Newbie needs help

[Upnorthcabin]

Hi all, I think our cabin will use about 5-7 kwh/per day. We need to run a small fridge 24 hrs (the energy star sticker says 378kwh/per year), a tv maybe 3 hrs/per day, LED ceiling lights occasionally, and a 12v water pump (took it out of our camper). We may run a fan some and charge cell phones and run small kitchen appliances such as a can opener. What size solar system would we need to power that amount? We are in lower northern Michigan in the woods. Thank you

 

[MichaelK]

Plan for a system that can fully supply your needs on the shortest day of the year. Let's assume you need 7kWh on 12/21, and you get 3 sun hours on that day. You need 7000W/3 sun hours =2333W of solar panels, let's call it 2400W. How many days of autonomy do you want? That is how many days can you go with no sun, 2, 3? Let's say you chose two days with no sun. Your battery needs to be (7000W per day X 2 days)/ system voltage.

With either a 24 or 48V battery bank those numbers would be 14000Wh/24V= 583Ah or 14000Wh/48V=292Ah. Assume you can't drain the battery bank all the way, maybe only 50% for lead-acid and maybe 70% for Li.

That works out to 583Ah/.5X=1166Ah for lead at 24V, and 583Ah/.7X=832Ah for Li. At 48V those numbers work out to be 292Ah/.5X = 584Ah for lead, and 292Ah/.7X= 417Ah for Li.

Looks like a 48V system might be a good choice for you, but with a 584Ah battery you'd need 584Ah X 0.125C X 52V charging X 1.25 loss factor=4750W of panels.

All these numbers are argumentable, but that's a starting point. Call it somewhere between a 24V system with 2400W of panels, and a 48V system with 4800W of panels, depending on how much excess capacity you want, and how often you're willing to run a generator on rainy days.

There's the math. What you might want to decide is whether the 7000W number is realistic, or could you get by with half that? The frig is likely to run less in winter when your place might be cooler. It will run more in the heat of summer, but your solar resources will be higher then. If you decide that your winter needs are only 3500W, then cut all the numbers above in half.

 

[Johncfii]

As MichaelK indicates, it is a complex question, with a lot to consider.

As another consideration, unless you want to be sure you “overkill it” in the beginning, think about designing and installing your system to allow for future expansion with as little fuss as possible . . . Space for more batteries, more solar panels, another charge controller, with extra unused wiring terminals, etc.

 

[DThames]

As MichaelK indicates, it is a complex question, with a lot to consider.

As another consideration, unless you want to be sure you “overkill it” in the beginning, think about designing and installing your system to allow for future expansion with as little fuss as possible . . . Space for more batteries, more solar panels, another charge controller, with extra unused wiring terminals, etc.

Yes, indeed. Putting in conduit that will allow extra wires, or putting the extra wires in from the beginning is wise. I have a 1" conduit going to my emergency power solar array. It already has 3 pair of 10ga wire in it and I can add another pair if needed. Any shade on the panels will kill your power production calculations. Find a place for the panels without any shade if you want your production calculations to be even close.

Consider how many days batteries must carry you in the event of cloudy days. If you have a generator, your batteries can be a lot smaller. If you can save larger power demands (if you have any...crock pot example) for daylight hours, having enough panels to charge batteries while you are cooking with the crock pot would be something to consider.

 

[MichaelK]

Both John and DT make good points about expansion. I myself am in the "overboard" category, having repeatedly expanded my systems over the years with bigger batteries, more solar arrays, more electrical demand. The neighbors now joke that I am the local utility, and I should sell power to them.

One important point is wiring. Yes copper wire is expensive, and you might be tempted to go with the lightest gauge that will get the job done. I went with overkill, and selected the largest wire (lowest gauge) that would fit in the terminals. As my system has grown over the years, I haven't had to rip out smaller wiring, because I started out large to begin with.

The thing I can tell you is do the wiring once, and be done with it. And, make sure you color code each and every connection so you can see plus vs minus. For me, it was a nightmare helping a neighbor troubleshoot his dead system when it was a mass of mismatched spaghetti of whatever color wire he had at the moment. If you get a good deal on black 4 gauge wire, thats fine. Just wrap some red electrical tape on the termini of that wire so some dumb-ass like me can troubleshoot it. I've even gone to the extent of spray painting my 0/4 wire red so I could connect the positive battery terminal.

What I might do at this point is shift gears and present to you a modest, starter system that could eventually grow by swapping out components. I'll make it 24V, because that's were all the serious electronics start.

4-225Ah 6V golf-cart style batteries 130$ each
1 40Amp Epever MPPT charge controller
4 240W-250W grid-tie Craigslist solar panels 60$ each (wire them in a 2S2P configuration)
24V Samlex sine-wave inverter 520$ (smallest inverter that Samlex makes that can be hard-wired into your main electrical panel) You want a fairly large inverter to start induction loads like your refrigerator.

