diy solar

diy solar

Designing small backup solar for essential home electric loads.

Vigilant24

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Aug 23, 2022
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Hello, I've enjoyed reading about the many projects here and finally joined up to dip my toe in. nThanks in advance for any assistance. I'm designing a small (cheap) solar backup system primarily to have emergency power if our grid goes down (regional ice storm or worse). It's just for the very, very basics (well water, light, tools, a fan, etc)

The total electrical loads would be modest, I'm figuring about:
- 20 minutes of submerged well pump use per day (115VAC, 1/2 HP. load: 900 watts when running BUT starting load of about 6000 watts)=300 Wh
- Recharge tools, fan, flashlights, radios, etc: = 400 Wh
So, about 700 Wh per day. No refrigerators, AC, etc. Some of that won't need to go into battery storage (e.g I can recharge all the tools, flashlights, fan during the day)

- The system I have in mind:
- Panels:
2 x 100W panels, I plan to run them in parallel, for a "12V" (nominal) system. So, in theory, up to 1000 wh of collection in perfect conditions.
- Storage: Right now I intend to use a single 12V marine "deep cycle" battery of 105 AH capacity (= 630 WH if discharged to 50%). It's not ideal, but it is inexpensive, available, sturdy (no BMS electronics to crump out), and if need be I can lug it to a car for an emergency recharging from the car alternator in a pinch.
- Charge controller: TBD. I own a 10A Renogy Wanderer PWM controller now.
- Inverter: TBD. I'll get a true sine wave model. If I must accomodate the 50 amp (120VAC) starting load of my well pump I might be able to get by with a 3000 watt (6000 wat surge) high frquency inverter. If I can use a soft-start device to bring down that starting surge I could use a much smaller (and less expensive) inverter.

Observation: That starting load of the well pump is the killer.

Some of my newby questions:
1) Charge Controller: Will a PWM controller provide DC voltage at something close enough to 12VDC to allow me to run an inverter off the battery charging leads? I know this sounds basic, but some descriptions I read indicate a PWM controller merely supplies the battery with the same voltage it gets from the panels, turning it on and off rapidly so the wattage going into the battery is the right amount for the battery's particular state of charge. Is that right? Would a MPPT charge controller provide voltage more suitable for running an inverter in the daytime?
So, I'm wondering if should I get a 20A PWM controller? Spend more for a 20A MPPT controller?

2) While my fully charged battery might have a useable capacity (to 50% discharge) of 630 Wh, if I power that 800 watt well pump with it for 20 minutes, the discharge rate/Peukert equation indicate I'll probably be lucky to get the 300WH I need for 20 minutes of pumping for one day. Buying another 105 AH battery might be a good idea, lowering the discharge rate of each one and providing at least one day of pumping if we get zero solar input.

3) Dealing with the pump starting surge. Any ideas on how best to do this? My well pump is a two wire 115 VAC 1/2 HP. I'm not too excited about the idea of paying thousands of dollars for a (very nice) Grundfoss DC pump. It appears that a Schneider Electric soft start device (their model number 6VMD4, cost $183) might work with my existing pump, and would let me go with a much smaller inverter. I've even considered the idea of snaking a small 12v sampling pump into my well beside my existing well line, power wires, etc. Something like this "Tornado" sampling pump (about $340) could give 1 GPM from a depth of 70 feet. Claimed motor life of about 400 hours, so that would be good for a few months of use (3 hour per day). It draws up to 210 watts, but if I've got good sun then that's not much more than the panels are producing, so little draw from the battery and no load on the AC inverter at all. If I wanted pressurized water in the house, I'd need to separate pump to do that, but an inexpensive 12V RV pulse pump could do that.

4) If I just use my existing 115VAC pump with no soft-start, the starting surge load will be about 50A (6000W). The marine battery has a claimed 500 Cold Cranking Amp capacity = 6000 watts. After inverter losses, probably not enough. Can I safely run that 12V battery in parallel with a regular automobile 12V starting battery >just for the starting period< of the motor? I know it;s generally not a good idea to attach batteries of different constructions or capacities in parallel, but I'd just do this to cover the motor starting load, then disconnect the car starting battery and charge it separately later. So, there shouldn't be much swapping of electrons between the batteries, unbalanced charging, etc.

Anyway, that's the outline of my little project. Any thoughts, harpoons, suggestions are welcome.

