How much solar power will I need?

[sailingchiro]

I will try giving my example. I started in hopes of running air conditioning from solar.

My a/c is 15 amps at 120 volts, This P=IE or current x voltage= 1800 watt/hour. Now we have to equate our batteries to the watts. Remember, there will be some loss 10-15% with inverter and wire. Using the same formula but backwards 1800/24volts (house bank)=75 amps (big wires). How long will I run my a/c? my wife likes 11 hours. Therefore 1800 x 11 =19.8kwatts. I have read, you will need x2 kwatts in batteries, therefore 39kwatts of batteries for overcast days. Seems like we get solar for roughly 7 hours per day and probably we get about 60-70% of rated output. If I am wrong on my numbers please enlighten me. Therefore, I would need at least 19800/7=2828 watts of panels plus the 15% loss=3252 watts and also the loss in panels of 30-40% of rated output. I will show the 30% to be fair. This equates to 4227 watts or 10-11 400 watt panels. This does not account for overcast days or boom shading etc. . The batteries will handle 2 days just in case. Now for the batteries. I would think it would be hard to have anything but Lifepo4 batteries. If one battleborn is 1200 watts or 100 amp hr. x voltage (12volts)= 1200 watts. this would equate to roughly 32 batteries at $900 a piece or $29,000. I know these numbers are not real world but convinced me very quickly that I could not afford solar a/c. I have 7kwatts of battery and 900 watts of solar. I have to admit the batteries and solar are a drop in the bucket to the other parts needed. I will still be installing this easy peezy solution for several weekends. I don't know if it will ever be completely finished. Charge controllers, inverters, busses, huge wires, wire ends, fuses, breakers, battery mounts, battery management system, chargers appropriate for batteries (lifepo4 likes the juice like 200 amps), charger for house bank, battery switches, current meters, amp hour log, etc. etc. I just want my refrigerator to run off it and invert power for the house. I will still need to monitor microwave, water heater, and wifes hair dryer. I believe if an a/c was less than 1000 watts or 8 amps, maybe but still couldn't run it 24/7. It is quite a feat to over come. Sorry. One guy on the forum has 32kwatts of power. Please remember the inverter produces alot of heat and most likely be the weakest link if used this much. Plus, as the inverter heats up, it becomes less efficient.

[sailingchiro]

That's what I'm trying to do now. And why I'm trying to see if I can realistically add AC. I figure, there wouldn't be so many different marine AC units if it wasn't feasible. And the one I'm looking at is only 3500 BTUs, the aft cabin is only 82 cubic feet (not square feet) and takes about 29amp. So if I do the math. A 6v golf cart battery has 210ah. So 210ah/29.2A= 7.19 hours. Realistically, so I don't kill my battery bank, I won't want to drain more then 50%, so one battery gives me 3.59 hours of run time with the AC. Now, i understand that batteries are a lot less officiant when discharged quickly. I think the 210ah is rated at a much smaller draw then 29.2a. I think you lose something like 40% efficiency when you draw current that quickly,


Could I ask at 3500 btu's, and 29 amps, is that 12 volts, 24 volts, or 120volts a/c? If it is 12 volts, it will take 2 6 volt batteries to give you the 210ah or roughly 105ah of use. So at 12 volts, you will only get 52.5 ah of use or less than 1.81 hours of use from each battery or 3.62 hours from two batteries if the 29 amps is at 12 volts. Plus the non-Lifepo4 batteries will shut down the a/c if low voltage is set to 12.5 volts very quickly.

[Hillbillybuddha]

Originally Posted by sailingchiro

I will try giving my example. I started in hopes of running air conditioning from solar.