This comes out to be <1400$. Add a hundred or so for wiring, fuses, breakers, ect and call it 1500$. Add a 500$ HomeDepot generator for 2000$

This should make you 3.0kWh/day of power in December, assuming you have clear weather. As your needs grow, replace with larger batteries, a second or larger charge controller, more panels, a nicer 120/240V split-phase sine-wave inverter.

BTW, I built a single-pole array frame that holds 1000W of panels. I can rotate this left and right to track the daily sun, so in February, I measured 6.1kWh of production. A rotating frame might help you eke out a bit more production with a smaller array.

 

[Upnorthcabin]

Yes thank you for all the info. We would like to start and then likely add to it as we need. I appreciate the set up you listed it sounds good. Thank you for the pic of the rotating stand. We were talking about having something moveable like that to track with he sun.

 

[Upnorthcabin]

Yes, indeed. Putting in conduit that will allow extra wires, or putting the extra wires in from the beginning is wise. I have a 1" conduit going to my emergency power solar array. It already has 3 pair of 10ga wire in it and I can add another pair if needed. Any shade on the panels will kill your power production calculations. Find a place for the panels without any shade if you want your production calculations to be even close.

Consider how many days batteries must carry you in the event of cloudy days. If you have a generator, your batteries can be a lot smaller. If you can save larger power demands (if you have any...crock pot example) for daylight hours, having enough panels to charge batteries while you are cooking with the crock pot would be something to consider.

Thank you for the response. We are going to clear a spot near the cabin to get more light but there will likely be times when there is some shade and cloudy days.we do have a generator. I have not tried to use the crock pot up north cause the generator would have to run all day.

 

[Upnorthcabin]

Plan for a system that can fully supply your needs on the shortest day of the year. Let's assume you need 7kWh on 12/21, and you get 3 sun hours on that day. You need 7000W/3 sun hours =2333W of solar panels, let's call it 2400W. How many days of autonomy do you want? That is how many days can you go with no sun, 2, 3? Let's say you chose two days with no sun. Your battery needs to be (7000W per day X 2 days)/ system voltage.

With either a 24 or 48V battery bank those numbers would be 14000Wh/24V= 583Ah or 14000Wh/48V=292Ah. Assume you can't drain the battery bank all the way, maybe only 50% for lead-acid and maybe 70% for Li.

That works out to 583Ah/.5X=1166Ah for lead at 24V, and 583Ah/.7X=832Ah for Li. At 48V those numbers work out to be 292Ah/.5X = 584Ah for lead, and 292Ah/.7X= 417Ah for Li.

Looks like a 48V system might be a good choice for you, but with a 584Ah battery you'd need 584Ah X 0.125C X 52V charging X 1.25 loss factor=4750W of panels.

All these numbers are argumentable, but that's a starting point. Call it somewhere between a 24V system with 2400W of panels, and a 48V system with 4800W of panels, depending on how much excess capacity you want, and how often you're willing to run a generator on rainy days.

There's the math. What you might want to decide is whether the 7000W number is realistic, or could you get by with half that? The frig is likely to run less in winter when your place might be cooler. It will run more in the heat of summer, but your solar resources will be higher then. If you decide that your winter needs are only 3500W, then cut all the numbers above in half.

Thank you for all the great information. I think we can get by with about half of what I figured. We might only need 3kwhs. I bought Will's book and have been reading it like a Bible. Lol

 

[Japera]

Plan for a system that can fully supply your needs on the shortest day of the year. Let's assume you need 7kWh on 12/21, and you get 3 sun hours on that day. You need 7000W/3 sun hours =2333W of solar panels, let's call it 2400W. How many days of autonomy do you want? That is how many days can you go with no sun, 2, 3? Let's say you chose two days with no sun. Your battery needs to be (7000W per day X 2 days)/ system voltage.

With either a 24 or 48V battery bank those numbers would be 14000Wh/24V= 583Ah or 14000Wh/48V=292Ah. Assume you can't drain the battery bank all the way, maybe only 50% for lead-acid and maybe 70% for Li.

That works out to 583Ah/.5X=1166Ah for lead at 24V, and 583Ah/.7X=832Ah for Li. At 48V those numbers work out to be 292Ah/.5X = 584Ah for lead, and 292Ah/.7X= 417Ah for Li.

Looks like a 48V system might be a good choice for you, but with a 584Ah battery you'd need 584Ah X 0.125C X 52V charging X 1.25 loss factor=4750W of panels.

All these numbers are argumentable, but that's a starting point. Call it somewhere between a 24V system with 2400W of panels, and a 48V system with 4800W of panels, depending on how much excess capacity you want, and how often you're willing to run a generator on rainy days.

