Would this off-grid system work?

Welcome to the forum Chrispy,

You can do a lot with off grid solar power... But it does cost a lot to run mechanical (i.e., water pumping, A/C and heat pump, etc.) and electrical heating... In the USA where grid power is something like $0.10 to $0.40 per kWH, off grid runs around $1.00 to $2.00+ kWH (can be less, can be more)... And the farther from the equator, the larger your arrays need to be, and the worse your winter/non-sunny day harvest.

I will use 3,300 WH per day (3.3 kWH per day or ~100 kWH per month). This is enough to power an off grid cabin (full size refrigerator, LED lighting, solar friendly water pump/well pump, clothes washer, LED TV, laptop computer, cell phone/tablet charger... With lots of conservation (I call it a "near normal" electrical existence).

I would suggest around a 1,200-1,500 Watt AC inverter for such a system--In reality, your peak power is limited by battery bank design/capacity in AH and working voltage (12/24/48 volts)... It is really easy to put a 3,000 Watt @ 12 volt AIMS inverter on some deep cycle flooded cell lead acid batteries (FLA)--But if you are looking for a "reliable" system over time and usage--It is not the best way to approach the problem.

So--I like to do a few paper designs first--And see if they meet your needs (paper/computer bytes are cheap). Once you have a good paper design, then you can select the hardware that will meet those needs.

For paper designs, scaling is easy... If you need 50% more kWH per day, simply multiply the results by 1.5x and check some simple rules of thumbs...

First, the "heart" of your system is the battery bank. You look at your power needs (conservation is almost always cheaper than "building a bigger system" for "inefficient loads"). For example, we start at 3,300 WH per day, 2 days of battery storage (kind of optimum from our experience for FLA batteries) and 50% maximum discharge for longer battery life. So, sizing the battery bank will look like:

• 3,300 WH per day * 1/0.85 AC inverter eff * 1/24 volt battery bank * 2 days storage * 1/0.50 max discharge = 647 AH @ 24 volt battery bank

I cheated here a bit because I knew the answer for the minimum battery bank voltage. In general, we have 12/24/48 volt to choose from. And I highly suggest that you keep the battery bank below ~800 AH for various reasons... Keeps the wiring AWG smaller and uses fewer solar charge controllers.

Next, we need to make two solar array calculations. One is based on Battery AH and voltage calculations (larger battery banks need more solar charging current) and the second based on your loads an hours of day of sun (your location, clear of trees and buildings, local weather, etc...---Any shading and clouds greatly reduce or even your kill solar harvest).

First suggest 5% to 13%-20% rate of charge... 5% minimum (weekend/sunny weather usage) and 10%+ for full time off grid use.

• 647 AH * 29 volts charging * 1/0.77 panel+controller * 0.05 rate of charge = 1,218 Watt array minimum
• 647 AH * 29 volts charging * 1/0.77 panel+controller * 0.10 rate of charge = 2,437 Watt array nominal
• 647 AH * 29 volts charging * 1/0.77 panel+controller * 0.13 rate of charge = 3,168 Watt array "typical cost effective maximum"

Then there is sizing based on your daily energy usage, location of system, and seasonal usage... Let's say you are around Stockholm Sweden, fixed array:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Stockholm
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 31° angle:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
0.93	1.96	3.42	4.56	5.19	5.03
Jul	Aug	Sep	Oct	Nov	Dec
4.97	4.47	3.60	2.16	1.26	0.71

And this is a pretty "dark" location (it appears to have "poor weather", farther north, etc.)... Nominally, I would suggest that you would not use much power in winter, and/or genset... So toss the bottom three months gives us 1.96 hours of average February sun:

• 3,300 WH per day * 1/0.52 off grid system eff * 1/1.96 hours of Feb sun = 3,238 Watt array February "break even"

So--For that region, just a typical (medium sized) cabin solar power system would require ~3,238 Watt array and a generator (for winter usage). During summer you have more sun and "excess energy"... But your base loads should be closer to 50% to 65% of your predicted loads (conserve during poor weather, run washing machine and water pumping during sunny weather).

Roughly, the suggested AC inverter would be 250 to 500 Watts per 100 AH of 24 volt FLA battery capacity.

