have thicker lead plates amongst other things) and you may only have 50% usable capacity. "Leisure batteries" are unsuitable for UPS by the way as are automotive batteries of any sort.
