Help with inverter battery sizing

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mister

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Hi Guys,

I measured my average usage with an Efergy monitor over 3 days and got this:

Ave Per 24 Hr: 1.669 Kwh
Max Draw: 1033 Watts (laser printer when printing)

I would like to go with a 2000 Watt pure sine inverter which would need 24 volts

So if I had 2 X 102AH Deep Cycle 12 Volt Batteries, how long could I run the inverter until I get to 50% discharge?

I’ve done some googling but just ended up with a headache. Can somebody please explain the maths?

Much appreciated!
 
without a bit more details here is a stab in the dark.

about 1 hour of back up at 1000watt, Therefore 30 minutes to reach 50% battery discharge.
 
Your batteries store 2*12*102=2448Kwh. Half of that (50%) is 1224 Kwh.

You should be able to run for a day, not considering any other inefficiencies and overheads
.
 
T-Man said:
Your batteries store 2*12*102=2448Kwh. Half of that (50%) is 1224 Kwh.

You should be able to run for a day, not considering any other inefficiencies and overheads
.
Click to expand...

The batteries are in series as the voltage required is 24 V ......
 
Don't forget you don't want to drain lead acid more than 50% otherwise you're killing them, so half the actual storage.
They're also about 85% efficient (power in / out) Eg put in 100W, get 85W out.

So
12v @ 100A = 1200w x 2 = 2400W total capacity.
2400w / 2 (50% max drain) = 1200W usable capacity (50% of total discharge so the batteries are usable more than a few times)
1000W usable out of that 1200W +- as they're about 85% efficient

So 1KW = 1HR nominal usable capacity

Assuming 1.7KW/ hr requirements, about 17minutes runtime to take the UPS to 50% of safe usable capacity.
(1700W / 60 = 28.33 - thats your per minute usage).
500W = 50% of 1KW, so 500/28.3 = 17minutes.
Can go to 100% usable @ 34 minutes. (which is also safe)
 
Last edited:
Ummm...

Let me clarify:

I said 1.669 kilowatt-hours per 24 hours. In other words I used 1.669 kilowatt-hours over a 24 hour period.

That's an average draw of 69 watts over 24 hours including the odd peak at 1033 watts when printing on the laser printer.

69 watts / 1000 = 0.069 kilowatts * 24 hours = 1.656 kilowatt-hours

I want a 2000 watt pure sine inverter which would need 24 volts.

2 X 102AH Deep Cycle 12 Volt Batteries would last how long if I discharge to 50%?

I hope I'm making sense and don't have this all wrong!
 
Last edited:
Time = AH x 24Volt/watts drawn

time = 102 x 24/69 = 35 hours for complete discharge

50% discharge = half that time = 18 hours

Ignoring the bursts at 1033watts
Ignoring efficiency factors
 
Geoff.D said:
Time = AH x 24Volt/watts drawn
time = 102 x 24/69 = 35 hours for complete discharge
50% discharge = half that time = 18 hours
Ignoring the bursts at 1033watts
Ignoring efficiency factors
Click to expand...

Hi Geoff

Thank you so much. I'm getting a similar result from another calculator.

Using: https://www.batterystuff.com/kb/tools/ac-to-dc-amperage-conversion-run-through-an-inverter.html

AC Voltage: 220 VAC
AC Amperage: 0.32 Amps AC
Wattage: 70.4 Watts
DC Voltage: 24V
DC Amperage: 3.24 Amps DC

Then I plug the DC amps into: https://www.batterystuff.com/kb/tools/calculator-sizing-a-battery-to-a-load.html

Load Size: 3.24 DC Amps
Battery Size: 102 AH @ 20Hrs
Gives me 15 hours runtime at 50% discharge

Quite a complicated way of doing it, and to be honest it goes right over my head.
 
Well that calculation takes into account the efficiency of the inverter which will reduce the stand by time.

More accurate certainly. But in the end an estimate is all you are after. There are many other variables which will reduce the time as well.
 
@mister

Its the same math, just plug in the time change. I've done it below for you.

UPS hasn't changed -

12v @ 100A = 1200w x 2 = 2400W total capacity.
2400w / 2 (50% max drain) = 1200W usable capacity (50% of total discharge so the batteries are usable more than a few times)
1000W usable out of that 1200W +- as they're about 85% efficient

So 1KW = 1HR nominal usable capacity in the UPS.


