The simple math shows that if you need 12.5 A at 120 V, you will need 125 A at 12 V. This is assuming 100% efficiency of the inverter. I bet that they are around 80%, so add 20% to that number (1.2 x 125 A = 150 A). Run this for 12 h and you will use 150 A x 12 h = 1800 Ah. In order to accomplish that, you need a 3600 Ah battery bank (lead-acid), or around 2250 Ah if lithium.
In short, you will need a small power station to power it (a 2000 W generator will recharge that battery bank in more than 12 h). These calculations did not include the Paukert effect or potential issues related to heat generated when discharging or charging the batteries. The only good news I could think of is that you won't need 12.5 A all the time (a good chance is that after the initial run, the AC will be cycling on and off, and will run about half of the time). Which means that a smaller battery bank will be needed (2000 Ah (L-A)/1250 Ah (Li)). There might be other considerations that might help reducing that size (e.g., AC that draws less current). Hope it helps Marek Ottawa, ON Sent from my Android-based can on a string -------- Original message -------- From: CHARLES SCHEAFFER via CnC-List <cnc-list@cnc-list.com> Date: 2023-10-18 20:18 (GMT+01:00) To: Stus-List <cnc-list@cnc-list.com> Cc: "Della Barba, Joe" <joe.della.ba...@ssa.gov>, CHARLES SCHEAFFER <cscheaf...@comcast.net> Subject: Stus-List Re: [EXTERNAL] Re: Sizing an inverter -------- Original message -------- From: CHARLES SCHEAFFER via CnC-List <cnc-list@cnc-list.com> Date: 2023-10-18 20:18 (GMT+01:00) To: Stus-List <cnc-list@cnc-list.com> Cc: "Della Barba, Joe" <joe.della.ba...@ssa.gov>, CHARLES SCHEAFFER <cscheaf...@comcast.net> Subject: Stus-List Re: [EXTERNAL] Re: Sizing an inverter Very helpful. What is the proper math to determine battery bank size needed to run our AC unit for twelve hours? I know a softstart would make it possible, and the rated amp draw is 12.5 amps at 120vac. I'm considering getting a lithium 200 ah and keeping a 100 ah AGM as a start battery. Solar panels are being considered also. Thanks, Chuck On 10/18/2023 12:50 PM EDT Della Barba, Joe via CnC-List <cnc-list@cnc-list.com> wrote: If it helps, think of the inverter as a special kind of extension cord. It passes power from Point A to Point B. It does not create or store power. It needs to be bigger than the expected load put on it. The batteries need to be up to the task, but they do not create any power, they store it. If you want to use a lot of power, you have to generate a lot of power somehow. A huge battery bank, say big enough to run an air conditioner overnight, on a C&C with a stock charging system would work one night and then you would need to motor about 8-10 hours if you wanted to do it again. Joe Coquina From: Dreuge via CnC-List <cnc-list@cnc-list.com> Sent: Wednesday, October 18, 2023 12:06 PM To: Richard Bush <bushma...@aol.com> Cc: cnc-list@cnc-list.com; Dreuge <dre...@gmail.com> Subject: [EXTERNAL] Stus-List Re: Sizing an inverter Richard, My point is that one should not size an inverter relative a battery, but size it to ones expected needs. Folks often mix up or misuse terminology. If the maximum power consumed from an electric coffee pot is 1000 Watts. Then one would need at least a 1kW inverter, but should get something larger to avoid maxing the operating limits. Now the question is what battery can provide a constant 1000W for the 5 minutes to brew coffee. Of course battery needs are greater than brewing coffee and an energy audit would give a better estimate, but for the sake of simplicity, let’s only consider coffee needs. For a 12V battery system (12.5V lead acid), the 1000W of power (P = voltage* current) is provided by a battery current of 1000W/12.5V = 80A. So one would need a battery that can provide 80A of current for 5 minutes. A single 100A*hr lead acid type battery would be a poor choice as it would end up nearly depleted. Relative to a 100 A*hr LiFePO4 battery, the LiFePO4 battery would brew about 10 times more coffee! - Paul E. 1981 C&C Landfall 38 S/V Johanna Rose Fort Walton Beach, FL http://svjohannarose.blogspot.com/ On Oct 18, 2023, at 8:55 AM, Richard Bush <bushma...