New to Solar and New Setup

KPowers · August 2023

I'm installing solar in a remote Alaskan cabin. I will only be using the cabin, May - September. I have 3-330 panels and 4 - 6V 415 AH agm batteries. I plan to run a few lights, a frig and maybe a small freezer. Will this setup meet that need or do I need to plan to use my generator?

706jim · August 2023

I think you will have an adequate system for a fridge but not a freezer too. My 24' fridge/freezer averages 1.9kw-hr per day.

BB. · August 2023

Can you give us a (rough) location (nearest town or Lat/Long) for the cabin--Can calculator the "hours of sun" per day your site will receive.

Also, any trees/mountains that block "hours of sun" per day? ANY SHADING on solar panels pretty much kills panel output power. Triming trees, placing array 100's of feet in clear meadow, etc. can help).

Refrigerators and Freezers are what typically drive small solar power systems to be medium/large solar power systems....

A typical energy efficient home freezer may draw 1,000 to 2,000 WH per day. A very efficient small (RV style) chest freezer may draw 500 WH or so per day).

Using a Kill-a-Watt meter to test power draw is helpful (run for a few days to check WH usage):

https://www.amazon.com/kill-a-watt/s?k=kill-a-watt (120 VAC power)
https://www.amazon.com/s?k=dc+ah+wh+meter

Typically for a "reliable" off grid system, would suggest using only 50% to 65% of the "predicted system harvest" per day to allow for cloudy weather and/or occasional high power loads (water pumping, visitors, vacuuming, etc.).

-Bill

KPowers · August 2023

We are about 5 miles south of Homer Alaska. We have a clear view of the sky but lose the sun at about 2 pm, as it falls below a tall mountain ridge.

I appreciate the responses.

One more question, would it help to add more panels?

BB. · August 2023

Ok... The PVWatts calculations for Homer Alaska, fixed array facing south at 60 degrees tilt:

https://pvwatts.nrel.gov/pvwatts.php

Month	Solar Radiation ( kWh / m2 / day )	AC Energy ( kWh per 1,000 Watts of panels)
January	0.93	27
February	1.54	42
March	3.32	99
April	4.40	127
May	5.21	146
June	4.17	114
July	3.84	109
August	4.11	115
September	2.81	80
October	2.12	64
November	0.97	28
December	1.05	30
Annual	2.87	981

Obviously, September is pushing the solar system with less than ideal hours of sun per day... 990 Watts of panels and 4 - 6V 415 AH agm batteries... Guessing you have a 12 or 24 VDC setup? Although, for larger systems (this is getting larger), 24 or 48 volt battery bus allows these of smaller AWG cables and the solar charge controllers being "fixed max current output" can supply more power at high bus voltages (i.e., p=v*I; p=12 volts * 40 amps = 480 Watts of solar; 24 volts * 40 amps = 960 Watts of solar; etc.).

Some more quick sanity calculations:

24 volt example: 2 series * 2 parallel batteries (series/parallel changes do not change battery stored total energy). Note that 5% minimum rate of charge for weekend system, 10-13+% minimum for full time off grid--Up to 20-25% suggested cost effective max charging:

415 AH * 29 volts charging (4s * 1p config) * 1/0.77 solar+controller deratings * 0.05 rate of charge = 781 Watt array minimum
415 AH * 29 volts charging (4s * 1p config) * 1/0.77 solar+controller deratings * 0.10 rate of charge = 1,563 Watt array min full time off grid
415 AH * 29 volts charging (4s * 1p config) * 1/0.77 solar+controller deratings * 0.13 rate of charge = 2,032 Watt array nominal full time OG

Then there is how much energy you will harvest... PVWatts has a 24 hour per day harvest output that you can put into a spread sheet and you can see what a 2pm sun cutoff does you your harvest (which months, etc.). Anyway, the September (long term average data) calculation for a 990 Watt array looks like:

990 Watt array * 0.61 AGM off grid AC system eff * 2.81 hours of sun per day (2pm cutoff?) = 1,697 Watt*Hours per ave September day

That is not too bad if you look at an "RV" style fridge/freezer (500 to 1,000 WH per day???)--Always double check/measure the actual consumption. Depending on how warm you keep the cabin, keeping a freezer outdoors can reduce energy usage... There is always the issue of a refrigerator out in very cold weather--The fridge section may freeze in cold rooms/conditions.

