A simple, inexpensive, yet powerful solar storage system

[NW-Bound]

As mentioned in another thread, I will describe a simple off-grid solar system that should be helpful for people who face frequent power outages, such as the current blackouts in California for powerline fire prevention. Such a system should be sufficiently powerful to run a refrigerator, several lights, and various electronic devices such as a TV and PC, laptops, etc... And it should have sufficient storage capacity to run the above overnight.

Here's a system that will give you 2,400W output max, and a usable storage capacity of 3.2kWh, with the major components costing $2K total.

How do you relate the Watt and kWh numbers above? My big 30 cu.ft. fridge draws only 280W when the compressor runs. A large modern TV draws less than 100W. A laptop draws perhaps 30W. Anything that you plug into a wall outlet is limited to 1500W max by definition. LED lights draw a few watts each.

That means you can run a microwave oven, or an induction portable cooktop, or a toaster oven (but only one at a time), in conjunction with your fridge and your lights and electronics. And you can sleep through the night without the noise of a generator.

I will describe the following items further, but here they are and their costs.

Solar Panels 327W x 6 ($108 x 6 = $648, shipping extra)
Solar charger / AC charger / inverter - 2400W ($618)
Lead-acid batteries 3.2kWh storage ($88 x 8 = $704)

When not having an outage, you will want to use the power from this system to reduce the utility bill. In a place like San Jose, it is expected to provide 3,400 kWh/year, which is worth $408 at 12c/kWh, or $816 at 24c/kWh. However, with this off-grid system, you have to manage to use all of that power to reap the benefits, because it is not grid-tied and you cannot sell the surplus.

 

[NW-Bound]

As you can see, the heart of the system is the Solar charger/AC charger/inverter. Traditionally, these can be 3 separate units.

First, I will talk about the solar charge controller.

The solar charge controller accepts the output of the solar panels to charge the batteries. You don't wire the panels directly to the batteries for 2 reasons.

First, the panel output voltage is not compatible with the battery voltage. Modern charge controllers are all of the MPPT (maximum power point tracking) type, which is a DC/DC converter that reduces the voltage of the panel to match that of the battery, while also boosting the current.

Secondly, the controller must terminate the charge when the battery is full, lest the battery gets ruined. Electrolyte would boil over in the case of lead-acid battery, or violent flames would erupt from a lithium battery.

 

[NW-Bound]

Next, the AC charger is what you use to charge the battery using electricity from the grid, or from a generator. You are likely to still need a standby generator in case you do not get enough sunlight, or your usage is more than the panels can provide.

Having a good bank of batteries is nice, so that you do not have to run a generator non-stop. Hopefully, if everything is sized right and you know how to limit consumption and there is enough sunlight, you would not need the generator that often.

 

[NW-Bound]

Finally, the inverter is what takes the battery juice and converts it to AC voltage for your household appliances.

Modern inverters are of the pure sine wave type. Accept nothing less. This new type does not cost a lot more than the "modified sine wave" (MSW) type of yesteryear. This old type can cause grief with some appliances or electronics, and there's no telling what appliances will work and what will overheat without trying them out. You do not want this hassle. Avoid the MSW inverters.

 

[NW-Bound]

What is exciting is that the above 3 electronic devices (Solar charger/AC charger/inverter) are now rolled into one unit, which you can get for a measly $618. Beautiful!

So, all you need now is wires, to connect the solar panels to this single unit. You also wire the battery bank to it. Then, you connect a beefy extension cord to it, and enjoy its 2400W output.

That's it.

Oh, I have to describe what to look out for, so that you will not let smoke out of this unit. Smoke has to be contained in all electronic devices. When smoke escapes, things stop working.

 

[NW-Bound]

Forgot to say that this system can run a small window AC. No problem! A small 5,000-BTU/hr window AC draws about 700W when the outside temperature is 120F, and less than that when it is not as hot. You simply cannot do this while also using the microwave, or another appliance, because the 2,400W limit would be exceeded.

In contrast with devices like microwave ovens or induction cooktops or toaster ovens, the induction motor such as what is inside an AC draws a large surge current when it starts, and it can be 3x to 4x the running current. You will need to leave headroom for that, and the 2,400W capacity is good enough to handle it.

 

[Sunset]

Could you state where you priced these.

I need a small system to run a sewage pump probably once per day for 1 min. The catch is my system has to be removable as it will be stolen if left in an isolated cabin.

I'm guessing 200 Watts of solar panels, and 2 batteries might do it ?

 

[Jerry1]

Could you state where you priced these.

