I have a cabin that is way out in the middle of no where, it would cost 000's of dollars to get power to it. If I have a 8,000 BTU air conditioner that runs on 25 volts and 5 amps (or 875 watts)How many solar panels will I need to run this during the day, plus charge the batteries to run it all night? How many batteries would I need
Unless you have a good charge controller, the battery will never charge, as a lead acid battery requires 5 volts to charge. Ans you don't tell us the capacity of the battery? Is the solar cell only 5 watts? That is amp if it develops 5 volts. Assuming the solar panel is good for 5 watts and 5 volts, that is only amp. so you are charging the battery with amp and discharging it with 5 amps, so how long you can do that depends on the size of the battery. But it will discharge in at most 5 hours with a large battery, and the solar panel will take several days to charge it up again at amp. The best you will be able to do with that 5 amp load is perhaps hour a day, which is 2 watt-hours. The solar panel will be able put back amp for 2 hours at 5 volts, which is 5 watt-hours, which will give you perhaps 2 watt-hours in the battery. .
You are probably going to need about 60 panles and 60 batteries....you will also need a very good power inverter. Also consider the power it takes to invert 2 volt DC battery power in to 20 volts AC power required for an air conditioner. You might consider also trying to find a 2 or a 24 volt unit cooler. You would be better off buying a generator. Solar panels and batteries are not going to run an air conditioner for long if at all.....it might be cheaper to have the power company run the power.
OK, in addition to the solar panels, you wil need an inverter (DC to AC; 2V t0 20V) capable of delivering 20A continuously and as much as 40A for the compressor starting surge, and a fairly large battery bank capable of delivering something over 2000 watts continuously (losses in the charging and conversion process) as well as the surge. So, if your actual demand is now calculated around 2000 watt/hour, and you need to both run the unit and charge batteries during the daylight hours - and considering that the ideal charging rate for a lead-acid battery is on the order of 0% of its capacity, you will need a huge reserve on the battery section and a huge excess on the solar section to make up for the 5 hours maximum input. So, the average high-quality deep-discharge battery will do about 400Ah @ 2.6V. Which comes to 5040 watts for one hour. For round figures, it will give you 2000 watts for 2.5 hours. You will need a minimum of eight of them just to make your system run for the 9 hours you are anticipating. Given that you do not want to run the batteries to zero each night, a 50% reserve would be a wise investment - comes to now twelve (2) batteries. Now, you will have to make 2000 watts (to run the unit) and also produce and another 8600 (per hour for five hours) watts to charge the batteries for the next night. Again, for round figures, you will need 0,000 watts of solar power, or seventy (70) panels. At ~0 watts/s.f. (00 watts/meter), that comes to about 000 square feet or 00 square meters. Modern non-crystaline panels may reduce this by 20%, or so but no more than that. You will be charging the batteries at roughly 20% of their discharge capacity - which will pretty much cut their service live in half, or so. As the average deep-discharge battery has about an 8-year life span under ideal conditions (and your situation will be far from ideal) you may expect to replace them every four years or so with good care. Much less with no care.
You would need 6 300 watt solar panels, US price around $300 per, And a minimum of 4 batteries. You are going to have major problems only having 5 hours of sun though.I am not sure you could get it to work with only that much sunlight. Along with an inverter and some other controller units. If you live in an area that has some wind, a wind generator would knock it down to where you would only need 2 solar panels and a small 500 watt wind generator. The problem you are going to have is trying to run it all the time. I had an AC that size in my camping van, it took about 20 minute to run two batteries down so low that the inverter started to sound the low battery alert. If I left it running it did ok, and I had an over-sized alternator for it. I had a 50 amp heavy truck alternator. A swamp cooler would be a lot less energy, you could easily recycle about 95% of the water. I know this is the 2st century and this may sound a little nuts, but with only 5 hours of daylight, if you have some trees you could use for burning you could build an outdoor steam engine cheaper and it would charge a lot more batteries in a lot less time. Plus you could run off the steam engines generator for as long as you had a fire day or night. Even a small 55 gallon water tank would give you plenty of head for the steam. You could easily run a 6000 watt generator off of it and have plenty of electricity while it was running. Just a thought, I had to resort to some extreme measures when I bought some Montana land in 2000, it was 6 miles to the nearest utility pole.
It would just be cheaper to run the power lines back to the house but what ever you do DONT take electricity from the drop above the meter for free at night. That would be wrong! Lol sticking it to the man!