Draw has changed -
Assuming 1.7KW/ day requirements, about 70.83W /hr. (1700W/ 24hrs)

(70.83W / 60min = 1.1805W/min- thats your per minute usage).
500W = 50% of 1KW, so 500W/1.1805W = 423.54minutes runtime (or 7 hours) to 50% of safe usable capacity.
Or 14 hours total to 100% of safe usable capacity.

Will be less though, as the inverter won't be 100% efficient, more likely 92-95%, on top of the battery 85% efficiency, and the UPS is also going to be draining current to run itself in a power failure. Maybe 50w/hr on top for the UPS, so add that into your calculations for Draw.

So 78.83/hr + 50w = 128w /hr (lets call it 130W)

500w / 130W = 3.84hrs to 50% of safe, or 100% of safe to 7.68hrs.
So in reality, you'll probably get about 8hrs out of the UPS.
 
Last edited:
lsheed_cn said:
@mister

Its the same math, just plug in the time change. I've done it below for you.

UPS hasn't changed -

12v @ 100A = 1200w x 2 = 2400W total capacity.
2400w / 2 (50% max drain) = 1200W usable capacity (50% of total discharge so the batteries are usable more than a few times)
1000W usable out of that 1200W +- as they're about 85% efficient

So 1KW = 1HR nominal usable capacity in the UPS.


Draw has changed -
Assuming 1.7KW/ day requirements, about 70.83W /hr. (1700W/ 24hrs)

(70.83W / 60min = 1.1805W/min- thats your per minute usage).
500W = 50% of 1KW, so 500W/1.1805W = 423.54minutes runtime (or 7 hours) to 50% of safe usable capacity.
Or 14 hours total to 100% of safe usable capacity.

Will be less though, as the inverter won't be 100% efficient, more likely 92-95%, on top of the battery 85% efficiency, and the UPS is also going to be draining current to run itself in a power failure. Maybe 50w/hr on top for the UPS, so add that into your calculations for Draw.

So 78.83/hr + 50w = 128w /hr (lets call it 130W)

500w / 130W = 3.84hrs to 50% of safe, or 100% of safe to 7.68hrs.
So in reality, you'll probably get about 8hrs out of the UPS.
Click to expand...

If you can't distinguish a W from a Wh, or a UPS from an inverter, you should not be giving advice.
 
lsheed_cn said:
Whatever.

Inverter + Batteries = UPS.
Click to expand...

Inverter + Batteries = What you solar nutjobs use and gets drained every single day. 5 year battery life at 50% maximum DoD... if you're lucky.

UPS = What normal people use and gets drained when it needs to. 10 year battery life at 100% maximum DoD, easily (with quality batteries).
 
Here we go again. As soon as the fanatics find a thread the personal attacks start. The OP asked a simple question that now turns into a battle. There is no need to sling mud around guys. You are not helping.
 
lsheed_cn said:
@mister
Its the same math, just plug in the time change. I've done it below for you.
Click to expand...

Hi LSheed

Thanks for replying. I know you are very knowledgeable about solar & batteries, but would you mind altering your answer to use the correct metrics so I can understand the maths better?

For instance you say:
"12v @ 100A = 1200w x 2 = 2400W total capacity"
Would this not be 100 amp-hours, and watt-hours capacity?
 
Geoff.D said:
Well that calculation takes into account the efficiency of the inverter which will reduce the stand by time.

More accurate certainly. But in the end an estimate is all you are after. There are many other variables which will reduce the time as well.
Click to expand...

Thanks very much for helping. I appreciate it.
 
Geoff.D said:
Here we go again. As soon as the fanatics find a thread the personal attacks start. The OP asked a simple question that now turns into a battle. There is no need to sling mud around guys. You are not helping.
Click to expand...

See the post below? This is why "the fanatics" want accurate, precise language. And never mind the language, half his calculations are wrong.

mister said:
Hi LSheed

Thanks for replying. I know you are very knowledgeable about solar & batteries, but would you mind altering your answer to use the correct metrics so I can understand the maths better?

For instance you say:
"12v @ 100A = 1200w x 2 = 2400W total capacity"
Would this not be 100 amp-hours, and watt-hours capacity?
Click to expand...
 
His calculations are not wrong, they are different. Yes possibly making a few assumptions along the way.
 
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