Clueless solar newbie- how much can a solar system support?

Hey all-
I'm a complete idiot when it comes to electrical (including solar) and I'd really appreciate some advice from you smart folks.
My husband and I will be beginning to build on our homestead this summer. In the woods where we are building, we are a ways from the power lines and we'd be looking at about $6k just to run the lines to connect. In addition, our power company is really expensive- roughly 270/mo for your typical house (wood heat and propane stove).
I like the idea of simply setting up a solar system instead, I feel like 6k could get us a good head start on a solar system instead of just paying to connect to the grid.
But how much can a solar system support? I know the answer is "it depends", but if, for example, you can power a 'typical' house with say 4 panels (I made that up, I don't know how many you need), could you just buy twice as many panels and batteries and then use an electric water heater instead of a gas one, say? Or triple the size of the system and heat your home with an electric tankless water heater running radiant in floor heat? (We're in northern MN- heat is a huge cost in our area)
If you can power more electric appliances by buying more panels and batteries, it seems like it'd be a worthwhile investment to build a bigger system off the bat and save in the long run.
My idea was to combine a bunch of solar panels with wind turbines and have a generator as backup.
But I'm sure I'm missing something, like I said, electricity is not my strong point.
Can anyone give some advice? Or point me toward some educational resources so I can wrap my head around what is feasible and what I need?
Thank you guys so much!

Start by doing an energy audit of your current needs in the same climate. Look at your power bill, and make note of the KwHs used. The average household in the US uses something like 20 or 30 kwh each day. A system to generate that much juice is going to easily run into the tens of thousands of dollars, even in a good site for solar.

A system that you put together for 6 grand would probably produce 5 or 6 kwh in the summer, and as little as .1 on a cloudy gray winter day. That is based on my experiences in Oregon, which is pretty rainy. I would suggest you would need to get your usage down to 1 or 2 kwh per day. You could likely run a small chest fridge, lighting, some electronics, some water pumping, and maybe a small washing machine. Heating and hot water would have to come from some other source. AC would also likely be a non-starter. If you add a generator that is used with any regularity, the price of fuel is going to make the grid connection look more appealing.  

The big expense is batteries. You need to have enough capacity to get you through several days of poor output. The best way to make it cheaper is to use less power.

If you come up with a baseline power consumption number, I am sure some people would be able to pencil out how much it would cost.

Hi Betsy, I will try to help out a bit.  But first some math:

Volts x Amps = Watts

I have a small, cheap solar panel that is rated for 28 watts and on a bright, sunny day will run at 20 volts.  Mathematically this looks like this:

Watts/Volts = Amps

OR

28 Watts/20 Volts = 1.4 Amps

OK, so now we have our normal peak performance specs, but the Wattage is what we will pay most attention to.

Just to make things easy, let’s say your panels are 100 watt panels.  You can easily find panels in the 300 watt range or greater but we will stick with 100 watts to make thing easy (Math=Not My Thing!)

From here you are going to need to find your power consumption.  Realistically you need to find your something closer to your peak power consumption.  Just to give you an idea of the power we are talking about, a single LED bulb is about 10 watts (I rounded).  10 of these will consume one 100 watt solar panel.  My microwave (an average one) takes just a smidge over 1500 watts!  That’s 15 solar panels!  Surely there are other considerations.  TV, Computer, Washing Machine, Dryer (even just the electrical part), vacuum, and the list goes on and on.  Basically you need to find that magical total power consumption and aim for a bit higher.

Without knowing much, my guess is that you need at least a 4000 watt system, or 40 100 watt solar panels.  We got a quote for a 7000 watt system for my house years ago but the cost was prohibitive.

Figure up the cost for that many solar panels and you have a start.

In addition, since you would be off grid, consider that you will also need the following (among other things)

Lots of batteries
Inverter to make 120 volt electricity
Charge controller to safely charge your batteries

I know I am throwing a lot of variables at you, but I can only make very crude guesses without knowing your power consumption.  Years ago (prices were higher then), I was quoted $12000 for a system with no batteries (called grid-connect) that would reduce my power consumption by a whopping 10%!  A similar $30,000 system would have replaced 50%!

I am not trying to dissuade you, anything but.  But without knowing the total electrical consumption I just can’t go much further.  I may be wrong, but I would be surprised if you could get an adequate system for less than the connection fee.  Of course that does not account for savings due to no power bill, but we have not talked about things like the length of day during winter or solar availability on dark, cloudy days.  Again, I just don’t have that information.

Betsy, I have thrown a LOT of information at you but at least you may have a starting point.

Have any more questions, fire away!

Eric

Carl posted while I was writing.  I agree with everything he said.

