Using solar power to charge a large battery
November 13, 2015 7:26 AM Subscribe
We have a small cabin out in the middle of nowheresville, USA that is probably five miles from the nearest human being. No electricity, no indoor plumbing, wood stove for heat, that type of thing. We recently decided to use a big ol' marine battery to power some electric lights because the lp gas lantern lights really just don't cut it anymore. To charge those batteries (we have two), we are currently using a generator, but we would like to use solar instead. What are we looking at for equipment and cost?
How much electricity do you need to generate?
posted by JackBurden at 8:19 AM on November 13, 2015
posted by JackBurden at 8:19 AM on November 13, 2015
If it's really just a few lights, and maybe a small laptop, but not eg, a fridge or other large power sink, and if you don't need to completely guarantee power at all times (some weeks are just cloudy and 2 marine batteries can only go so far), you should be able to make do with 128 watts or so of solar panels (probably a few hundred dollars) and a small charge controller (my xantrex controller was $150).
That's my experience anyway. You can find solar calculators that you can plug the numbers into and get an estimate of just how much solar capacity you need, covering all worst cases. But, it's easy to start with one panel and one battery and expand if you under-estimated, so take those calculators with a grain of salt.
posted by joeyh at 8:19 AM on November 13, 2015
That's my experience anyway. You can find solar calculators that you can plug the numbers into and get an estimate of just how much solar capacity you need, covering all worst cases. But, it's easy to start with one panel and one battery and expand if you under-estimated, so take those calculators with a grain of salt.
posted by joeyh at 8:19 AM on November 13, 2015
Response by poster: Do you stay at the cabin for long enough stretches that you need to charge them more than once per visit?
Sometimes up to a month at a time of constant living (by various brothers as their vacation times sometimes do not overlap) in the cabin. Usually the Fall because of the hunting seasons and the lake fishing is best then. The Summer months see heavy usage as well. Once the snow starts flying, though, there's no way to get to it.
posted by NoMich at 8:19 AM on November 13, 2015
Sometimes up to a month at a time of constant living (by various brothers as their vacation times sometimes do not overlap) in the cabin. Usually the Fall because of the hunting seasons and the lake fishing is best then. The Summer months see heavy usage as well. Once the snow starts flying, though, there's no way to get to it.
posted by NoMich at 8:19 AM on November 13, 2015
Response by poster: How much electricity do you need to generate?
I think joeyh's post about what we'll be powering sums it up about right. Overhead LED lights, a coffee maker and they are looking to add a microwave oven (nothing huge or industrial, of course). But there are no plans for a TV or a home stereo or anything frivolous like that.
posted by NoMich at 8:22 AM on November 13, 2015
I think joeyh's post about what we'll be powering sums it up about right. Overhead LED lights, a coffee maker and they are looking to add a microwave oven (nothing huge or industrial, of course). But there are no plans for a TV or a home stereo or anything frivolous like that.
posted by NoMich at 8:22 AM on November 13, 2015
Microwaves use a lot of power. That basically pushes you from the few-hundred-dollar RV/boat/camping level system to the few-thousand-dollar off-grid home level.
The things you'll need are:
solar panels
batteries
battery charge controller
inverter (preferably pure sine)
Use one of the many online load calculators to determine how to size your system.
posted by bradf at 8:50 AM on November 13, 2015 [2 favorites]
The things you'll need are:
solar panels
batteries
battery charge controller
inverter (preferably pure sine)
Use one of the many online load calculators to determine how to size your system.
posted by bradf at 8:50 AM on November 13, 2015 [2 favorites]
Check out the solar units made to charge electric boat lifts... They might work for your needs..
posted by HuronBob at 9:45 AM on November 13, 2015
posted by HuronBob at 9:45 AM on November 13, 2015
Also, following on from bradf's list, you'll be getting up to the scale that is beyond sealed RV batteries. The big PV lead-acid batteries spit acid, belch explosive gases, are heavy as all get out, and really, really, really don't like to go below freezing.
posted by scruss at 10:55 AM on November 13, 2015
posted by scruss at 10:55 AM on November 13, 2015
Microwaves use a lot of power. That basically pushes you from the few-hundred-dollar RV/boat/camping level system to the few-thousand-dollar off-grid home level.
Not necessarily. Microwaves do use a lot of power, but they don't use a lot of power for a long time. Say you have a 1200W microwave that you use for 10 minutes a day. 1200W@120V=10A draw. 10 minutes is 1/6th (.166) of an hour, so the total battery consumption is 10*.166, or 1.6Ahr.
