Calcuating watt hours /storage and usage

[Natasha Lee]

Hi Everyone!

My question may or may not be silly, unsure if I am simply overthinking it or not!

So, we have a 9KW solar system, 18x 500w panels. 2 Charge controllers, 1 Inverter and 16 x 300ah LiFePo4 cells wired together to make a 48v battery pack. When the batteries are full, they are at 56.8volts.

I have calculated the battery storage capacity as 300ah x 56.8v = 17,040 watts.

The battery pack is suppling power to a cabin, which is drawing AC and uses 220volts.

So my question is, if my cabin is using 220v, but my battery pack is 56.8v does this mean that a 4watt hour light is still drawing 4 watts per hour? Or does this figure change because it is being operated on a 220v system? And is my battery storage figure of 17,040 watts accurate, or does that also change?

Please help! I hope this was written clearly enough to understand.

Thanks for any help you can provide.

 

[ianganderton]

The universal truth is that watts are watts, they don’t change in the way that amps and voltage do with their relations ship to each other.

so yet a 12V 4 watt light is using the same amount of energy as a 220V 4 watt light.

I’d suggest you may be over calculating your battery bank watts by using your maximum charged voltage because as the batteries charge reduces with use the voltage will also reduce. To keep things simple I’d prefer to use the nominal voltage (48V) to calculate the watts available. This would factor in some wiggle room and be slightly underestimating the power available. Over estimating can be disappointing.

 

[Forbisher]

I have calculated the battery storage capacity as 300ah x 56.8v = 17,040 watts.

The battery pack is suppling power to a cabin, which is drawing AC and uses 220volts.

So my question is, if my cabin is using 220v, but my battery pack is 56.8v does this mean that a 4watt hour light is still drawing 4 watts per hour? Or does this figure change because it is being operated on a 220v system? And is my battery storage figure of 17,040 watts accurate, or does that also change?

Please help! I hope this was written clearly enough to understand.

Thanks for any help you can provide.

16 x 3.2V cells is 51.2V nominal aka 48V battery.
51.2V x 300Ah is 15,360 watt hours.

 

[wattmatters]

I have calculated the battery storage capacity as 300ah x 56.8v = 17,040 watts.

Since amps x volts = watts, you left out the hours, so the energy capacity on the right has units of watt hours (Wh), not watts.

300Ah x 56.8V = 17,040Wh

As others have mentioned, it's better to use a nominal voltage for the battery when calculating storage capacity, and also consider how much of that capacity is functionally useable.

e.g. if you wish for the batteries to have a long life span and achieve a greater number of charge - discharge cycles before needing replacement, it would be a good idea to set upper and lower limits on capacity stored in the battery, e.g. you might choose to operate between ~20% capacity at the lower end and ~90% at the upper end, meaning useable capacity is about 70% of the nominal capacity.

So my question is, if my cabin is using 220v, but my battery pack is 56.8v does this mean that a 4watt hour light is still drawing 4 watts per hour?

Yes.

However because the inverter is not 100% efficient at converting DC energy to AC, then a 4W draw on the AC 220V side will draw a bit more than that on the DC input side. How much more depends on the efficiency of the specific inverter and other factors affecting efficiency, but you might for instance draw 4.5 - 5W on the DC input side.

The other factor is the round trip efficiency of the batteries/inverter. In general you need to put more energy into charging a battery than you'll be able to get back out during discharge, e.g. you might need to put 1.1kWh into a battery in order to get 1kWh out.

It's the 2nd law of thermodynamics in action. Each time there's a process to transfer energy from one location or form to another location or form, some will be lost in the process along the way.

 

[Natasha Lee]

The universal truth is that watts are watts, they don’t change in the way that amps and voltage do with their relations ship to each other.

so yet a 12V 4 watt light is using the same amount of energy as a 220V 4 watt light.

I’d suggest you may be over calculating your battery bank watts by using your maximum charged voltage because as the batteries charge reduces with use the voltage will also reduce. To keep things simple I’d prefer to use the nominal voltage (48V) to calculate the watts available. This would factor in some wiggle room and be slightly underestimating the power available. Over estimating can be disappointing.

This response was really helpful and clarified my doubts and questions perfectly.

Thank you for taking the time to reply and educate me on this, and I also agree and appreciate the extra information regarding the battery bank. Thanks again!