DonkeyOatie :
5040 watt minutes, is 10 minutes at 500 watts. If the 500 watts includes everything in your system, including the monitor, then at full power it will give you ten minutes. However, as the batteries age, this will drop to less, but your system is unlikely to be running at full blast as you try to shut it down, so you could easily get fifteen minutes. This also assumes that the battery is fully charged and that the ambient temperatures are sensible. Batteries are chemical devices and are affected by high and low temperatures. I get power outages here from time-to time. My system takes less than two minutes to shut down.
Don't forget to allow for conversion inefficiencies too. Even the best inverters cannot achieve 100% efficiency, just like PSUs. Also, if the PSU is running at full load, then it will consume more at the socket. For example, a good PSU supplying 400W on the 12v rail could easily draw 480W from the mains, provided that it is 80+ certified.
Your FX 6300 and GTX 760 consume ~265W together. Allowing for other power draws, we can safely say that your system will draw about 320W maximum under load.
To find out power draw at the socket, we use:
X*80%=320
X = 400W, provided that your PSU is 80+ certified. Therefore, your PSU will draw 400W from the socket to power your PC under full load.
Next is the inverter. Inverter efficiency is decided by many things. For starters, pure sine wave inverters are less efficient than modified sine wave ones. Also, most inverter efficiency specs are obtained when operating at or near full power. Second thing is they are rated for pure resistive loads and not reactive loads. So if you are operating at say half power and reactive loads you will not achieve maximum efficiency. Thus, allowing for such differences, we can say that on the lower end you can expect ~70-75% efficiency. Here is the calculation:
X*85%=400
X=533W
Therefore, assuming you have a cheaper, less efficient inverter, and a 80+ certified PSU at least, we can say that under full load, your PC will draw about ~530-540W. The rest of the calculations are easy:
5040/540= 9.3 minutes
Therefore, under full load you can expect the PC to last around 8-9 minutes before draining the battery. However, there are a lot of variables involved, so if your PSU is 80+ gold certified, or on the other hand a generic non-certified brand, you could be looking at a longer or shorter battery power time. Same goes for the inverter, if you have a better quality one it may be more efficient, conversely if you have a cheap one it may result in a lower battery power time. This is only a theoretical calculation to give a rough idea of the battery power times you will have; in practice, it may be substantially different. The only way to know for sure is to try it out with the equipment itself.
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