Eric

Thank you guys so much for the info!
Sounds like solar is going to be more costly than I had anticipated, though if I can keep my usage down I'd be willing to spend more upfront if I could prevent those huge power bills.
Fwiw, in my area I've seen bundles of solar panels called a 10.8kw system, 27 panels, for sale under 10k. If Im doing this right that would mean it's 27 400w panels? If it's something even remotely feasible I'd jump on board with that.
Since the house isn't build yet I'll need to do some research to see what our usage would be with various appliances; thank you guys so much! I really appreciate it!

A good question is do you want a whole-house system like would run all appliances, the same as your current use or do you want an off-grid situation when you scale down appliances to conserve energy?

Anne-
I had originally been hoping for a whole house system though I'm afraid that might have been a little bit of false hope lol. It would be the ideal, but I think I was underestimating how much power appliances use. We wouldn't have an A/C but things like washer dryer water heater fridge freezer etc...

Here is a post that might offer some info:

https://permies.com/t/168204/Solar-panel-beginner-set#1321810

This one might be helpful:

https://permies.com/t/68921/learning-solar

I'm off-grid solar and while I like it. It's very expensive to maintain. We have two houses so our needs are higher, but just for perspective, we replaced our battery bank last year and it cost $37,000. And we still have to use the generator in the winter and last year during extreme smoke days.

I don't know but to me $6000 to connect to the grid doesn't seem that much, and $270 a month in power bills is much lower than I paid in California. I would connect and get grid-tied solar, but that's just my 2 cents.

I can tell you what I put together that works as expected.  I can't tell you how much power you will use, but I can tell you how much I am using.  With a 18cuft refrigerator, lights, TV, computer, clothes washer, and whatnot, I consume about 3500Wh per day, call that 3.5kWh.  I'd guestimate that your needs would be similar, though that's for you to determine.

Here is a bit of math for you.  Use this as the scaffold for your own design.  That is if you find your number are higher, swap in your numbers.  If your numbers are lower, then swap in those.  The numbers may change, but the math stays the same.

Let's say you want to bump up the numbers slightly to 4000Wh per day.  Let's start with your location's sunhours.  Sunhours, sh, is NOT the number of hours the sun is shinning.  It's the multiplicative factor you use to determine how many solar panels you need.  Let's keep it simple.  Let's say you get 2.5sh in December, and 5.0sh in June.  You want to plan your system so that you get what you need on the worst day of the year, not the best.

4000Wh/2.5sh = 1600W of panels.  That's not too much.  I'm going to pick up some 260W REC panels next week at 65$ each, so that works out to be 390$ for about that power.  Here's pic of an array frame I built that can hold 6 of those.  Maybe you might not get the deal I get.  Plan on paying 100$ per panel in the 250-300W range.  Look for them on Craigslist.

You want a battery that can supply you with at least 4000Wh, but you don't want to drain lead batteries more then 50% to maximize life, and you want to include at least 1 extra day of power if a storm blows through.  Let's say you go with a 24V battery based system. The math would be...

(4000Wh X 2days X 2fold battery size)/24V = 667Ah battery.  Umm, that's pretty big.  How about 48V instead?

(4000Wh X 2days X 2fold battery size)/48V = 333Ah battery.  That's just about the size of a 6V Trojan L-16 RE-B battery (it's 377Ah).  So, you need to wire eight of those in series to get 48V.  At 350$ each, that works out to be 2800$.

You need a MPPT charge controller to take the raw solar voltage and transform it down to battery charging voltage.  Assume your 16000W of panels gets transformed down to 50V charging.  1600W/50V = 32A, so you need a controller that can handle more than 32A.  Take a look at Epever's 5415AN.  On Ebay it's about 280$ right now.  It has a max amperage rating of 50, and a max voltage of 150.  If you bought 6 panels, you could wire two parallel strings of three panels wired in series.  (write that as 3S2P).  Each parallel string of panels would be putting out ~8.8A at 90V, so two in parallel would be 17.8A at 90V.  The controller takes that power in and transforms it down to ~32A at 50V.

Finally, you need an inverter to convert the battery DC into household AC.  Here is an inverter I have for my workshop.  it is sine-wave, so it can power things with an electric motor. Schneider's Conext 4048, at ~1500$ right now.  BTW, it's split-phase, so it puts out both  120 and 240VAC.  It also has a built-in generator charging circuit.

Avoid anything advertized as modified, or square-wave.  It will kill a motor.  They are cheap for a reason.

So, here's what will get you start
1600W of panels, 400-600$
377Ah Trojan battery at 48V, 2800$
50A MPPT charge controller, 280$
4000W sine-wave inverter, 1500$
Let's through in 1000$ extra for wire, breakers, electrical panels, and framing.

