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# Posted: 8 Dec 2017 10:42

I posted this on a solar board and essentially got called an idiot. Hopefully there are a few on here that will be a bit kinder than that.

I'm building a small workshop that is off-grid. I need sufficient power to run a small heater (350W for 2 hours) maybe once a week, plus an occasional power tool and some LED lighting. Neither of the latter will be extensive, so very little overall power draw. (if I need more power, I have a small generator).

My thought is to run a single ~300W panel into 3 ~100Ah, 12V batteries (total capacity ~3000W), through a charge controller. Running power tools (small drill press or bandsaw or shopvac) would require some surge capacity, so I would look at an inverter that could handle ~1500W continuous and 3000W surge.

I get plenty of sun (average 5.5 hours/day) and have calculated the overall power budget so that I will have plenty of input power to cover the average demand.

So, looking for thoughts on whether this design is workable and, if not, what could/should be changed. I'm looking to keep the cost reasonable, so planning on a square wave inverter (unless there is a reason to go with the pure sine wave). I will not be using any sensitive electronics that require pure sine wave. And looking for suggestions on components. As background, I've been off and on using solar for over 30 years, so not new to the concepts.

Thanks for any input.

# Posted: 8 Dec 2017 12:32

I don't see anything wrong with it but an option would be to forgo the solar panels and charge controller and just use the generator for a short period of time to charge up the batteries. Might be more cost effective.

# Posted: 8 Dec 2017 13:42

This is where idiots like me gained a ton of knowledge...still on the up curve but this is indeed a great place.

Stay tuned, I'm sure more will chime in

# Posted: 8 Dec 2017 16:50

Heating with electricity is the least efficient method possible, seriously ! ANY resistance device (coil on stove, element in water heater, strip coils in portable heaters) are plain and simply BAD, you'll kill the batteries in no time and that's silly.. Infrared is better, Quartz NO.

I have mentioned this before, try and get a small furnace out of an RV/Camper which runs on propane, scavenge the regulator too and with 2 20# / 30# bottles you'll last a long time. Most run on 12V (ususally what an older RV has), some run 120V and others can be either or. They are most often vented with their own air intake, so no worries about gassing yourself.

LED Lights and light power equipment will run off batteries as long as you have enough. Pure Sine Wave is best all around and won't harm electronics, square & mod sine wave not so good and can in some cases damage motors & pumps.

Single 300W panel not likely gonna cut it... double that up and you should be ok. Batteries ! If they could freeze ( low charge / depleted & cold below 30F) you need to take precautions to prevent that.

IF your staying 12V then you can also use the system as battery minder for other batteries, have a couple of leads with an OFF switch that you can turn on and use for boosting if needed and more... There is more & more Low Voltage stuff coming on the market all the time as there is no real need in many cases for Big Volts to do a job or run equipment for living.

# Posted: 8 Dec 2017 17:14

dsmith, thanks for that thought. It had occurred to me to do this as well and may be the most cost-effective way to go.

Steve S, also thanks for the input. I know that electric is the worst way to go for heat in general. What I didn't mention is that this is a preheater on a small airplane, and the only real option in my case is electric resistance. I only need it once or twice a week for a couple hours each time and expect to have to size my battery array appropriately to stay above ~25% DoD. So, might end up with more than the three I have initially planned.

I am somewhat concerned about using the tools on MSW power but will definitely not be doing any electronics on that inverter should I end up going that way. Wish I could find a pure sine inverter that wasn't several times the cost of MSW.... but may end up springing for the expense.

Would like to know the rationale for a second panel. My calculated usage is about 400W/day on average but will have some days where I use 700-800W (when the heater is on). With >5 days of sun on average it would seem a single 300W panel would suffice (~1500W/day) but maybe I'm missing something (which is why I have posted here).

I would also consider going to a 24V system (I have such at the cabin already, but not within striking distance of the shop) if there is a good reason. I know it would decrease the wiring size. One reason for 12V is that many of the MSW inverters seem to be based on 12V. If I went to pure sine, then 24V might make more sense.


# Posted: 8 Dec 2017 17:54

Re the MSW inverter. Many motors do not like MSW. A motor with brushes will probably be okay, but the type of motor on a drill press will likely run hotter and have a reduced service life. Back when I was looking at getting solar up and running I used a KillAWatt meter to measure the power factor of several tools I was wanting to use. Many used 20% more power on MSW compared to grid or PSW inverter power. One motor used 30% more. That extra power goes out of the motor in the form of heat.

# Posted: 8 Dec 2017 19:48

The airplane heater is an important detail LOL.. ok, dunno about that, I thought shop - as in small bldg.