I need a small system to run a sewage pump probably once per day for 1 min. The catch is my system has to be removable as it will be stolen if left in an isolated cabin.

I'm guessing 200 Watts of solar panels, and 2 batteries might do it ?

I’m no expert, but if it has to be portable, I would probably just use a small gas generator. They also have propane generators.

However, I’ve started seeing these:

https://www.amazon.com/Jackery-Port...efix=Portable+electric+battery,aps,164&sr=8-3

Basically a battery and an inverter. Not sure it would be enough to start your pump. But, if it were, you could get a single panel from a place like Harbor Freight to keep it charged (depending on usage and sun light of course). You could also just get an inverter and hook it up to your car.

What is the main reason you want to go solar? Quiet? No hauling fuel?

 

[Sunset]

I'd like solar as is quiet, and could charge up my cell phone easily (no electricity at cabin).
The portable part would be set up and leave for weeks, then take it down, and take the panels, controller, batteries, and inverter with me. Right now I move a large generator, which is harder/heavier as is all 1 piece.
I could leave the wiring in place.

The sewage pump can run on it's own via a floating switch with solar, otherwise I'll have to make a 9v battery powered alarm/light to tell me to start up the generator to run the pump, and this could happen once per day, to once every 2 days.

Can't use a car as the cabin is on an island.

Those Jackery devices are neat, but I'll need about 2,150 watts to start , and 1050 running for the pump. Even though it will only run about 1 minute.

 

[marko]

FWIW, the only thing I'd be careful of is the fridge.

The newer/better ones apparently have all sorts of sensitive electronics. We had just a very minor power swing a few years ago and it wiped out the entire system (on the hottest day of the year of course).

 

[folivier]

A couple points to consider with the above: first when designing the battery system you should plan for getting only 40% amphour capacity out of each battery. Any more than that and they won't last very long. Another point is that not all lithium battery chemistry is dangerous and can cause fires if overcharged. Lithium Iron Phosphate (LiFePO4) chemistry is very safe, actually safer than lead acid batteries and is the preferred type in RV's.
Another point is to use a pure sine wave inverter to eliminate problems with electronics.

 

[Senator]

I’m no expert, but if it has to be portable, I would probably just use a small gas generator.

My Honda 2200 can run 10+ hours on Eco mode on under a gallon of gas. I can hook up an extended run gas can that only needs to be filled every 2-3 days. And if I need more power, I can parallel 2+ together to get even more watts. I use that setup in my RV all the time.

Solar is great, too. I know that there are many homes "Off the grid" that use it, and they are very nice places. They are just too far off the power poles to justify a line to them.

 

[NW-Bound]

I'd like solar as is quiet, and could charge up my cell phone easily (no electricity at cabin).
The portable part would be set up and leave for weeks, then take it down, and take the panels, controller, batteries, and inverter with me. Right now I move a large generator, which is harder/heavier as is all 1 piece.
I could leave the wiring in place.

The sewage pump can run on it's own via a floating switch with solar, otherwise I'll have to make a 9v battery powered alarm/light to tell me to start up the generator to run the pump, and this could happen once per day, to once every 2 days.

Can't use a car as the cabin is on an island.

Those Jackery devices are neat, but I'll need about 2,150 watts to start , and 1050 running for the pump. Even though it will only run about 1 minute.

Supplying the 2kW surge is not too hard, and if the run time is so short, a single common 12V marine battery can support that.

Portability may be the problem. The 327W solar panel I am going to recommend is 61" x 41" and weighs 40 lbs. There are flexible panels that can be rolled up. They cost more, and produce less power for the same square footage (low efficiency). I do not have experience with them.

 

[NW-Bound]

A couple points to consider with the above: first when designing the battery system you should plan for getting only 40% amphour capacity out of each battery. Any more than that and they won't last very long. Another point is that not all lithium battery chemistry is dangerous and can cause fires if overcharged. Lithium Iron Phosphate (LiFePO4) chemistry is very safe, actually safer than lead acid batteries and is the preferred type in RV's.
Another point is to use a pure sine wave inverter to eliminate problems with electronics.

LFP is the battery chemistry that I used for my 22kWh storage battery.

However, most people will cringe at the cost, if they were to buy off-the-shelf. They typically run as high as $1K for 1 kWh for a 12V turnkey battery. Buying large-format single cells and assembling them into a large battery will bring the cost down to $500/kWh. This may still be too expensive for many.

All lithium batteries require some care and a BMS (battery management system) to avoid ruining them, and in the worst case causing them to burst into flames.