Total 6180$

One last thing.  If you put your panels on a rotating array like what I pictured, expect your sunhours to double if you rotate your array east to west over the course of the day.  So, with rotation you can expect 8000Wh in winter, and 16,000Wh in summer.

I can expand a bit on my first post.  That is more of a lower end system, that will meet your basic needs.  At my own cabin, I have an even bigger system, that handles far more then what I just outlined.  In addition to the day to day loads I mentioned above, in the summer, I use my solar system to pump water to irrigate my orchard, which is about 120 trees right now.  I need a system that can run my 1hp 240VAC Grunfos pump, and I need to put out 2000+W from 8am to 4pm on any given day.  In summer I make between 20-25 kWh of power when I'm running the pump.

It all works, but it has 4500W of panels instead of 1600W, a XW+6848 inverter instead of the 4048, and Rolls 568Ah batteries instead of the Trojans.  So, besides the 3500Wh base load, my large system can run the well pump, and also a 8000BTU air-con upstairs in the bedroom.

A few terms to help with electrical planning.  Amps X volts = Watts.  So, an automotive headlamp with 10A of consumption uses 10A X 12V = 120W.  A kitchen toaster that uses 9A consumes 9A X 120V = 1080W.

In series, volts add while amps stay the same.  In parallel, amps add while volts stay the same.

The older charge controller technology is called "Pulse Wave Modulation", or PWM.  It is basically an on/off switch that shuts off panel current once the battery is full.  You must match the voltage of the panels closely to the battery voltage or the battery gets damaged.  Today they are relagated to low-end budget 12V systems of just 100-300W.  Newer "Maximal Power Point Tracking" or MPPT controllers act as a transformer, converting raw high solar voltage to exactly the voltage the battery wants.  This allows you to run high voltage strings from panels hundreds of feet away with only thin copper wire, because voltage drop is the biggest enemy of 12V systems.

I apply this myself by placing my arrays in the sunniest spots as much as 130' away from the controller, and run 120VDC to the MPPT controller.  It takes that raw 120VDC and transforms it down to the 50-55VDC the battery needs to charge.  Compare this to a simple 12V system with a PWM controller.  You can't use panels with a voltage >18VDC, and can only be spaced a few feet away from the controller because of voltage drop.

The last thing I can mention at this point is shop locally for your solar panels, instead of online.  Shipping is the killer for solar panels.  Shop online and you'll pay 1-2$/watt for solar.  Locally, you can get 3-4W/$.  Don't worry about buying used panels.  The MEASURED production of quality used panels I bought is higher than the retail Renogy panels I bought new.

Don't buy a cheap inverter off Amazon or Ebay.  Buy your electronics from a reputable solar company.  I bought from https://ressupply.com/.  I would stick with brand names like Midnight, MorningStar, Outback, Schneider, and Victron.  Epever though is a mid-quality brand for charge controllers that you can buy on Ebay.  Beware though that there are Chinese fakes out there.  Some other solar beginner here just bought a fake controller.  Money totally wasted.  If the shipping weight of some electronic product is 8oz, it's a low-budget fake.  If it weights 8lbs, you are getting a real unit.

Michael,
Wow! Thank you so much for all the great info! Since I don't have any appliances yet to judge their power usage I've just been plugging them into solar estimate websites to try and get an idea of how much power different appliances eat. Looking at what you've been accomplishing, I feel pretty confident that a 10.8kw system should be able to keep up with a small household pretty well.
Do you run solar only?
Thank you again!

Lots of good info already. I would add to check your most recent electricity bill to get an idea of your usage.

I'm familiar with using solar panels in MN so I'll add it's a very tall order to rely only on solar there all year long. Especially winter. It's doable but plan very carefully. Please give yourself a good margin of error because winter is brutal there.

Here's my small-scale experience as an example of what you can run off solar. I have 520 watts of panels, an MPPT charger, and two Trojan t-105s (220Ah @12V).  My records show about $800 into it, though panels and batteries were used. This runs my freezer and charges my drill batteries. That's it. It'll last about a day an a half without sun before I get down to the 50% mark. And I'm in Colorado where we get a lot of sun even in the winter. I think I'm long on solar so I believe I could get more out of the system just by adding more batteries. I could also get more out of the system if I had batteries that could charge faster. Lead-acid batteries like a slow charge meaning even if I have lots of solar capacity all at once I can't utilize it all to charge the batteries. I believe Lithium can charge faster. They're more expensive but supposedly have a much longer useful life.

I've rarely gotten more than half the rated capacity out of my panels. Plus they're not at the optimum orientation, plus trees limit the time they get sun.

And of course, never ever plan to use solar to produce heat.

Your mileage will vary.

