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Old 02-19-2019, 11:33 PM   #11
STB
Guru
 
City: Gulf coast
Join Date: Aug 2016
Posts: 2,271
"Back of an envelop" estimate for running an AC unit or heat pump off an inverter. /Really/ rough numbers.

Really roughly, I approximate an AC unit or heat pump as having a wattage power requirement of a little more than 10% of its BTU rating. So, for example, I'd ballpark a 16,0000btu unit at 1600W + "a bit", so maybe 1700W.

1700W is doable by many common inverters. It doesn't leave much headroom, but it is clearly doable.

But, how much battery is required? 1700W @ 12V is about 140A. That's about as much energy as one can draw from a pair of 6V golf cart batteries (parallel for 12V) in an hour at about 75% discharge. Again, really rough.

So, again really roughly, one would need 2 golf cart batteries for each hour one wants to operate a 16000btu AC unit or heat pump for heating. Want to run it for 4 hours per day? You'll need about 8 batteries. 6 hours per day? 12 golf cart batteries.

Golf cart batteries cost $150/each. A box for each pair of batteries, probably costs $60, or $30/battery. Maybe another $20/battery in wire. $200/battery in all. $2,400 for the battery bank. Maybe another $500 for the inverter. With taxes, etc, probably right around $3,000 for the set up.

If you want half as much AC capacity, you can cut that in half, 50% more, multiply by 1.5. Run it for half as long per day, cut it in half. Twice as long per day, double it. You get the idea.

But...don't forget that you need to recharge those batteries. 12 batteries might need 1200 amp-hours @ 12V. If one could dump the whole of 2x60A alternators into them, that would be 10 hours to fully charge. But, the charging efficiency might be, in really rough numbers, 50%. That would be 20 hours to charge. Not at all reasonable.

What if we just charge them to 70%. Well, then we are discharging from 70% to 30%, using 40% of the battery capacity. Before we were using 70% of the battery capacity (100% - 30%). So, we need 70/40= 1.75x as many batteries -- 22 batteries and boxes. Wildly guessing as above, $5,250 in cost.

What about the charge time? Since batteries charge ore efficiently earlier on, charging to 70% probably only takes really roughly 15hrs. Upgrade from 60A alternators to 150+A alternators, and that is down to 6 hours.

Lets say we want to charge the batteries in just 4 hours. Anything more than that would be hard on the batteries. That would be 1200 amp-hours / 4 hours = 300 amps/hour. That probably means north of 2x 175A alternators -- basically the equivalent of a 4.2kw generator! Those might be $750/alternator. Maybe $1000/alternator after making other needed adjustments. We're up to $7,750 to do this without a generator -- and the need for batteries every few years. I'd think for $7,750 one can get a rebuilt generator installed.

Even if I introduce one friendly piece of reality -- the HVAC might only have a 60% duty cycle (compressor on 60% of the time, wildly guessing), this $7,750 is still just a little shy of $5,000.

These numbers are obviously very, very, very approximate and rough and compound a bunch of error. But, my point is this. It is easy to get enough inverter to run an AC unit, even a large one. But, it takes a huge amount of battery to run one for any length of time. And, that is a much bigger expense that needs to be repeated every few years as the batteries need replaced.

But, even with that, there is just no fast way to charge that much battery from that depleted a state to a near charged state on a daily basis underway. It just takes time.

It is very reasonable to operate without a generator. But, this does involve limiting long-running major loads like HVAC. Even short bursts of loads, like a few minutes of microwave can be fine. But tings like AC? Would need to be really limited.
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