I think lead-acid battery is the safest, cheapest, and easiest to work with for people just learning the rope. People who know how to work with lithium battery would not need this thread. One can graduate to the LFP later. In another year or two, more will be imported and the price will go down further.

About pure-sine-wave inverters, I strongly recommended this earlier.

PS. My 3.2kWh number for the 8 lead-acid batteries is for 50% depth of discharge.

 

[NW-Bound]

My Honda 2200 can run 10+ hours on Eco mode on under a gallon of gas. I can hook up an extended run gas can that only needs to be filled every 2-3 days. And if I need more power, I can parallel 2+ together to get even more watts. I use that setup in my RV all the time.

Solar is great, too. I know that there are many homes "Off the grid" that use it, and they are very nice places. They are just too far off the power poles to justify a line to them.

For emergency use, it's hard to beat an inverter generator. It's simplest, most expedient and portable.

The off-grid solar system is for longer-term use. And for places of high electricity cost, it can pay for itself in a few years when used to offset the power draw from the grid.

I started my system with a 5.5kW solar array and 22kWh battery as a lark. My electricity rate is only about 1/2 of the rate Californians are paying, so I am doing this more for fun. I need a hobby when I am not traveling.

My system has been providing about 20kWh each day, and that's enough to run a 1.5-ton minisplit AC 24 hours. It helps offset my power bill. In the hottest day of the year when it hit 120F, my home uses 100kWh each day. Sounds like a lot, but that's only 4 kW x 24 hours, and that's what it takes to run a 5-ton central AC.

 

[NW-Bound]

OK, now let's talk about sourcing the components I described in post #1.

First, the simplest item: batteries.

Your local Costco will have the marine 12V batteries at $88/each for the 27DC format. The specs say reserve capacity of 160 min, which works out to 67 Ah. The nominal energy is then 800Wh. We will draw only 50% of that for longer battery life, hence the 3.2kWh for all 8 batteries.

Yep. Lithium batteries will beat the pant off lead-acid batteries for the size and weight, but see my caveat in the earlier post. Plus, you will pay at best $500/kWh, compared to $220/kWh. For the entire battery life cycle, the lithium battery will win out big, but I still think a starter may learn the rope better with just a couple of lead-acid batteries, then graduate to the expensive batteries later when he is convinced that the system works.

And by the way, you can also start out with a lower number of panels, and add more as you like it. I am looking for places to put up more panels myself.

 

[CaptTom]

...I think lead-acid battery is the safest, cheapest, and easiest to work with for people just learning the rope...

PS. My 3.2kWh number for the 8 lead-acid batteries is for 50% depth of discharge.

Can you post a link to the batteries you're using for this calculation?

A lot of boaters are now using 6-Volt golf cart batteries in series for their 12V house bank. Pairs of batteries can then be joined in parallel to increase the Ah capacity. This can then be converted to kWh (with the 50% load factor that you mention, less any inverter losses.)

I'm just wondering if this is the path you're recommending, or if you've got a better option for deep-cycle 12V batteries.

Note that so-called "marine" batteries are generally NOT deep-cycle, but a compromise somewhere between starting and deep-cycle. Golf cart batteries are designed for lots of charge-deep discharge cycles.

 

[NW-Bound]

Next is the solar panels. Where I bought mine: eBay. Shipping cost is high, but I was fortunate to be able to buy locally where I could pick them up in person.

The price of solar panels has dropped dramatically in the last few years, particularly as used panels appear on the market. The source of these panels is somewhat of a mystery. I searched the Web for info, and some said that they came from commercial solar farms that replaced old panels with newer panels of better efficiency.

Indeed, I can get used 250W panels for $50 a piece. A common size is 65" x 40", and the weight is 40 lbs. These are of the polychrystalline type.

However, most people are space-limited. A better choice would be the higher-efficiency monochrystalline type. Mono panels also have a better temperature coefficient, and their output drops less at high temperature. When a black panel is exposed to sunlight in 120F weather, the cells are at 150-160F temperature. Their output will be 75%, or worse, than what they put out when at room temperature. Poly panels will do worse.

A name brand I prefer is SunPower, an American company. A SunPower panel of 61" x 41" has the specs of 327W. The price is $108 on eBay for a used panel.

Now, you will ask "but are these used panels any good"? I guess that may depend on your local panel monger.

I bought some used ones along with more expensive brand-new ones in order to test them side-by-side. The used ones put out between 5% to 10% lower power, which is about right or perhaps a bit worse than the specs for their age. Half-price to 1/4 the price for 10% less power. You decide if it's a good deal.

