A/C from battery solar (actually realistic scenario?)

[ERTF]

My trawler already has 500w solar on aft hardtop. I have space for 1200w more on my flybridge hardtop (that I've been itching to fill).

I just installed 860ah of new lead acid gc batteries, but i have space to bump that up to 1075. On a Miami summer day, I estimate I could reasonably expect to capturre 800 amps @ 13volts (from 1700w). Way more power than I need for the usual stuff...which has me thinking about air conditioning.

Here's my idea for a summer routine. Run generator & a/c all night. Wake up in morning nice and cool with batteries fully charged. Turn off gen & a/c. With that much solar, even at 8am i will be pulling in a surplus over refrigeration & fan draws. Since I'm waking up cool I can then tolerate the heat for several hours.

Then around noon or so I turn on the a/c thru inverter. My aft cabin 10k btu claims 8-10amps, so just call that 100amps per hour @ 12v? With a 1075 ah bank, i should be able to run the a/c til 5 or 6pm. Then turn off a/c and hangout outside in the shade until bedtime, at which point i fire up the generator & a/c to run all night.

I'm trying to semi rationalize this economically. Figure over a 100 day summer, I'm saving 500-600 hours of generator. Call it $1/hr in fuel. So over 5 years I've saved $3k in fuel and 3000 gen hours. Plus additional savings captured during the other 9 months of the year, because I would literally never need to run the gen except to heat water for shower. Now i'm aware this will be at the cost of deeper cycles on the batteries (only during summer a/c use), but lead acid aren't that expensive in comparison to the fuel savings. Also it would likely require me to get a larger inverter. And of course buy the panels, multiple controllers, and wire. Still, it seems like i will be getting a brand new larger inverter and surplus of free energy (fall thru spring), completely subsidized by summer fuel savings.

Is anybody doing anything similar? Can anybody poke holes in my summer routine plan?

 

[rslifkin]

Running A/C from solar during the day is reasonable in your case. 1700w will keep up with your 10k BTU unit for a good portion of the day before it starts to draw the batteries down later in the afternoon.

Personally, if the weather is such that 24/7 A/C is basically necessary, I try to spend as much time at docks with shore power as possible. Compared to the fuel burn, noise, wear and tear, etc. of running the generator a ton (and I don't like running mine overnight), the cost of dockage isn't terrible in most cases. Although it does depend on where you are. In some places, it's just too expensive to be worth it.

 

[ERTF]

Personally, if the weather is such that 24/7 A/C is basically necessary, I try to spend as much time at docks with shore power as possible. Compared to the fuel burn, noise, wear and tear, etc. of running the generator a ton (and I don't like running mine overnight), the cost of dockage isn't terrible in most cases. Although it does depend on where you are. In some places, it's just too expensive to be worth it.

I'm in Miami so a slip would be $$$$...and I'd MUCH rather be on an anchor anyway. I figure 10hrs a day for a 100 day summer will be less than a grand in gas. As for wear and tear on generator, I bought a 3800 watt honda knockoff for exactly this purpose. It only cost $650 so even if it only lasts 1000hrs, it can be considered a cheap disposable.

 

[JWellington]

If I lived there in Miami, I'd want an Alva 60 boat..20KwH of solar panels and massive Li battery storage.

 

[EngNate]

One thing to keep in mind is those solar wattage numbers are best case max - unobstructed, unfiltered sun directly overhead. With the sun at an angle to the panels it falls off quite a bit. So your expected energy gain may be unrealistic.

 

[ERTF]

One thing to keep in mind is those solar wattage numbers are best case max - unobstructed, unfiltered sun directly overhead. With the sun at an angle to the panels it falls off quite a bit. So your expected energy gain may be unrealistic.

My estimated amp hour potential was roughly based upon what i get now with the 500w I already have. My current panels are 14 years old and the controller is an inefficient pwm. I'm still getting over 150ah in a day. I'm very confident 1200w of new panels with mppt controllers will yield me an additional 500ah. So that would bring me to 650ah at least.

 

[diver dave]

I would be considering a 48Vdc system. To reduce wire size, fuse sizes, and get a marginal increase in conversion efficiency.

 

[Lucky Chucky]

Captain,

I put 4 x 300 watt panels with an Outback 60 amp invertor on my ship 2 years ago in La Paz, Mexico. We are now in Costa Rica. Over the last 128 days, my invertor has logged between 19 amps and 175 amps. While on the hook, we consume around 300 amp hours per day, and that's not including the 220v systems.

Essentially, I was hoping for more amp hours but am happy it's not less.

~Lucky Chucky

 

[mvweebles]

I have a condo in Yucatan. While it's grid-tied, it is net zero and air-conditioned with full monitoring.

By my calculations, your 1700 watts of panels will net you about 8500 WH/day of power on average. A 10k Btu marine A/C pulls around 8 amps plus another 2 amps for the pump. Call it 10 amps @ 120vac, or 1200 watts. On a perfect day, gives you 7 hours of A/C from sunshine alone. Your 1050 AH FLA battery bank will give you about 6000 WH of usable power (50% DoD), or another 5 hours of A/C run time at which time it will take a helluva battery charger over 8 hours to recharge. In 3-5 years, your batteries will be shot and need to be replaced.

The suggestion to consider 48v is a good one as there are better chargers for this size battery bank. Also suggest you consider LFP as they will last much longer and have a much greater DoD.

Please update with whatever you decide and how it works out. A/C on solar is the Holy Grail for many. You are likely on the right track, but doubt a marine 12vdc backbone with FLA batteries will get you where you want to go.

Peter