Ah to Kwh conversion clarification

Thread Starter

humptydumptyaac

Joined Nov 3, 2021
24
Hi, newbie here.

I was searching around for a home battery, something to store electricity in to use later on.

I found a 100Ah battery, at 12V output. (I assume DC)

And I was thinking I could get a 12V to 220V inverter (at a high enough W rate, perhaps around 3000W) to be able to use the battery for regular household appliances.

Anyway, I was trying to figure out how many kWh this battery fits and that's where I got confused (being a newbie and all)

According to the formula, 100Ah at 220V = 22kWh.
But 100Ah at 12V = 1.2 kWh.

So it comes out like if you use low power devices you get less electricity?! (stupid assumption, I know)

But help me out here please, with a battery of 100Ah, a 12V to 220V inverter, how many kWh of usable electricity I would get for household appliances?

For example, if I would plug in a 2000W electric grill and leave it on, how many hours would it run on this battery, assuming it goes from full charge to full discharge.

Thank you.
 

BobaMosfet

Joined Jul 1, 2009
1,951
In order to see exactly what the capability actually is for the battery in question, we need to know who made it and exactly what battery it is, so a datasheet can be pulled.
 

Thread Starter

humptydumptyaac

Joined Nov 3, 2021
24

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MrChips

Joined Oct 2, 2009
25,928
You cannot get more energy than what you were given.

Assuming 100% efficiency (which will never be the case), 100Ah @ 12V battery will deliver 5A @ 12V for 20 hours (using the C/20 discharge rate).

5A @ 12V = 60W
This translates to 0.27A @ 220V or 0.22A @ 220V at 80% efficiency.

Hence you can draw about 0.2A @ 220V for 20 hours.
 

MrChips

Joined Oct 2, 2009
25,928
2200W electric grill will draw 10A @ 220V.
This will require 184A @ 12V, or closer to 200A @ 12V.
The 100Ah @ 12V battery will run for less than 30 minutes.
 

crutschow

Joined Mar 14, 2008
29,504
if I would plug in a 2000W electric grill and leave it on, how many hours would it run on this battery, assuming it goes from full charge to full discharge
Less than an hour.

The theoretical maximum energy rating (not attainable in practice) would be the Ah rating times the battery voltage.
The 220V converter output voltage has nothing to do with that, as you do not have 100Ah @ 220V.
The converter cannot create energy.

So a 12V,100Ah battery has a maximum energy capacity of 1200 watt-hours, as you calculated.
Thus, ignoring the less than 100% efficiency of your 220V converter, the 2000W grill will operate for 1200/2000 = 0.6Hr until the battery is stone dead (and likely will not tolerate that for very many charge-discharge cycles)
 

Thread Starter

humptydumptyaac

Joined Nov 3, 2021
24
Less than an hour.

The theoretical maximum energy rating (not attainable in practice) would be the Ah rating times the battery voltage.
The 220V converter output voltage has nothing to do with that, as you do not have 100Ah @ 220V.
The converter cannot create energy.

So a 12V,100Ah battery has a maximum energy capacity of 1200 watt-hours, as you calculated.
Thus, ignoring the less than 100% efficiency of your 220V converter, the 2000W grill will operate for 1200/2000 = 0.6Hr until the battery is stone dead (and likely will not tolerate that for very many charge-discharge cycles)
The battery voltage... that's the part that I was missing. I was misdirected at the inverter voltage. That clears it up really well. Thank you!
 

BobaMosfet

Joined Jul 1, 2009
1,951
The vendor is using the confusion over the true meaning of Ah to disguise the truth.

What they are actually saying is that for 3600 seconds (1 hour), it will deliver 27.77777777777mA each second if the load only requires 12V across it. In other words, over 1 hour's time, 100A will be delivered at 12V.
 
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