Amp hours

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Amp hours

Postby Grub » Thu Aug 07, 2014 7:58 pm

Hi guys I need to understand amp hours a bit more if you can help we have bought a freezer (240v) and we are running it on our off grid solar it says 1amp on the back and the energy rating sticker on the front reads energy consumption 281 kwh per year. We have 6 x 120 amp hour batteries so 720 amp hours (is that right) so does that mean it only uses 1 amp per hour????? Also if it was say 2.8 amps how do you convert that to amp hours??
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Re: Amp hours

Postby davidg » Thu Aug 07, 2014 9:09 pm

Grub wrote:We have 6 x 120 amp hour batteries so 720 amp hours (is that right) so does that mean it only uses 1 amp per hour????? Also if it was say 2.8 amps how do you convert that to amp hours??

281 kWh is pretty good, is it a small freezer? Did it tell at what temp that was? If run colder then it will certainly use more, likely to use more anyway, in real life a typical estimate might be approx 2kWh allowing for losses, based on what you posted about it. For fridges & freezers, rule of thumb could be running for 6-8 hours per day roughly.

"ah = amp hours" No unless the batteries are all in parallel, does not quite equate anyway, but near enough for now. It's better to work in kWh's. ie 12Vdc x 720ah = 8640Wh's = 8.64Kwh's now assuming a DOD of 30% for PB batteries, would be approx 2.59kWh's no losses which = 24Vdc x 360ah which = 48Vdc x 180ah which is again the same number of kWh's approximately. DOD remains consistent, just depends on how the batteries are connected together.

2.8 a x 1 hr's = 2.8 ah's simple.

A quick overview, does not allow for losses in the charge to discharge ratio and the efficiency of the equipment used, which critical to good performance of a system. Car batteries or deep cycle car batteries do not do well for this sort of use. PB (Lead) typically needs 15% more energy input (charge/recharge) to get a battery back to where it started roughly.
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Re: Amp hours

Postby Tracker » Fri Aug 08, 2014 9:11 am

Grub wrote:.... We have 6 x 120 amp hour batteries so 720 amp hours (is that right) so does that mean it only uses 1 amp per hour????? Also if it was say 2.8 amps how do you convert that to amp hours??


The important thing is to correctly state what this figures are for.. eg. 2.8A... what is this... :?:
If that is on the rear of the freezer, then that is the typical AC current..
2.8A @ 240V = about 700W of power
So... to run via an inverter, you will likely need more than that from the battery.. (depending on efficiencies)

Again more info needed .. 6 batteries.. at what voltage. 2V, 6V, 12V???

Indeed you have a total capacity of 720AH... but this value changes with the load.. and is meaningless without voltage..
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Re: Amp hours

Postby Benny » Sat Aug 09, 2014 10:03 am

Grub - your fridge uses 281kWh per day. So thats about 0.7kWh per day. Or 700Wh/day.

If you used a 12V battery to supply it, this would mean 700/12 = 58Ah/day.
If you used 24V, 29Ah etc.
Remember W(power) = V x I

The 1A on the spec label doesn't really mean much. Its probably a max current draw so at 240V the max power is 240 x 1 = 240W. Its much more likely to actually be using 125W while the compressor is operating but could also have a big short spike at startup. Many others have commented her on inverters needing a high rating to cope with compressor startups. Have you got a power meter to measure the steady state power ?

So you need to consider 2 things - how much power does the fridge use while operating (incl startup) and how much energy (Wh) does it use over time.

If it is say a 125W compressor, then this only needs to run for 700Wh/125W = 5.6h to provide the 700Wh the spec says it needs per day. So thats a 23% duty cycle which sounds about right for a chest freezer.
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