Mark W


Dayton, OH
 
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For the pump, I suggest a 24V system, including inverter, and a hefty battery bank. You will need to produce massive DC amps to feed your inverter and power your pump motor to surge to 6000 Watts.
 
General question: If I have a panel putting out 20V to my PWM charge controller, will the charge controller step that down to a lower voltage for charging the battery so that I can use the output (at the battery terminals) to run an inverter (12VDC--> 115VAC)?

Thanks for the info and post, Risky Rob! I guess I'd focused on 12V so that I could get by with 1 battery in a pinch (again, this is a backup-I-can't-get-replacement-parts application). Maybe best to go with 24V.
 
PWM connects the solar panel to the battery, nothing else. The battery drags down the solar panels voltage. It turns off when the voltage gets too high.

Backup systems are just a waste of money. Design a system that does something useful every day that can bewitched over for backup power. I have one that heats water and should the day come it can supply power.
 
Backup systems are just a waste of money.
That kind of depends on the circumstances. For some it might be life and death, e.g. life support equipment, or say some heating or to enable a gas system to operate (many require electrical power for ignition and fans), or pumps for water supply. For others it means being able to continue working and earn an income. Others may have investments they need to protect with a reliable power supply e.g. perishables, animals (such as fish tanks or incubators).

Backup value is kind of subjective as well. We get regular grid outages, so backup matters more to us.

That said, I did also design something to be useful in other ways, but the primary design was to provide backup.
 
Backup systems are just a waste of money. Design a system that does something useful every day that can bewitched over for backup power. I have one that heats water and should the day come it can supply power.
I understand and agree with the rationale ("You paid for the stuff, might as well put it to work and get some payback."). But, my suburban lot has plenty of trees, and no practical spot on my property gets more than 2-3 hours of sun per day. If our house is without power, I won't mind AT ALL moving them around to get full sun all day, but I sure won't move them in "normal times" to save 7 cents per day on my electric bill. Plus, if it's a windstorm or ice storm that brings down our local grid, there's a good chance there will be tree limbs falling all over outside. In this case. better to have these two panels safely inside, in their boxes. I honestly do hope it turns out to be a waste of money--cheap insurance.

Your use of PV to heat water is a fine application for existing "just-in-case" PV capability, especially if you'd otherwise be heating with electricity. Where I live, the panel would pay for itself in about 8 years, which is not bad.
 
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That kind of depends on the circumstances. For some it might be life and death, e.g. life support equipment, or say some heating or to enable a gas system to operate (many require electrical power for ignition and fans), or pumps for water supply. For others it means being able to continue working and earn an income. Others may have investments they need to protect with a reliable power supply e.g. perishables, animals (such as fish tanks or incubators).

Backup value is kind of subjective as well. We get regular grid outages, so backup matters more to us.

That said, I did also design something to be useful in other ways, but the primary design was to provide backup.
Our grid power is generally stable, but if we get a bad ice storm (bringing down lots of branches and utility lines) the grid could stay down for a week or two. Other anticipatable calamaties could result in longer outages. We're on a private well, so that means no water. We heat with natural gas, but require power for ignition and fans. If I can produce about 1000 wh per day on most days and practically store about 600-1000 wh to get through lulls in production I think it would make a pretty big difference to us, in a pinch
 
Our grid power is generally stable, but if we get a bad ice storm (bringing down lots of branches and utility lines) the grid could stay down for a week or two. Other anticipatable calamaties could result in longer outages. We're on a private well, so that means no water. We heat with natural gas, but require power for ignition and fans. If I can produce about 1000 wh per day on most days and practically store about 600-1000 wh to get through lulls in production I think it would make a pretty big difference to us, in a pinch
So pretty much a good chunk of the factors I mentioned. Good for you. 1 kWh/day and about the same again for storage is not a bit ask but you'll need more than a couple of 100 W panels.

Starting pumps is problematic, so I hope the soft starter or small backup pump does the trick for you. I think you might need to look at 24 V or even 48 V system though if your surge power demand is high.

Fortunately we have mains water and a tank for emergency use so don't have that pumping problem. And waste water is to a gravity fed septic system. I'm not into pumping sh** uphill.

Our grid also is fairly stable but I have years of stats on our outages and it averages one longer outage per month. We can go six months with not an issue, then the next month might have several outages. It's usually storm related, rural power lines are more susceptible to damage.

Not a cold climate where I am but we wanted energy security, and a few years back added an extra dwelling for my elderly mum so it became even more important. A multi-day outage which screwed up a family Christmas some years back was the proverbial final straw.
 
