Run AC off large solar array while boondocking

NomadLife

We have a class A rv running (10) 405 watt Canadian Solar panels that I can tilt in the winter to get maximum efficiency. I built the array using actuators to tilt the panels as they take up the complete roof of our 42 foot coach. 

I am currently running victron mppt charge controllers and a Cerbo. Our coach came with a Magnum 2800 watt pure sine inverter which currently runs everything on the coach except for the (3) rooftop AC units. The coach is also an all electric coach. 

I have (4) LiFeBlue 300 amp hour lithium batteries and my current setup is all 12 volt. 

I have setup the coaches breaker panel to run everything except for the AC units. 

My question is two fold.  

1:  I am thinking of installing (2) Victron multiplus 3000 watt inverters so I can run 2 of my AC units off the solar. My thought was to run the inverters and wire them to a 50 amp female plug. That way all I need to do is plug the coach in and run the 2 AC units off the Victron inverters. The rest of the coach will stay on the magnum 2800 inverter. 

2: I will also look at adding an additional 600 amp hours of lithium batteries to the mix. 

3: is it better to change my system to 24 or 48 volts since the AC units pull about 3500 watts on average?
Lastly we stay off the grid 99% of the time so we rarely plug in to power. 

Any advise will be welcome. Currently in Quartzsite, Az till end of March 2023
thx

Photowhit

I think I would first look at the load!

What BTU are air conditioners that use;

NomadLife said:

 AC units pull about 3500 watts on average?

That is a ridiculous load! Even if that was together...

Maybe you are referring to startup? locked-rotor amps?

I think the Midea 12000 btu window unit runs <1000 watts running. I suspect your RV is split up into multiple spaces. You might consider a mini split with multiple indoor units.

BB.

Welcome to the forum NomadLife,

In general, I highly suggest that you run at 24 or 48 volts--The DC wiring for 12 volts gets very large (AWG), and thick (difficult to bend), and 12 VDC voltage drop is a pain (heavy current, and you have maybe 0.5 to 1.0 volts of allowable voltage drop which means heavy/short battery cables).

The downside to a 24 or 48 volt system, is if you use a lot of 12 volt power for other things/appliances in your RV. You may end up with 12 volt down converters or even a 12 volt system for those devices--Or changing out 12 volt for 120 VAC (typically) appliances--Which in the longer term is usually not a bad choice other than the upfront money and work to replace DC with AC appliances.

Next--Need to know more about your loads... With solar, conservation of energy (find the more efficient/smallest loads that meet your needs) is usually cheaper in the long run vs just running "inefficient" loads (such as A/C, refrigerator, lighting, computers, etc.).

And towards that end, knowing more about the energy usage of your loads is pretty critical here. You don't want too large of system (expensive, takes up space and weight, etc.) and too small of system that does not meet your needs is "worthless" to you.

A good place to start is using a Kill-a-Watt type meter (or similar other brands) for your 120 VAC plug in loads and measure your average wattage and Watts*Hours per day of usage:

https://www.amazon.com/s?k=kill-a-watt

For larger appliances/hardwired systems, there are "whole house" units too (note, links are just suggested starting points for your reseach):

https://www.theenergydetective.com/

You can get units that measure the whole home/RV loads, or even have multiple sensors that can measure each branch circuit's consumption.

Your Cerbo mahy give you much of this information--At least a piece at a time--Such as just running one of your roof top A/C systems from your existing system to gather the measurements.

The issue with A/C systems is they are real energy hogs... Newer units (such as mini-splits and other "inverter" based A/C systems) can be really nice--They can be setup to run at lower average power usage for many hours a day--Vs the older (typically non-inverter type) that cycle on, draw lots of Watts for 10 minutes, then cycle off 10 minutes).

And the heat pump versions of these newer units are pretty amazing too... If you are in cold climates, the heat pump versions can use 1/2 to 1/3 the Watt*Hours of energy vs electric strip (resistance) heaters even down to near freezing weather or below.

Anyway... Lets do some math and see how much power (Watts or rate of energy usage like miles per hour) and energy (Watt*Hours or total energy used like total miles driven) we are talking about...

With solar power systems, we like to start with the loads/energy needs (Watts and Watt*Hours per day). That defines the battery bank. Then we figure out where you are (RV, where you will be spending most of your time), how many hours per day of sun per season, and the size of solar array needed to "keep the battery bank happy". And remember, solar panels need full sun to work properly at full harvest... Any trees, shading, wires overhead, etc. dramatically reduces amount of harvested energy)--I.e., choice of parking under trees for shade/avoid heating the RV from sun, vs parking in full sun to get full sun on solar array.

Regarding your system--You do have the choice of 1x solar system (all batteries, panels, inverter(s) together)--Or using your existing 12 volt system unchanged and adding a second 24/48 VDC system for your A/C energy needs... For the moment, I am going to just assume 1x large system to keep the math, options, and confusion to a minimum. Later if you want a split system we can talk more (I suggest 1x large system vs 2x split systems--But that is a choice).

Say you have 6x (total) 12 volt @ 300 AH batteries. And configure as 2x series (for 24 volt) and 2x parallel (for two strings @ 300 AH per string). Gives 24 volts @ 600 AH battery bank (that is thesame amount of stored energy as 12 volts @ 1,200 AH battery bank where Power=Voltage*Current. You can double voltage or double current--Same number at the end).