1.6 amp-hours is not that a big draw on the battery. A 300W solar panel generates more than that every hour. A 120W lightbulb draws, if it's on for 12 hours, 12Ahr. A 12W LED fixture running for 15 hours draws 1.5 Ahr. Given that a 12W LED lamp put out about what a single 100W incandescent does, it's pretty obvious that the thing that gets you is lighting. Four of those lamps is 6Ahr -- well over three times as much as nuking the oatmeal every day.
Yeah, if you run the microwave for an hour, you're looking at 10Ahr and starting to reach a significant draw. But who does that?
The thing that kills you in battery power isn't short high-current usage, it's long usage. That's when the 1Ahr/hr devices turn ugly on you. If your running something 12-15 hours, like a lamp, you need it to be low draw or you need much larger storage and recharge.
posted by eriko at 11:04 AM on November 13, 2015 [1 favorite]
Not necessarily. Microwaves do use a lot of power, but they don't use a lot of power for a long time. Say you have a 1200W microwave that you use for 10 minutes a day. 1200W@120V=10A draw. 10 minutes is 1/6th (.166) of an hour, so the total battery consumption is 10*.166, or 1.6Ahr.
1.6 amp-hours is not that a big draw on the battery. A 300W solar panel generates more than that every hour. A 120W lightbulb draws, if it's on for 12 hours, 12Ahr. A 12W LED fixture running for 15 hours draws 1.5 Ahr. Given that a 12W LED lamp put out about what a single 100W incandescent does, it's pretty obvious that the thing that gets you is lighting. Four of those lamps is 6Ahr -- well over three times as much as nuking the oatmeal every day.
Yeah, if you run the microwave for an hour, you're looking at 10Ahr and starting to reach a significant draw. But who does that?
The thing that kills you in battery power isn't short high-current usage, it's long usage. That's when the 1Ahr/hr devices turn ugly on you. If your running something 12-15 hours, like a lamp, you need it to be low draw or you need much larger storage and recharge.
posted by eriko at 11:04 AM on November 13, 2015 [1 favorite]
I know this is offtopic, but drip coffee makers also draw a lot of power: around 800 watts for 4 cups.
anything heat related you really want to do with gas, not electricity. a newer led LCD TV probably uses less power than a coffee maker...
posted by ennui.bz at 11:04 AM on November 13, 2015
anything heat related you really want to do with gas, not electricity. a newer led LCD TV probably uses less power than a coffee maker...
posted by ennui.bz at 11:04 AM on November 13, 2015
Lots of good battery, load, solar, charging info at The 12volt Side of Life. Don't miss the 2nd page.
posted by LoveHam at 11:16 AM on November 13, 2015 [1 favorite]
posted by LoveHam at 11:16 AM on November 13, 2015 [1 favorite]
Say you have a 1200W microwave that you use for 10 minutes a day. 1200W@120V=10A draw. 10 minutes is 1/6th (.166) of an hour, so the total battery consumption is 10*.166, or 1.6Ahr.
1200W@12V (typical lead acid batteries) = 100A draw. And that's not accounting for inverter losses. Deep cycle batteries are generally rated at C/20, and capacity is greatly reduced if they're discharged at 1-3C. Eg: A 100Ah battery may only be 30Ah at 1C (100A). In this scenario, 10 minutes of microwave usage would deplete the battery to 50% DOD.
posted by bradf at 11:19 AM on November 13, 2015
1200W@12V (typical lead acid batteries) = 100A draw. And that's not accounting for inverter losses. Deep cycle batteries are generally rated at C/20, and capacity is greatly reduced if they're discharged at 1-3C. Eg: A 100Ah battery may only be 30Ah at 1C (100A). In this scenario, 10 minutes of microwave usage would deplete the battery to 50% DOD.
posted by bradf at 11:19 AM on November 13, 2015
Read HandyBob's site carefully. He's living that life and has a practical engineering view.
posted by sammyo at 12:02 PM on November 13, 2015
posted by sammyo at 12:02 PM on November 13, 2015
A hybrid approach can make sense also since you already have a generator, use less gas, shorter charge times but do keep those batteries topped off.
posted by sammyo at 12:05 PM on November 13, 2015
posted by sammyo at 12:05 PM on November 13, 2015
We recently decided to use a big ol' marine battery to power some electric lights
This is just for electric lights? If you switch to DC powered LED lighting you can probably reduce your power needs by quite a bit.
posted by yohko at 1:49 PM on November 13, 2015
This is just for electric lights? If you switch to DC powered LED lighting you can probably reduce your power needs by quite a bit.