I have mini circuit breakers and have not heard until now that cause current resistance. I like that they function both, as switches, and will safeguard the system.
 
marine batteries (dual purpose) are only really intended for 20% discharge, not 50%.
Thanks. I'm trying to find more technical details on the battery I have now (an Exide 27MDC). Exide makes 3 lines of marine batteries: Marine Starting batteries, Marine Dual Purpose batteries, and Marine Deep Cycle batteries. I've got a Marine Deep Cycle battery. I'm sure it won't give the life of a true dedicated Trojan deep cycle battery. OTOH, I may need to (briefly) generate a lot of current to get my well pump started, so maybe some compromise is warranted in this case (this one is rated at 675 amps for 5 seconds). It's not what I'd buy to charge and discharge daily for years, but if they can last at least 250 cycles to a DoD of about 50% I'd be happy. And at the price I paid it might be cheaper to buy twice as many of them (so, lower DoD needed) than to buy the purpose-built deep cycle batteries. Running the well pump will require a draw of about 800 watts. According to the Peukert charts, that discharge rate will be murder on a single battery of this size, and I think the situation would be the same for a deep cycle battery. Better to have more of them to reduce the discharge rate on each for higher ultimate capacity.

But, clearly, I need to get more technical info on the batteries. You are right to point it out, thanks.
 
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marine batteries (dual purpose) are only really intended for 20% discharge, not 50%.
But for back up use why not take them all the way to LVD? In my experience, most batteries in this application die from calender life long before cycle life.
 
I'm
thanks, I didn't know that
I still haven't gotten the details on these particular "Marine Deep Cycle" batteries, but I found an interesting article about these >types< of batteries. It is written by a guy who apparently knows firsthand about >real< deep cycle batteries and has compared them to the cheaper, more common, and not as good RV or Marine "deep cycle" batteries usally sold (at least in the US) as Series 24, 27, and 31 (which really denotes the physical size)

https://marinehowto.com/what-is-a-deep-cycle-battery/

It's approx 40 pages, pictures of cut-open batteries etc. The upshot is that the "marine deep cycle" or dual purpose batteries, aren't nearly as robust inside as the batteries designed for golf carts, floor sweepers, fork lifts, and stationary standby power uses. The "marine deep cycle" batteries can do the deep cycles, they are better for this than typical car starting batteries, but compared to dedicated true deep cycle batteries their plates are thinner, often smaller. They have less open space below the plates (so stuff that falls off the plates with use can more readily short them out). A lot less lead. In a controlled lab environment, the Series 24, 27, 31 batteries cycled to 50% depth of discharge may last 300 - 500 cycles, while the true deep cycle batteries can go 800 -1200 cycles or so. Yes, the golf cart batteries are more expensive, but on a WH-per-cycle basis, they are cheaper.

Now, the other side: As OzSolar pointed out, in a typical backup role, lead acid batteries will usually die of calendar days age rather than cycles. If the batteries will perform the number of cycles I'll need over their calendar life, that's good enough. Also, in my use, being able to lift and carry a 12V battery to a car (for recharging, in a pinch) will be handy, and easier than two 6V batteries. Also, they have a surge rating (I can use to start my pesky well pump, or maybe a car, etc) while the manufacturers typically don't publish surge ratings for golf cart batteries, etc, and they discourage their use in this role. When the power is out in a whole neighborhood, flexibility can be important because it is hard to anticipate what the needs will be. 300-500 cycles would be enough for me, and if I can afford more of them to run in parallel it may mean each experiences lower peak loads for surges and lower overall depth of discharge when cycled. As a bonus, they'll fit in the compartment of my camping trailer and maybe see some use in the real world.

Right now I'm leaning toward these marine "deep cycle" batteries, but my case isn't a typical one.

Thanks again. Sorry for the long note.