There's the math. What you might want to decide is whether the 7000W number is realistic, or could you get by with half that? The frig is likely to run less in winter when your place might be cooler. It will run more in the heat of summer, but your solar resources will be higher then. If you decide that your winter needs are only 3500W, then cut all the numbers above in half.

Let's hold all things constants in this brilliant explanation you've given. Let's say you want to just charge your lifepo4 batteries via a generator in your days without sun. How would one go about determining what size generator and other equipment needed to charge the batteries?

 

[MichaelK]

Another formula, well, OK. You multiply the Ah capacity of your battery X selected charging rate X charging voltage X loss conversion X 2X-3X capacity factor.

Different styles of batteries want charging at different rates. For lead-acid it's 1/8 (0.125C), for AGM it's 1/5 (0.2C) and for Li it might be 1/4 to 1/2 (0.25C to 0.5C). The battery bank charges at a higher voltage than the nameplate. That is, a 12V bank bulk charges at ~13V, a 24V bank at 26V, and a 48V bank at ~52V.

The bulk rate is the charge the battery gets when it's depleted less the 80%, and as many amps as can go in are pumped. As the battery approached full charge though, the voltage continues to go up, by amperage goes down. So, you need the most amps (watts) at the beginning stages of charging, when the voltage is relatively low. This is largely true for lead batteries, and I think it is also true for Li, but I will defer to the opinion of the Li guys here.

So, let's say you have a 24V, 200Ah Li battery, and you decide you want to charge it at 1/4C. The math works out to be....
200Ah X 26Vcharging X 0.25C X 1.25 loss factor X 2X generator capacity = 3250W generator. With a lighter load on the generator that would be...
200Ah X 26Vcharging X 0.25C X 1.25 loss factor X 3X generator capacity = 4875W generator

So, call it a 3500W to 5000W generator. Something you can pick up at HomeDepot.

One important thing to think about. Depending on the electronics used to charge the batteries, those electronics might be very fussy about the quality of power they are receiving. With my own XW+ inverter, I can charge the batteries with the generator feeding amps into the ACin2 of the inverter, and then the inverter feeds the batteries. But, my XW+ doesn't like my cheap generator, and won't accept more than about 35amps from it. Higher than that and the inverter drops the generator power as being too dirty.

So, depending on your system, you are more likely going to be successful with the bigger generator than the smaller one, all things considered. All the better quality inverter brands, like Magnum, Outback, and Schneider all have built-in generator charging capacity. With a cheaper quality inverter you might need a golf-cart charger you can plug into the generator.

 

[Japera]

What about getting a larger array and battery bank? I want to start of with 24v 280Ah. If I increase to 24v 1120Ah in the future,
1. what wire size would work from the battery to the inverter assuming a 8kw-12kw surge?
2. What if I cap my usage to just 4kw out the same 8kw-12kw surge?

 

[MichaelK]

I think at this point, you should get comfortable with doing the math yourself. Just take all those example formulas above and start plugging in the numbers yourself. Here's a chart that documents what amperage a certain gauge of wire can safely handle. Go ahead and do the math and tell ME what you think you should be using?

Here's an important point. The numbers in the chart are for steady, continuous current, flowing for minutes to hours. The surge current can safely be ignored. In fact, engineers have demonstrated that regular wires can handle even thousands of amps if it's only for a tiny fraction of a second. That applies to breakers too.

So, for a 4000W inverter, that's pulling 4000W of power out of a 24V battery, how many amps would that be? The formula would be A x V =W. Once you determine what that number is, you can look at the chart and select the proper gauge. The rule of thumb is if your value falls half-way between two gauges, then you select the lower (thicker) gauge.

 

[Japera]

I can calculate very well. However, I don't just do what people tell me without solid information. Thanks for your time. In the meantime here are some things to ponder if yuo'd like to indulge.

https://www.wiringproducts.com/battery-cable and

this https://invertersrus.com/product-category/power-inverter-accessories/power-inverter-cables/ which I believe is utter foolishness.

 

[Upnorthcabin]

As MichaelK indicates, it is a complex question, with a lot to consider.

As another consideration, unless you want to be sure you “overkill it” in the beginning, think about designing and installing your system to allow for future expansion with as little fuss as possible . . . Space for more batteries, more solar panels, another charge controller, with extra unused wiring terminals, etc.