posted by yohko at 1:49 PM on November 13, 2015
I have an off-grid cabin with a cheap (read: amorphous) 60-watt solar system that I put together myself without really knowing a lot. Panel prices have dropped so much that I will probably upgrade my panels next summer and use the old ones elsewhere. It powers lights, radio, ceiling fan, and a refrigerator(!). I have to pay a certain amount of attention to my power consumption if I'm there for more than a weekend, but I'm pretty routinely there for a week at a time and have never really had a problem. Close call once when the first four days were completely overcast. This is with no generator for backup. Normally I about break even on power during the summer when I am there, and quickly make up for any discharge in the batteries when I am not there.
It is easier to save power than it is to make it. LED bulbs are your best friend. Anything 12 volt is better than running off from an inverter. I do use an inverter, but sparingly.
My refrigerator is a Sundanzer 5.8 cubic foot which is designed to run off from solar, 12v or 24v. It was expensive to buy, but it's fantastic. It is by far the majority of my power consumption. I leave it on all summer. Cold beer waiting for me on arrival.
Spend extra on an MPPT charge controller, and on your battery bank. If you use more than one battery, they should be the same size/make/age and ideally from the same lot. Two 6v deep cycle (golf cart) batteries wired in series to make 12v works great. If you combine different batteries or if one goes bad, the weakest one will screw up the others. If you don't discharge your batteries too much, they should last a long time. May as well start with your existing one until it dies or you want more capacity. My system in 5+ years old and going strong.
Total cost for batteries, panels, and charge controller should be under $500. You can get started with a 100w panel and charge controller for under $200. Do it.
I also use a couple of digital "Watt's Up" meters to track how much power I make and use. This is worth the investment if for no other reason that you can tell at a glance if your panels are working correctly.
Put in a couple of inline fuses, minimally one between the panels and the charge controller and another between the battery and the load. If you don't know much about electricity, a friend or family member with even basic knowledge will be a huge help.
I cannot imagine having my cabin without solar, but I do have the benefit of pretty good southern exposure where I'm located. This is in northern Minnesota, which pretty much proves that it can work anywhere. I obviously make much less power in the winter, but it doesn't matter because I don't use the fridge then. Feel free to MeMail me if you have any specific questions.
posted by LowellLarson at 2:33 PM on November 13, 2015 [6 favorites]
It is easier to save power than it is to make it. LED bulbs are your best friend. Anything 12 volt is better than running off from an inverter. I do use an inverter, but sparingly.
My refrigerator is a Sundanzer 5.8 cubic foot which is designed to run off from solar, 12v or 24v. It was expensive to buy, but it's fantastic. It is by far the majority of my power consumption. I leave it on all summer. Cold beer waiting for me on arrival.
Spend extra on an MPPT charge controller, and on your battery bank. If you use more than one battery, they should be the same size/make/age and ideally from the same lot. Two 6v deep cycle (golf cart) batteries wired in series to make 12v works great. If you combine different batteries or if one goes bad, the weakest one will screw up the others. If you don't discharge your batteries too much, they should last a long time. May as well start with your existing one until it dies or you want more capacity. My system in 5+ years old and going strong.
Total cost for batteries, panels, and charge controller should be under $500. You can get started with a 100w panel and charge controller for under $200. Do it.
I also use a couple of digital "Watt's Up" meters to track how much power I make and use. This is worth the investment if for no other reason that you can tell at a glance if your panels are working correctly.
Put in a couple of inline fuses, minimally one between the panels and the charge controller and another between the battery and the load. If you don't know much about electricity, a friend or family member with even basic knowledge will be a huge help.
I cannot imagine having my cabin without solar, but I do have the benefit of pretty good southern exposure where I'm located. This is in northern Minnesota, which pretty much proves that it can work anywhere. I obviously make much less power in the winter, but it doesn't matter because I don't use the fridge then. Feel free to MeMail me if you have any specific questions.
posted by LowellLarson at 2:33 PM on November 13, 2015 [6 favorites]
I helped someone go through these calculations a couple of months ago for a similar situation (cabin for weekend use) and the answer was to stay with a generator for now. Especially considering winter snow (not much solar generation under several feet of snow) and complexity/ease of use, our analysis kept ending up with the gasoline powered generator, even though we had wanted it to turn to solar.