Mark
 
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So pretty much a good chunk of the factors I mentioned. Good for you. 1 kWh/day and about the same again for storage is not a bit ask but you'll need more than a couple of 100 W panels.
Yes, more panels will be better, and not very pricey now.
I'm seeing how this works--I keep addressing up the bottlenecks in my "system" and soon I've come full cycle and talked myself into upgrading each bit in turn. :)
Starting pumps is problematic, so I hope the soft starter or small backup pump does the trick for you. I think you might need to look at 24 V or even 48 V system though if your surge power demand is high.
I'm still checking on the soft starter. The well pump rep wouldn't bless it, but suggested I call the manufacturer of the soft-start. I don't think I was talking to a real engineer.
The backup-backup plan for the well is a hand-operated pump fabricated from PVC. Fairly primitive--the pumped water goes into a bucket. I only need to lift it about 70 feet, and it wouldn't be fun (or pressurized), but if it comes to that we'd be happy to have it.
If it gets very cold and natural gas is still available and if solar production/stored capcity runs short, I might have to decide between running the well pump and running the furnace/fan. Thinking about that choice makes me loosen the pursestrings for an extra solar panel or two.
Not a cold climate where I am but we wanted energy security, and a few years back added an extra dwelling for my elderly mum so it became even more important. A multi-day outage which screwed up a family Christmas some years back was the proverbial final straw.
Yes, the family civilizes us. I suppose if I were single I'd be happy living in a small cinderblock house with plastic sheet over the windows . When needed, I'd wash it out with a hose into a floordrain, like a kennel. I like things better the way things are, with curtains and everything. At least most days I do....

Thanks again!
Mark
 
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Appears to be way under sized if you are planning on running it during an ice storm in, I assume, winter.

Solar panels produce only a fraction of the rated power during winter.

If you are depending on this emergency system for your water you may wish to test it, during December, before you need it. (Assuming you are in the Northern Hemisphere).
 
Appears to be way under sized if you are planning on running it during an ice storm in, I assume, winter.

Solar panels produce only a fraction of the rated power during winter.

If you are depending on this emergency system for your water you may wish to test it, during December, before you need it. (Assuming you are in the Northern Hemisphere).
I'm coming to believe you are probably right. Based on the input so far, I've evolved things a bit:

Revised project load scope (daily):
- Well pump: 20 minutes of submerged well pump use per day (115VAC, 1/2 HP. load: 900 watts when running BUT starting load of about 6000 watts)=300 Wh Note: this is conservative. I'll have enough water storage to allow skipping this for up to 2 days.
- Water transfer pump (12VDC, small RV-type diaphragm pump: 8A, 5 GPM, up to 55 PSI). To pump 200 gallons = 72 Wh
- Recharge tools, fan, flashlights, radios, telephones, etc: = 400 Wh

So, a projected load of about 800 Wh per day. If production permits, maybe run the freezer. Some of that won't need to go into battery storage (e.g I can recharge all the tools, flashlights, fan during the day). And, as noted, I can skip a couple of days of well pumping if needed.

Revised major system components:
- Panels:
2 4 x 100W panels, I plan to run them 2P2S for a nominal planned maximum of approx 40 V, 10 amps into the charge controller. So, in theory, up to 2000 wh per day of collection in perfect conditions. Note, these won't be in a fixed installation. This is emergency use, and I'll be moving them a few times a day to keep them out of shadows and closer to optimum alignment for collection.
- Storage: Right now I intend to use a single two12V marine "deep cycle" battery of 105 AH capacity each (= 630 1260 WH if discharged to 50%). It's not ideal, but it is inexpensive, available, sturdy (no BMS electronics to crump out), and if need be I can lug each battery to a car or to available grid current if it is available and recharge them.
Unresolved: Two 12V batteries in parallel, or run them in series for 24VDC? I'm leaning toward keeping them in parallel at 12VDC
-- Parallel 12VDC: Bad: Much bigger cables to the inverter (2 feet?). Requires a higher amperage charge controller. Good: Easier to find appliances, USB chargers, etc to run on 12V. Gives in-extremis option of running entire load off one battery for a very brief time.
--Series 24VDC: Good--Much smaller cables from batteries to inverter. Also, a lower amperage charge controller compared to 12V
- Charge controller: 30A MPPT (brand, required featerues undetermined).
- Inverter: It'll be a true sine wave model, but rigt now it is Unresolved: Options: Use a soft-start device for my well pump and an approx $300 2000W HF inverter (4000W surge) OR an approx 3000W LF inverter (like this Vevor model: presently unavailable, priced at $380 a few days ago) with a 6000W surge capability to cover the well pump startup surge. I need to learn if a soft-start device will work with my well pump and, if so, what the resultant surge current would be.

I'm at about $1000 now,and that's without cables, fuses, circuit breakers, etc. Ouch. But, I'll be a hero if the power goes out for a while!

Thanks for the input to date. Comments/better ideas/laughter welcome.

Mark
 
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