If you figure the extra wear / hardness on your equipment and failure, MSW will end up costing you more (tools + New inverter), but you'll likely get frustrated with it and buy a PSW anyways, so...

Inverter, you don't need a Rolls Royce when a Geo Metro will do ! wink wink... You can get a cheaper offshore of decent quality one, won't be as efficient, but your only using it for a short time & shutting it off removes the drain on your batteries...

24/12 well, either will do, just gotta size it right, you did that. Future potential though, FLA batteries the weakest link brings the lot down thing, as you know, so adding new later isn't great. 25% dod is not good that will shorten their lifespan and efficiency. A lot depends on the type of batteries used too, so.

I suggested 2 panels as more is better, shade, clouds, dust & pollen etc... weather is never guaranteed either and less so as things progress. You already use solar and know your spot so your probably ok anyways. If going up in 24v & bigger batteries then probably not a bad idea... you might also wind up using the shop more too, as you'll have "power" in your hands ! AND without the gas fumes from a genny to boot.

# Posted: 9 Dec 2017 10:36

Aren't the solar forums fun. ha ha ha. I got kicked off one for daring to post the link to the princeton study on lithium.

so you know where this is going.

Your battery pack is small for what you're trying to do. Also. you're running motors and lots of them. These will run cooler and happier with pure sine wave. there are lots of cheap chinese "dsp" style pure sine wave inverters now. for the 100 bucks you may or may not save. my .02

Depending on how much you plan to use your workshop there are two good reasons to start with 2 solar panels. 1) more power! 2) higher voltage.

With the larger draw appliances you plan to use it would be a good idea to use a higher voltage battery pack. I would recommend 48v. Get a good mppt solar controller. You will need 2 panels of 300w class to get the voltage to run the charger. (48v plus 10v to make the controller happy charging the higher voltage battery pack).

Go lithium: See the "question for creeky" post for an ebay link to Volt batteries. Even at double that price you are getting excellent value for $$$. And one would give you more power than your 3 lead acid batts. Not to mention last 4x longer etc etc

and, as others have noted, the saving on gas will quickly pay for the additions to your system.

if you do go smaller. and that's not a bad idea, one panel, small battery, etc. Use the genny to run the bandsaw et al for that limited time you're using it. Put in a transfer switch. they're cheap.

Have fun.

# Posted: 9 Dec 2017 10:57

Thanks all. I guess a PSW inverter is worth the cost of an upgrade. I've seen a few that seem to get good reviews (Cotek is one) that aren't too expensive. And you are right that only using it occasionally rather than continuously should prolong the life. And I'm leaning toward oversizing the system, particularly the batteries. If I go with ~7000W of storage capacity, then I would only ever draw down to 90% (~10% DoD) which should allow them to last a long time.

Steve, the shop is fairly small (12x12 feet) but probably not going to be heated by electricity. It's just the airplane that I would like to preheat in winter occasionally. I'm hoping to be able to figure out a remote start or timer system of some sort to have it heated and ready rather than having to drive to the airport and sit around in the cold for 2 hours while it warms up.

Creeky, been looking at those batteries but wonder if there is any way to determine their condition? I have a bit of distrust of ebay, particularly if there is a no return policy of the seller. Lithium would be great since they can be discharged a lot deeper than lead thus requiring less battery capacity. But if one of these would replace 3 lead batteries, maybe it's worth a try for $350. Any other caveats on used Li batteries? Gotta think about this one a bit....

# Posted: 9 Dec 2017 13:05

Lithium means you have to watch your voltages. They are different from Lead.

A Volt lithium 48v is charges to an "absorb" of 48v. A lead 48v is actually 50.8 and charges at "absorb" of 57.6. Very different mind set.

(Its actually really fun to see your batteries charge at the level of charge. Ie. 46 while charging is 46 if you turn the charger off. While with FLA you charge to 57.6 but if you turn off the charger the voltage may "settle" down to 42v or 44. Or 46. Or. You really can't tell what lead is by voltage. While with lithium you can. Sweet)

Almost all of the Volt batteries I've seen are good. The nice thing with the ebay fella there is he's done all the hard work. I can't speak to the condition of the modules tho.

It is far superior tech. Even if there are the issues with adapting older equipment. Watching voltages. Tuning systems. Worth it. So worth it.

Mike 870
# Posted: 9 Dec 2017 15:07

Lol the forums are crazy. I got kicked out of the largest Solar Powered Off Grid RV Boondocking Facebook group for basically telling people what their users manuals said when they asked for help. The group owner, let’s just say his name rhymes with Ryan Loon, didn’t like the fact that I knew more about solar than he did.

As others have said, you can decrease your cost by doing your space heating with propane, kerosene wood etc.

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