@aol.com<mailto:bushma...@aol.com>> wrote: Paul, If I understand your point, it is that we shouldn't be relying on battery power for any appliances or coffee pots...etc.; is there any alternative that doesn't involve solar? thanks! Richard 1985 C&C 37 CB; Ohio River, Mile 584: Richard N. Bush Law Offices 2950 Breckenridge Lane, Suite Nine Louisville, Kentucky 40220 (502) 584-7255 On Wednesday, October 18, 2023 at 07:12:19 AM EDT, Dreuge via CnC-List <cnc-list@cnc-list.com<mailto:cnc-list@cnc-list.com>> wrote: Hi Chuck, First off, 12V * 100 A*hr = 1200 W*hr. But keep in mind that A*hr battery ratings for a 100 A*hr battery is tested typical at 5 amps(or 0.05C). That is your 100 A*hr is 5A * 20hr. At this rate the battery is only providing 12.5V * 5A = 63 Watts. At higher current loads, your effective battery capacity will be much much less due to Peuket Effect losses. In fact, a 100 A*hr rated lead-acid battery at 100 amp load would only provide about 20 A*hr of energy(and lots of heat). To add to the insult, lead acid based batteries like AGM should not be depleted below 50% capacity to limit damage, so reserving 1 battery for start leaves only 50 A*hr for your house bank at loads of about 5 amps. At a 20A load, the 100 A*hr (50 A*hr effective) only provides 60 A*hr (30 A*hr effective). That’s about 30 A*hr / 20A = 1.5 hrs of use. A 1000 W inverter could draw a max of 1000W / 12.5V = 80 Amps (but likely higher due to a voltage drop). This would drain your battery in 6 - 10 minutes. I gave a talk to some local Hams about Solar Off-Grid Systems. I posted the slides on my blog (see link below). On slides 5 & 6, I discuss battery discharge capacity and bank sizes. https://svjohannarose.blogspot.com/2021/10/solar-off-grid-system.html On Johanna Rose, I have a 2000W inverter charger. When I run my microwave, it draws about 100A from a 560 A*hr LiFePO4 battery. Like Dennis, I have my inverter connected to my panel powering all AC loads except for my A/C unit and water heater. The inverter has a built in transfer switch which automatically (and smoothly) transfers to shore power when available. - Paul E. 1981 C&C Landfall 38 S/V Johanna Rose Fort Walton Beach, FL http://svjohannarose.blogspot.com/ On Oct 17, 2023, at 7:27 PM, via CnC-List <cnc-list@cnc-list.com<mailto:cnc-list@cnc-list.com>> wrote: OK, an DC/AC Pure Sine Wave inverter looks attractive I've heard some boats have 1000 watt, some 2000watt and some 3000 or more. What size inverter is right for a boat equipped with two 100Ah AGM batteries? One battery provides 12v x 100ah: 1200watts. My system can provide 2400 watts but I usually reserve one battery to restart the engine and run on the other battery. Does the inverter get fed from a bus common to both batteries, or to the selector switch marked, "Off, 1, both, 2"? Thanks, Chuck S Please show your appreciation for this list and the Photo Album site and help me pay the associated bills. Make a contribution at: https://www.paypal.me/stumurray Thanks for your help. Stu Please show your appreciation for this list and the Photo Album site and help me pay the associated bills. Make a contribution at: https://www.paypal.me/stumurray Thanks for your help. Stu
Please show your appreciation for this list and the Photo Album site and help me pay the associated bills. Make a contribution at: https://www.paypal.me/stumurray Thanks for your help. Stu