Thank you for the response. I like the idea of planning to add more. If I want to start with 400 Watt solar array but in the future add 400 more watts would I need an 80amp charge controller? If yes which would be recommended? Also, would it be eaiser or better to run 2 40amp CC or find one that works with over paneling?
Thank You

 

[Johncfii]

Thank you for the response. I like the idea of planning to add more. If I want to start with 400 Watt solar array but in the future add 400 more watts would I need an 80amp charge controller? If yes which would be recommended? Also, would it be eaiser or better to run 2 40amp CC or find one that works with over paneling?
Thank You

It will you a lot in this project to really understand the “ohm’s law” formulas. They are pretty simple, and there are a ton of good resources on line to help you get comfortable with them. There are only two basic formulas to apply.
The first is: E = I x R ........ which says that Voltage = Current (in amps) x Resistance (in ohms)
The second is: P = E x I ...... which says that Power (in watts) = voltage x current (in amps)
These two formulas can be re-arranged in several useful ways.

To get the “rough cut” answer to your question about the size of charge controller you need, just use the second formula to solve for amps.
Re-arrange the formula to be. Amps = Power divided by voltage.
400 watts divided by 24 volts equals 16.7 amps.
800 watts divided by 24 volts equals 33.3 amps.
So, generally, a 40 amp rated charge controller should be big enough to handle the power output of 800 watts of panels.
But then you look at the fine points to decide if a particular CC fits your needs.

To answer the second part of your question, what would be better, 2 x 40 amp charge controllers, or a single 80 amp CC?
It is generally going to be significantly less cost to buy the single larger charge controller, compared to two smaller controllers. One advantage that might lead some people to install a pair of 40 amp charge controllers is that, if you are trying to plan for a WTSHTF possibility, you have some redundancy in case one fails. At least you then would have partial power ( disregarding that a single 40 amp controller would be large enough in your example).

Specific charge controllers are thoroughly discussed in many other threads on this forum. The search function of this forum is also your friend that you should use liberally.

 

[Hedges]

PV panels are so cheap (used) that I would go straight to 800W. That might cost $200, more or less.

Panels are cheaper than rotating mounts. Instead of tilting from morning sun to afternoon, aim half the array (400W) at morning and half at afternoon. That 800W array will now present capture about 560W (if 90 degree angle between them, 0.7x the footprint), but will produce for more hours. Cheaper, simpler than a tracker, and more reliable. "Overpaneling" without clipping.

Batteries have a maximum charge current. In the case of FLA, it has an optimum charge current.
Overpaneling and sizing SCC to battery is one way to hit that current. But, when inverter draws power, charge rate is less.
Some SCC (Victron) have accessories to make use of a battery shunt and regulate battery current while providing extra to loads when needed.
Some hybrids do that internally.

 

[Upnorthcabin]

Both John and DT make good points about expansion. I myself am in the "overboard" category, having repeatedly expanded my systems over the years with bigger batteries, more solar arrays, more electrical demand. The neighbors now joke that I am the local utility, and I should sell power to them.

One important point is wiring. Yes copper wire is expensive, and you might be tempted to go with the lightest gauge that will get the job done. I went with overkill, and selected the largest wire (lowest gauge) that would fit in the terminals. As my system has grown over the years, I haven't had to rip out smaller wiring, because I started out large to begin with.

The thing I can tell you is do the wiring once, and be done with it. And, make sure you color code each and every connection so you can see plus vs minus. For me, it was a nightmare helping a neighbor troubleshoot his dead system when it was a mass of mismatched spaghetti of whatever color wire he had at the moment. If you get a good deal on black 4 gauge wire, thats fine. Just wrap some red electrical tape on the termini of that wire so some dumb-ass like me can troubleshoot it. I've even gone to the extent of spray painting my 0/4 wire red so I could connect the positive battery terminal.

What I might do at this point is shift gears and present to you a modest, starter system that could eventually grow by swapping out components. I'll make it 24V, because that's were all the serious electronics start.

4-225Ah 6V golf-cart style batteries 130$ each
1 40Amp Epever MPPT charge controller
4 240W-250W grid-tie Craigslist solar panels 60$ each (wire them in a 2S2P configuration)
24V Samlex sine-wave inverter 520$ (smallest inverter that Samlex makes that can be hard-wired into your main electrical panel) You want a fairly large inverter to start induction loads like your refrigerator.

This comes out to be <1400$. Add a hundred or so for wiring, fuses, breakers, ect and call it 1500$. Add a 500$ HomeDepot generator for 2000$

This should make you 3.0kWh/day of power in December, assuming you have clear weather. As your needs grow, replace with larger batteries, a second or larger charge controller, more panels, a nicer 120/240V split-phase sine-wave inverter.

BTW, I built a single-pole array frame that holds 1000W of panels. I can rotate this left and right to track the daily sun, so in February, I measured 6.1kWh of production. A rotating frame might help you eke out a bit more production with a smaller array.

Do you have the plans for your ground mount it looks like what we would need. Thanks,

 

[MichaelK]

Do you have the plans for your ground mount it looks like what we would need. Thanks,

Not really. I just measure whatever panels I want to mount, and scale the frame to fit them. Just look at my pics. The construction is actually quite simple.