 

[NW-Bound]

Can you post a link to the batteries you're using for this calculation?

A lot of boaters are now using 6-Volt golf cart batteries in series for their 12V house bank. Pairs of batteries can then be joined in parallel to increase the Ah capacity. This can then be converted to kWh (with the 50% load factor that you mention, less any inverter losses.)

I'm just wondering if this is the path you're recommending, or if you've got a better option for deep-cycle 12V batteries.

Note that so-called "marine" batteries are generally NOT deep-cycle, but a compromise somewhere between starting and deep-cycle. Golf cart batteries are designed for lots of charge-deep discharge cycles.

The specs for the battery came off Costco's Web site.

As an RV'er myself, I was disappointed with the use of golf-cart batteries with my 2kW inverter. They lasted one RV trip of 6 weeks, and their performance was lousy when they worked. The 12V marine batteries work MUCH better. Why? It goes against the common wisdom.

I searched the Web, and found one guy with the exact same experience I had. His theory was that golf-cart batteries will work better than marine batteries, but for lower current drawn. For my RV application, I used a 2kW inverter to run the microwave to warm up food. The 6V golf-cart batteries in series could not handle the 150A current drawn. Two 12V marine batteries in parallel can easily handle 75A each.

Now, if you have enough strings of 6V golf-cart batteries in parallel in order to reduce the current through each string, they may work better than marine batteries.

PS. The golf-cart batteries also could not handle the high current that my upgraded RV charger puts out. My RV is a short class C, and I do not have room for more than 2 batteries.

I now run 3 lithium batteries of 100Ah each in my motorhome

 

[NW-Bound]

Another reason I recommend 12V marine batteries is that you only need 2 of them to test out the system to start. The combo charger/inverter I am going to recommend runs off 24V, and two marine batteries will get you started.

With 6V golf-cart batteries you will need 4 to start, and as my experience indicated, their life would be dramatically shortened with a 2.4kW inverter. A pair of 12V marine batteries may last a bit longer, but you would really need to exercise restraint and test that 2400W power very briefly. That's more than 100A that the poor batteries have to supply.

Golf-cart battery rating is at 25A current. A string of 4 batteries to get 24V will reliably provide 600W continuous. You will need 4 strings (16 batteries total) to feed a 2400W inverter.

 

[ls99]

FWIW my off grid system at my camp has 6 GC2 golf cart batteries. They do take up a fair amount of space. Now that I changed to Pure sine wave inverter 3KW rated, the system handles handles circular saw, induction stove, microwave as major loads just fine. No electric fridge. Runa an old RV propane fridge when there more than a few days.

 

[Spock]

I think this is the concept you're describing.
He provides parts lists, wiring diagrams, etc.

I started futzing by buying his inverter, but the 1st two bought off Amazon had screws rattling around inside the cover (an antique test method for catastrophic short circuits) so I returned them. Haven't looked around for a replacement inverter yet.

 

[IBWino]

It's a bit more expensive, but also take a look at Powerwall. The units are about the size of a thick card table, cost approximately $6K-$7K and can provide several days of power.

-Wino

 

[ncbill]

Another reason I recommend 12V marine batteries is that you only need 2 of them to test out the system to start. The combo charger/inverter I am going to recommend runs off 24V, and two marine batteries will get you started.

With 6V golf-cart batteries you will need 4 to start, and as my experience indicated, their life would be dramatically shortened with a 2.4kW inverter. A pair of 12V marine batteries may last a bit longer, but you would really need to exercise restraint and test that 2400W power very briefly. That's more than 100A that the poor batteries have to supply.

Golf-cart battery rating is at 25A current. A string of 4 batteries to get 24V will reliably provide 600W continuous. You will need 4 strings (16 batteries total) to feed a 2400W inverter.

Costco Interstate-branded 6V GC2 batteries get pretty poor reviews.

OTOH, Sam's Club has Duracell-branded 6V GC2 batteries for $90 at my local club & those get good reviews.

For those not space-limited a battery bank using 6V golf cart batteries would usually result in higher capacity than paralleling 12V "marine" batteries (which aren't really designed for deep cycling)

 

[NW-Bound]

FWIW my off grid system at my camp has 6 GC2 golf cart batteries. They do take up a fair amount of space. Now that I changed to Pure sine wave inverter 3KW rated, the system handles handles circular saw, induction stove, microwave as major loads just fine. No electric fridge. Runa an old RV propane fridge when there more than a few days.

An appliance that plugs into a regular outlet is limited to 1500W. So, 6 batteries will handle roughly 250W each. That should be OK.