My a/c is 15 amps at 120 volts, This P=IE or current x voltage= 1800 watt/hour. Now we have to equate our batteries to the watts. Remember, there will be some loss 10-15% with inverter and wire. Using the same formula but backwards 1800/24volts (house bank)=75 amps (big wires). How long will I run my a/c? my wife likes 11 hours. Therefore 1800 x 11 =19.8kwatts. I have read, you will need x2 kwatts in batteries, therefore 39kwatts of batteries for overcast days. Seems like we get solar for roughly 7 hours per day and probably we get about 60-70% of rated output. If I am wrong on my numbers please enlighten me. Therefore, I would need at least 19800/7=2828 watts of panels plus the 15% loss=3252 watts and also the loss in panels of 30-40% of rated output. I will show the 30% to be fair. This equates to 4227 watts or 10-11 400 watt panels. This does not account for overcast days or boom shading etc. . The batteries will handle 2 days just in case. Now for the batteries. I would think it would be hard to have anything but Lifepo4 batteries. If one battleborn is 1200 watts or 100 amp hr. x voltage (12volts)= 1200 watts. this would equate to roughly 32 batteries at $900 a piece or $29,000. I know these numbers are not real world but convinced me very quickly that I could not afford solar a/c. I have 7kwatts of battery and 900 watts of solar. I have to admit the batteries and solar are a drop in the bucket to the other parts needed. I will still be installing this easy peezy solution for several weekends. I don't know if it will ever be completely finished. Charge controllers, inverters, busses, huge wires, wire ends, fuses, breakers, battery mounts, battery management system, chargers appropriate for batteries (lifepo4 likes the juice like 200 amps), charger for house bank, battery switches, current meters, amp hour log, etc. etc. I just want my refrigerator to run off it and invert power for the house. I will still need to monitor microwave, water heater, and wifes hair dryer. I believe if an a/c was less than 1000 watts or 8 amps, maybe but still couldn't run it 24/7. It is quite a feat to over come. Sorry. One guy on the forum has 32kwatts of power. Please remember the inverter produces alot of heat and most likely be the weakest link if used this much. Plus, as the inverter heats up, it becomes less efficient.

The idea, right now, is to sail down to the sea of Cortez and spend about a year down there, give our take, and then head to either the Caribbean or The Polynesian Islands.

The idea with the AC wasn't to run it all the time but on for a couple hours on hot nights, just to knock the heat out of the air avs make getting to sleep a bit easier. So I doubt we'd ever use it on overcast days. But, I think I'm going to look at small generators at this point. I just didn't want to bother people with the sound of a genset but, I'm guessing, on those really hot nights, i won't be the only one with a generator running.

[sailingchiro]

I installed the Nextgen 5.5 genset and lined the enclosure with 3/16" lead with several layers of kil mat. It burns very little fuel and is very quiet inside and outside. The A/C compressor and fan are louder than genset. I have a blower in and blower out of the genset enclosure that keeps the heat out of the boat. If you want to run 2-3 hours per day on hot days, that would equate to about 2-3 quarts per day. 2 gallons runs my 10-11 hours depending upon load. We don't run it very often, we like the wind while sleeping. Maybe run it 1 day every 2 weeks. We have screens on our hatches to keep the bugs out and a very large wind scoop on windless nights to channel wind thru boat. I typically sleep in the cockpit or on the trampoline with mosquito netting if the winds have died down. We have a very cheap mosquito net that goes over our entire hard top of the cockpit.
For clarification, what was the voltage and amp load on the marine a/c unit you were looking at?

[sailingchiro]

Plus if you are running a genset a couple of hours per day, you could recharge batteries, heat water, run a/c, cook meals, cool frig./freezer, charge computers, watch a movie, etc. Alot of cruisers run an engine 2 hours per day. This will also decrease usage of your drive engine. Probably less than 1/2 gallon per day. 50 gallons of fuel will last 100 days?

I selected a genset that was the same type engine as my drive engines, therefore, I have all my spare parts if needed (starter, altenaters, fuel injectors, etc.) Kubota.

[Emmalina]

Loose the AC Acclimatize its healthier !

[sailingchiro]

Originally Posted by Emmalina

Loose the AC Acclimatize its healthier !

I agree but imagine the OP has the same fear I have. I hot/unhappy wife.

[mvweebles]

This webasto 6000 btu AC is rated at 4.5 amps 120vac. Or around 550 watts (does not include water pump which is non-trivial). Or in 12vdc terms, under 50 ah. Rough terms, one GC battery for each hour of run time though would run down to 50%. If OP can really fit that much solar aboard, during the day could run this for free.

https://www.westmarine.com/buy/webas...unit--17484742

[sailingchiro]

Interesting, I have one berth that is not conditioned. This may be an excellent option. It is not a heat pump but wouldn't need heat in that berth, it has genset and engine. But I really like the option. Thanks for the link.