Calculating refrigerator AH correctly

rp3703

So after years of design work, much of which involving numerous threads on this forum, we finally put our off grid solar system together along with all the loads it will be running at our cottage in northern Ontario. The system consists of 6 310 watt Rensola panels(1860 watts total) which get used to fill our 24V 430 AH bank of 8 batteries. The loads that the system is powering are an electric submersible water pump, an electric refrigerator, an electric front load washer and propane dryer, three LED light fixtures and cell phone and laptop charging for 8-10 people. 

So far the system has been working great. We reach float at around noon or 1 PM. The lights flicker a bit when the water pump comes on but I think that is due more to the bulb than excessive amps. The thing that worries me is that the charge controller is saying that we are using about 15-20% of our battery storage while in resting mode during the night. Since this is July, which I should state is the only time of year we ever spend at this cottage, the resting mode usually only lasts from 9-10 at night to 6-7 in the morning. So the 15-20% of 430AH or 65-86 AH of use in 8-10 hrs would mean the fridge uses 8-8.5 AH per hour. All the formulas I have used to calculate required AH of battery storage, require at least 2 days of storage. That would be 384-408 AH for the fridge alone. Does this sound like the correct way to go about calculating this?

Estragon

A reasonably efficient fridge/freezer, not a water-through-the-door, ice-maker, etc, but basic modern fridge might use ~1500 wh per day. That would be roughly 1500/25v =~60ah/day.

If it's using that or more just overnight, I'd suspect either very warm ambient, being left open, filled at night with lots of warm beer for the morning, etc which would cause more cycling, or a more solar specific issue. Some might include; electronics in the fridge or other loads preventing the inverter from going into sleep mode between cycles, or the sleep mode messing up the defrost cycle.

westbranch

1. I will bet that period at the fridge is defrosting, in the 'overnight'.. .norm is about 11.5 hrs between .5 hr defrost

2.  I have about 2 X your battery and almost the same PV and use about 7% with overnight loads, TV , reading, reharging |Smart phones , Tabs and laptop.... 

3.  But we do not have a water pump...

4. what is the Inverter draw?

5 how big is the fridge, Age?  We have a 3 year old,  plain Jane, 17 cu. ft G E that uses ~1.5 - 2.0 kWh per day

hth

rp3703

Westbranch, if you’re using 7% of your battery storage with twice the AH and I’m using 15% with half the AH, then we are pretty close to the same. I was only able to really monitor my loads for a week and a half before we had to evacuate due to an approaching forest fire. During that period, the Classic 150 showed the batteries at about 86% most mornings but once or twice at around 82%. The fridge is a brand new LG 20.2 Cu. Ft. LTNS20220W which, according to it’s Energy Star sticker, uses 387 KWH per year. It has an inverter compressor which I was told was best for solar use. 

My fear is that if the fridge eats up 15% of my battery storage in 10 hours, then I would go below 50% in 33 hours and that’s just on the fridge alone. Using the water pump which was not contributing to that 15% battery usage, would deplete them even faster. 

I do have 4 more batteries that I was going to add to the system but decided not to at the last minute. I guess I will have to add them after all. 

westbranch

our fridge rated out at 319 kWh and it is  a different compressor than the old ones but not an inverter but very close as it will peak at ~40 Amps ad then down to 1 amp before it shuts off, runs about 50% of the time....

Do you have a WBjr? a real must ,  'knowing; how much is used and put back in to the batts.

Yes to adding batts....   just heading back to town, will check when there...

rp3703

Yes, we put in a Wiz bang but really didn't have enough time to look at any of the info it was giving us. This year was a big remodel year as well so the solar was just a small part on our project list. We are gone now for the season so maybe next year we can do more diagnostic work.

Photowhit

Your sun may set at 9pm, but if you have a fixed array, you likely are running a net negative once you don't have direct sunlight on the panels. Do you check the battery capacity at 9pm?

rp3703

Sun sets at around 10 so even at 9 the panels are still generating power. Batteries at 100% and float before going to resting but since cocktails start at 5:30, my numbers could be a bit off. Again, I hope to pay more attention next season.

706jim

I have a fridge rated 311 Kw=hr/ year. It uses just over 1 Kw=hr per 24 hour day so it seems to be similar to what you have. Unless you have a tracker, your panel output is going to vary over the day. Morning and evening output will be minimal.

mcgivor

One piece of the puzzle is missing, the inverter self consumption which may account for a large portion of overall drain on the battery, perhaps up to 30%. of the total. Using a Kill-a-watt device one could see what the refrigerator itself uses over  24 hours as opposed to the net load, but the 1.5KW per day seems about right, 1Kw for the refrigerator 0.5 Kw Tare load.

westbranch

To add, a WBjr can provide the same data, Inverter draw in all modes, especially 'Sleep'.  just turn on the system without the Inverter on, and then look at the consumption 'Page' or plot Battery amps as well as PV Amps.easy peasy...