Daniel Kaplan wrote: I think I'm long on solar so I believe I could get more out of the system just by adding more batteries. I could also get more out of the system if I had batteries that could charge faster. Lead-acid batteries like a slow charge meaning even if I have lots of solar capacity all at once I can't utilize it all to charge the batteries. I believe Lithium can charge faster. They're more expensive but supposedly have a much longer useful life.

I've rarely gotten more than half the rated capacity out of my panels. Plus they're not at the optimum orientation, plus trees limit the time they get sun.
.

Hi Daniel

I started out with Trojan T-105s for my first system, so I'm very familar with them.  I got 5 years out of mine, and they weren't used up yet, so I gave them to a neighbor.  Here is the math I use for determining how much solar to use for a particular battery.  My Trojans were 250Ah.  Trojan recommends charging at 1/8th of C (0.125).

(250Ah X 0.125C) =31.25A  Assuming charging starts at 13V that would be 31.25A X 13Vcharging = 406W.  I always include a fudgefactor because I agree, you never see full output.  I use 85%.  So, 406W/85% = 478W.

So, it looks like what you already have in place should be about right?  I'm guessing the angle of your panels is nowhere close to optimum?  Are they laying flat on a roof?

A classic mistake that gets made with solar is increasing battery size, without increasing the solar, so I would discourage you from just adding more batteries.  What I would do however, is to increase both the batteries and the solar at the same time.  Here is what I did for my own system.  I wired four 245W panels in series to get 120VDC.  I then ran that about 130' from a very sunny spot over to my controller.  If you have shade problems too, then that might be the perfect solution for you.  If you make a rotating array like mine, which can also be adjusted for winter vs summer angle, then you can always have perfect alignment.

So, double your battery size, double your 520W to 1040W, then wire the four panels in series over to the sunniest spot you have.  What charge controller do you have now?  What is it's maximum Voc?  Most economy controllers max out at 100V, so you would need a 200V controller to do this.  Look at Epever's 8420AN Tracer.  I myself am using a Midnight200 for my systems.

About the batteries?  How old are they?  As a general rule of thumb, it's not a good idea to pair new batteries with old ones more then 1 year old.  The new batteries always quickly degrade to the level of the old ones.  Secondly, you are in Colorado.  The weak link for Lithium batteries is cold weather.  They can be destroyed if they receive charging current just one single time if they are cold, around freezing.  So, Li must be keep above freezing at all times, otherwise they get trashed.

As this is new *construction*, I'll offer up our tried and true method of going off-grid ... it closely matches where I think you are:

1. get propane on the site (500-gal tank)
2. get a duromax 12kw propane open-frame generator ($1100 or thereabouts)
 - you'll most likely need this for construction tasks
 - all the power you'd ever need, 120v/240v

If you stop here, you've got all the power you need for construction tasks or anything electric. Anytime you need power, flip on the generator; downside is it runs all the time you need any bit of power, but that's ok, you are in "construction" mode anyway.

3. get a 24v inverter/charger (magnum ms-4024-pae) ($2000 or thereabouts)
 - 120v/240v, all the power you'd ever need, feeding the house and everything else
4. get two ampere time 300ah LiFePO4 batteries ($1000/ea or thereabouts)
 - at many 1000's of cycles, and no maintenance, these are what you want

If you stop here, you are fully off-grid, and generating your own power. Generator fills the batteries only when needed (for us, about 3 - 4 hours per day); extra power always there when you need it (flip on the genny). No more crazy electric bills; we do have propane bills, but they are sane, compared to electric bills these days with fees and surcharges galore.

5. add solar panels, mppt

At this point, your genny only runs when solar isn't charging your batteries (weather, etc.), or you decide to use a bunch of electricity. Either way, no calculating of energy budgets ... everything fits into solar (your choices on how much you use), or the genny makes up the excess. Nothing you can't run ...

You are off-grid! At step 4, you should still be under the $6k figure for bringing electricity in from grid, and there will probably be other charges to complete the grid connection. Do step 5 & beyond at your leisure ...

Our estimated grid connection was $30k ... laughed once, and went the above route. We've been off-grid approaching 10 years ...

Hope this helps ...

Hi Michael,
My charge controller is an EPEver 40amp Mppt. It maxes out at 100v input. Problem is my panels are 37 v open circuit. I'd love to put a third one in series otherwise. And, yes, I know about not adding new batteries to old. I didn't know the one year part but mine were used when I got them, so rather more than a year old. Eventually I'd love to get LiFePO4 batts but I figured the longer I wait to get them the more reliable, higher capacity, and cheaper they'd be. Maybe when these batteries are worn out. Or maybe some day when I'm feeling rich.

Edited to add. My system is mostly designed as a backup to keep the freezer going if the power goes out. So more batteries without accompanying solar might make more sense. Of course, I've love to have more solar, too.