• 250 Watt * 647 AH / 100 AH (@ 24 volts) = 1,618 Watt AC inverter "nominal suggested"
• 500 Watt * 647 AH / 100 AH (@ 24 volts) = 3,235 Watt AC inverter "maximum suggested"

This is all based on FLA battery type... A ~600 AH battery bank can be made out of 4x 6 volt @ ~200 AH batteries in series (for 24 volts) and 3x parallel strings for (3*200AH=) 600 AH battery bank... In the US, "golf cart" batteries are relatively cheap and rugged. And probably most people "murder" their first battery bank.

You asked about Li Ion (typically LiFePO4 for off grid power usage). It is a possibility, but you need to measure/estimate/tell us more power your energy needs. Using a Kill-a-Watt type meter to measure "real" loads and your planned usage (get a hotplate and cook a day's worth of meals on it):

https://www.amazon.co.uk/s?k=kill+a+watt+meter

Do all of this for your planned cabin appliances... Using a 2,000 Watt hot plate for 20 minutes is 2,000w*1/3 hour = 667 WH per meal

Of course, there are thermostats and variable heat hot plates... You should probably look at induction cookers instead (more efficient):

https://www.amazon.co.uk/s?k=induction+cooker

And very popular with Asians are Thermos type slow cookers (heat/boil your stew/soup/etc. and let sit for 4 hours in Thermos to "cook"):

https://www.amazon.co.uk/s?k=vacuum+thermos+cooker

This is where I remind you that "extreme" conservation (especially in less than optimum solar conditions) is usually a good idea. And using propane/alcohol stoves is cheaper long term.

Batteries--There are a lot of options out there... FLA, AGM, some other chemistries, Li Ion, etc. For example, I think you are looking at 12 volt @ 100 AH LiFePO4 batteries (lithium Iron Phosphate--Relatively stable and "safe" for Li Ion chemistry). For your battery bank, you would need 2x 12 volt in series (for 24 volt) and 3x parallel strings for (3*100AH=) 300 AH battery bank. Or a total of 2*3=6 batteries total.

LiFePO4 batteries are getting better in pricing, and their performance is much better than FLA... However, they are expensive and many people use a BMS (Battery Monitor System) to ensure that they don't damage the LiFePO4 batteries (over discharging, over charging, you pretty much "kill" a cell/battery). So you have BMS questions and figure out if the BMS will work with your solar charger, battery choices, and AC inverter...

Other issues include getting a larger/cheaper AC inverter... A  3,000 Watt AIMS per sine wave AC inverter is not my first recommendation--But the issue with large PSW inverters is they take just a lot energy when turned on:

https://www.aimscorp.net/3000-Watt-Pure-Sine-Inverter-Charger-24-Volt.html
For a "medium" to small AC power system, 47 Watts is a lot if you plan on running 24x7:

• 46.5 watts * 24 hours per day = 1,116 Watt*Hours per day (AC inverter "on" and no loads)

That is literally 1/3rd of my suggested energy usage for an off grid "near normal" electrical existence... Or you would need to make the solar array and battery bank 1,116 WH larger (1.33x) just to "waste" power for the inverter.

I will stop here for the moment--With a warning that you not purchase any solar power equipment until you have done a few back of the envelope (above) and paper system designs (picking equipment, confirming specifications, local suppliers, etc.).

Your thoughts/questions?

-Bill

Another thing to look at is 100 Watt Vmp~18 volt panels vs using 200+ Watt Vmp~30 volts (or higher) panels.

In general, 200+ Watt panels are 1/2 the price (or less) than of 100 Watt panels. Look at the price per Watt.

If you use Vmp~30 volt panels (which are less expensive), you have to use an MPPT type charge controller, which is more expensive.

Overall, it is usually less expensive to use an MPPT controller+the less expensive large format panels. Plus the cost of wiring from the array to the charge controller is less.

Downside is that >175 Watt panels, you need two people to move them safely (large/heavy). Also, shipping is more expensive for large panels (generally must ship by truck on pallets).

Shop around and figure out what panels/equipment is available to you at the best price. Ordering everything from China has its downsides too... Support and warranty repairs are more difficult. And it may take a month or more to receive your product. Any many of the low cost hardware is of questionable quality (and reviews are rare/not always real).

If you have solar installations in your area, you should check with the people around you about their experiences.

-Bill