Note: If you wanted a 48 VDC battery bank, you would have to use 4x 12 volt batteries in series (i.e., the bank is groups of 4 series batteries--Or 4/8/12/etc. batteries).

With the Lithium, I suggest that you use (on an average day or two days) around 70% of battery capacity (30% for reserve, aging, etc.). For A/C usage--Lets say you use 70% per day (i.e., charge during day, discharge overnight). The "useful" energy storage would be:

• 24 volts * 600 AH * 0.85 AC inverter eff = 12,240 Watt*Hours per day

Just to give you an idea of what that could support... Say your A/C runs 12 hours overnight:

• 12,240 WH of AC energy / 12 hours of load = 1,020 Watts of average "over night" load

We can see here that "3,500 Watts" of A/C load is 3x more energy usage that even your proposed "larger" battery bank can sustain:

• 12,240 WH of AC energy / 3,500 Watts of A/C load = 3.5 hours of A/C usage

More than likely, your A/C units may "peak" at 3,500 Watts continuous load--But their average load is probably 1/2 or less... But until you measure that usage (hot day in RV, using as you typically require)--This is all guess work. A single larger Mini-Split/Inverter unit may average around 300-800 Watts (average) of energy usage.

Running 3x roof top A/C systems at full power from battery/solar may be more than can be expected... Conservation, choosing energy efficient units, insulation, just cooling areas needed (living area during day, bedroom at night) are all going to help here.

Next--Looking at your solar array. 4,050 Watt array... That is good sized for an RV (even larger than I can fit on my home's roof). So how much energy per day can that array harvest. Say we use Phoenix Az for this (simple) solar harvest website, and you can adjust the tilt of your array for best harvest from summer to winter--I.e., 72 degrees from vertical in summer to 42 degrees from vertical in winter):

http://www.solarelectricityhandbook.com/solar-irradiance.html

Phoenix
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel where the angle is adjusted each month to get optimum sunlight.

Jan	Feb	Mar	Apr	May	Jun
5.28	5.74	6.45	6.97	7.26	7.52
Jul	Aug	Sep	Oct	Nov	Dec
6.60	6.07	6.03	5.94	5.57	5.24

Estimate of solar harvest efficiency:

• 0.81 warm/hot panel derating * 0.95 solar charge controller eff * 0.98 lithium battery eff * 0.85 AC inverter eff = 0.64 (64% overall system efficiency)

Here we are in September--So compare with what you are measuring this month:

• 4,050 Watt array * 6.03 hours of Sept Sun (average) * 0.64 off grid system eff = 15,630 WH per day "average September daily harvest" (this is 120 VAC Energy per day)

That is the average "best case" harvest you can expect for September--Remembering that you have 12,240 WH of battery storage and whatever energy you use during the day (cooking, hot water, A/C, etc.). With solar, anything you cannot use/store that day, you "lose".

What we really need now is your actual measured (or best estimate) of daily energy usage. You have a large system with a nice lithium Ion battery bank... So you are off to a good start. Some guesses at your daily loads/energy needs (pure guesses on my part to show how the math works):

Lights: 10 lights * 15 Watts per light * 5 hours per evening = 750 WH per day
Refrigerator: 1,000 WH per day
Microwave: 1,200 Watts * 1/3rd hour per day (20 minutes) = 400 WH per day
Induction stove: 800 Watts (average) * 2 hours per day 1,600 WH per day
Laptop computer: 30 Watts * 10 hours per day (work & play) = 300 WH
Clothes washer: 1,000 WH per cycle = 1,000 WH
RV Water pump: 100 Watts * 1/3 hour per day = 33 WH
Electric Water heater: 1,500 Watts * 1 hour per day = 1,500 WH
Starlink Internet: 50 Watts (rectangular version, not freezing weather) * 10 hours per day = 500 WH
=========================================================================== 
Total estimated usage: 7,083 WH per day (7.1 kWH per day)

Available energy for A/C:

    15,630 WH of September Harvest - 7,083 WH of non-A/C loads = 8,547 WH of available (Sept Average) A/C power

Existing battery bank: 12 volts * 1,200 AH battery bank "useful" storage:

    12 volts * 1,200 WH * 0.70 battery "useful capacity" * 0.85 AC inverter eff = 8,568 WH of "over night" battery storage available

Anyway--This is a start of how to look at your system, daily loads, daily harvest, splitting loads between daytime (i.e., not using battery power) and night time (only battery power--To be recharged next day).

Your thoughts/questions/corrections to my guesses?

-Bill

Surfpath

Bill,
That was an awesome detailed post. Thanks for sharing your wisdom with all of us. SP

BB.

Thank you for your kind words Surpath.

Sincerely,
-Bill

NomadLife

Bill

thanks for the great write up as it is very helpful. I will be going through your list of suggestions and get more detailed information. Again thank you 

phil

BB.

You are very welcome Phil.

The above is a lot of generalities and pure guesstimates on energy usage and harvest.

With power in general, and solar+battery specifically, details matter. Your needs, what is traditionally available (components wise), etc. all matter.

Please feel free to ask more questions. Starting with energy measurements, reviewing for any replacement appliances that use less energy/more efficient... Then starting the system design. Starting with "paper designs", see the pluses and minus for the various options, refine. Then do a detailed paper design (possibly several options). Once you have the full up paper design, then start looking at buying hardware.

Buying hardware before the (relatively) detailed design is complete can end up costing lots of $$$$ as some items may not play well with others.

Hope to hear more from you in the future here on the forum and we can continue to be of help.

-Bill