Energy Audit factor

[JJJJ]

I decided to do an energy audit to see what the power requirements would be for some basic items. I used a Kill A Watt and carefully measured everything.

So far so good. Then I put the power usage in watts, estimated hours, estimated inverter efficiency and size battery in the calculations.

At this point, my mind turned to mush. I will be using a 24v battery.

So, at the risk of sounding like I have never seen a calculation I am asking for advice. Please let me know if I am completely missing it...

Determine Amps (or watts if going that route). If memory serves me correctly: Watts = Amps x Volts.

If I measure watts in AC, then if using a 24v battery, would I multiply AC watts x 5 (120/24)? to determine DC watts?

If this is correct, then I would take this number and divide by 24 to get amps.

For example: Something uses 60 watts.
Multiply by 120/24 = 5
Equals 300 watts d/c
Divide watts by volts to get amps 300/24=12.5 amps DC. Then if an inverter is 85% efficient 12.5/.85 = 14.7 battery amps required

Am I totally missing this???

 

[pollenface]

I don't really follow your formula.

The way I calculate

60w AC / battery voltage / 0.85 = DC amps

60 / 24 / 0.85 = 2.94 amps

 

[OffGridInTheCity]

Divide watts by volts to get amps 300/24=12.5 amps DC. Then if an inverter is 85% efficient 12.5/.85 = 14.7 battery amps required

14.7a * 24v = 352.8watts. 352.8w * 85% = 299.2w. Close enough.
Remember that batteries have a range of voltages... 24v nominal is not 24v but a representation of the overall range. For example, a lithium-ion 7s battery might range from 23.8v low to 29.1v hi which affects the calculations when you get down to very precise numbers.

However for general estimates, if you wan to run a 300w AC load for 10hrs on an inverter with 15% losses - you are correct, you need 353w * 10hrs = 3,530wh (3.53kwh) from your battery.

For battery sizing, you should factor in DOD (depth of discharge) you plan to run. For example, if you operate at 80% DOD, then you need a 4.4kwh battery battery to get 3,530watts total power out of it (e.g. 4,400wh * 80% = 3,520watts of useable power)

So I tend to think in terms of "what do I want my battery to deliver / how long do I want it to last". For example, in my home Powerwall of 18650 cells is designed for a 40% DOD for extra long life - so I need a battery twice as large as someone who plans 80% DOD operations.

 

[fafrd]

I decided to do an energy audit to see what the power requirements would be for some basic items. I used a Kill A Watt and carefully measured everything.

So far so good. Then I put the power usage in watts, estimated hours, estimated inverter efficiency and size battery in the calculations.

At this point, my mind turned to mush. I will be using a 24v battery.

So, at the risk of sounding like I have never seen a calculation I am asking for advice. Please let me know if I am completely missing it...

Determine Amps (or watts if going that route). If memory serves me correctly: Watts = Amps x Volts.

If I measure watts in AC, then if using a 24v battery, would I multiply AC watts x 5 (120/24)? to determine DC watts?

If this is correct, then I would take this number and divide by 24 to get amps.

For example: Something uses 60 watts.
Multiply by 120/24 = 5
Equals 300 watts d/c
Divide watts by volts to get amps 300/24=12.5 amps DC. Then if an inverter is 85% efficient 12.5/.85 = 14.7 battery amps required

Am I totally missing this???

Watts is Watts is Watts.

There is a bit of a nuance in the conversion of AC Watts to DC Watts but let’s shelve that for now and just consider that WAC = VADC.

So it’s not the Watts that change, it’s the current.

You will need 5 times the current (Amps) for the same Wattage @ 24VDC as the amount of current (Amps) for that same wattage @ 120VAC.

And if your inverter has an efficiency of 85%, current will need to increase to 5/0.85 = 5.9 times of what it is at 120VAC.

Or more correctly, 300WAC out of your inverter @ 120VAC will consume about 353WDC from your battery, or 14.7A @ 24VDC…

 

[upnorthandpersonal]

*Minus some losses.

Calculate/measure your energy usage in Wh. You can easily know the total amount of energy you have available by taking your Ah capacity and multiplying it with your battery nominal voltage.

 

[JJJJ]

Thank you everyone for your quick and helpful hints. It was good to know that I wasn't completely off the track. Hope you all have a good evening.