Your situation might be very different (in terms of climate, use, etc), but that is where we ended up. Lowering consumption is huge, as is being really honest in terms of usage and the technical sophistication of the people who will be using the cabin.
posted by Dip Flash at 7:03 PM on November 13, 2015
Your situation might be very different (in terms of climate, use, etc), but that is where we ended up. Lowering consumption is huge, as is being really honest in terms of usage and the technical sophistication of the people who will be using the cabin.
posted by Dip Flash at 7:03 PM on November 13, 2015
Backwoods Solar is pretty well regarded and have various worksheets to like this one: http://www.backwoodssolar.com/power-usage
I thought I'd use it to run the numbers, and compare with my actual situation. A lot of laptop use, some more for lights, a little bit of radio, and that's all the power I need. Using that page's table this came to 1100 watt-hours per day. Following their worksheet to the end, it estimated I need 385 watts of solar capacity.
My actual capacity? Some 256 watt panels that are over 15 years old, plus hail damage, so have certainly degraded below 128 watts.. my guess is to around 64 watts. With a battery bank that came out of an electric bus in 1997 and is still limping along, plus 1 new deep cell battery (very bad idea to mix batteries like this BTW). However, in real life, this is enough for me 99% of the time. Even in winter with snow (you have to brush the panels off, but then snow tends to bounce sunlight around and so if there are hills nearby, production actually goes up on a sunny day with snow)!
Which is why I say, look at these solar calcuators sure, but they are very rough estimates, aimed at guaranteeing more power than you need, under the worst conditions. When you're not there all the time and/or are willing to be flexible, such calculators could easily be off by a factor of 2x to 10x.
posted by joeyh at 6:37 AM on November 14, 2015
I thought I'd use it to run the numbers, and compare with my actual situation. A lot of laptop use, some more for lights, a little bit of radio, and that's all the power I need. Using that page's table this came to 1100 watt-hours per day. Following their worksheet to the end, it estimated I need 385 watts of solar capacity.
My actual capacity? Some 256 watt panels that are over 15 years old, plus hail damage, so have certainly degraded below 128 watts.. my guess is to around 64 watts. With a battery bank that came out of an electric bus in 1997 and is still limping along, plus 1 new deep cell battery (very bad idea to mix batteries like this BTW). However, in real life, this is enough for me 99% of the time. Even in winter with snow (you have to brush the panels off, but then snow tends to bounce sunlight around and so if there are hills nearby, production actually goes up on a sunny day with snow)!
Which is why I say, look at these solar calcuators sure, but they are very rough estimates, aimed at guaranteeing more power than you need, under the worst conditions. When you're not there all the time and/or are willing to be flexible, such calculators could easily be off by a factor of 2x to 10x.
posted by joeyh at 6:37 AM on November 14, 2015
The big PV lead-acid batteries spit acid, belch explosive gases, are heavy as all get out, and really, really, really don't like to go below freezing.
Huh, what? Everyone has lead-acid batteries in their cars and they don't spit acid and belch explosive gases. A charged battery will not freeze even at -50C as everyone who lives in someplace like Minnesota knows and leaves their car outside over night. Even a fully discharged battery will not freeze at -10C.
It is high temperatures rather than low temperatures that shorten the life of lead-acid batteries. Low temperatures can temporarily reduce capacity, but that capacity is restored by warming to room temperature. Some people will put their batteries in a compartment in the ground under the floor to maintain more even temperatures.
Lead-acid batteries are the most practical storage for small stationary PV systems. You don't care about weight and they are very cheap. They have the highest energy capacity per dollar.
posted by JackFlash at 10:53 AM on November 14, 2015
Huh, what? Everyone has lead-acid batteries in their cars and they don't spit acid and belch explosive gases. A charged battery will not freeze even at -50C as everyone who lives in someplace like Minnesota knows and leaves their car outside over night. Even a fully discharged battery will not freeze at -10C.
It is high temperatures rather than low temperatures that shorten the life of lead-acid batteries. Low temperatures can temporarily reduce capacity, but that capacity is restored by warming to room temperature. Some people will put their batteries in a compartment in the ground under the floor to maintain more even temperatures.
Lead-acid batteries are the most practical storage for small stationary PV systems. You don't care about weight and they are very cheap. They have the highest energy capacity per dollar.
posted by JackFlash at 10:53 AM on November 14, 2015
Response by poster: There's a lot to go through here. I sent the given links to my brothers for them to pore over as well. Thanks all!
posted by NoMich at 5:04 PM on November 19, 2015
posted by NoMich at 5:04 PM on November 19, 2015
This thread is closed to new comments.
posted by jon1270 at 8:10 AM on November 13, 2015