It looks like you may have marine layer issues--So variable amount of sun is probably an issue. Again, if you want to minimize the use of a backup genset, then using 50% to 65% of harvest for "must be powered" solar loads can look like this:

1,697 WH per day (September) * 0.65 solar fudge factor = 1,104 Watt*hours of derated solar fudge factor.

You can also calculate the solar array from loads. Say you want 2,000 WH per day:

2,000 WH per day * 1/0.62 AGM AC solar eff * 1/2.81 hour of Sept sun = 1,148 Watt array "September break even"
1,148 WH break even array / 0.65 solar weather fudge factor = 1,766 Watt array w/ fudge factor

In solar, more panels almost always help. Most people "murder" their battery banks by under charging/over discharging.

Having more solar panels vs a small 2 kWatt or so backup genset is a choice--What works best for you... And your May-September temperatures (40-60F?) tend to reduce the energy usage of freezers/fridges.

And there is always the question of what happens if you miss a few days of solar (storms/marine layer)... Can you not use other loads until sun returns?

For a 415 AH @ 24 volt battery bank battery bank... The "suggested" draw would be--Assuming 2 days of "no sun" and 50% typical discharge (for longer battery life--But you can go down to 20% SoC "when needed"):

415 AH * 24 volts * 1/2 days * 0.50 max planned discharge = 2,117 Watt hours per day (2 days with 30% reserve)
2,117 WH per day / 24 hours per day = 88 Watt load (i.e., over 24 hour "day")

Normally, I would be suggesting a 10% rate of charge minimum for AGM/FLA batteries... So that would be a 1,536 Watt array.

You can also do things like pointing that array to the South East--I.e, capture more morning sun as late afternoon sun is killed my the mountain shadow. But you have to be careful, facing SE (135 degrees) cuts overall harvest--So what you gain by facing SE vs what you lose by facing S is always a question. Again, you can download hour by hour harvest into a spread sheet and do your own checks:

Image: https://us.v-cdn.net/6024911/uploads/editor/t8/sbwxb555bkdq.png

With a "lower power" freezer--You can probably squeak by. Ideally, adding another 600 Watt of solar power would be a help (reduce genset usage, losing freezer of food, over discharging battery bank). Typically adding solar panels is the best bank for the buck solution for "under powered" system.

-Bill

MichaelK · October 2023

I'll take a slightly different tack to suggest you need more solar. Assuming your bank is 415Ah, and a very conservative charging rate for an AGM is 1/8th of C, the the amount of charging amps your bank should receive for a 24V system would be 415Ah/8 = 52A. Keep in mind that some AGM batteries want an charging rate as high as 1/5th C.

I like to include a fudgefactor of 85% for real-world production. With three 330W panels, that would be charging that ~25V or so, that would be only [(3 X 330W)/25V] X 0.85 = 34A. What you really need is (25V X 52A)/0.85 = 1529W. So, maybe a minimum of at least 4 panels, but better yet, 6 panels might serve you better.

With six panels, a charge controller that can handle at least 80A is appropriate.

SteveK · October 2023

I have been impressed with my cheapo Frigidaire 7 cu/ft chest freezer of 14 years. This thing sips power the likes of which BB mentioned above only in a full sized box.

The solar powered 15A circuit it is attached to also powers our washing machine and the (no longer made) Nyle Geyser heat pump water heater attachment. I monitor this circuit using a modified Emporia Vue2 energy monitoring device.

If you look at the graph below you will see the water heater cycle on/off two times during the day yesterday. These two cycles amount to ~1.65kWh of the total circuit use for that day.

The second graph shows total usage of the circuit for that day including the water heater with no wash cycles done that day.

So, total use for the day was 2.02kWh minus the water heater's contribution of ~1.65kWh which leaves the freezer consumption at right around 400/500Whr per day. Judging by the duty cycle though it's more like 700wH per day. I've not measured it alone but this thing impresses me .

Now, getting stuff frozen will take more energy but this is just maintaining temp. So, if you are putting up meat and fish constantly up there in AK you will use more energy and I don't have data on that type of use.

Added: As an aside I originally purchased this freezer with the intention of converting it to a fridge. It was quite a popular modification back then for the off-grid folks and i was using a much smaller solar system back then. I would encourage you to look into doing that by changing the thermostat in it. You will reduce your food storage demand considerably.

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