inverter choice

Billy

I have 2000 watts solar with outback flex 80 controller, on a 24 volt battery bank 900 or so ah.  Only running small lights and small fridge on two 1500 watt inverters on separate runs.

I would like to run a water well pump (1hp) 240v short time to fill tank and maybe a mini split AC with one inverter installed to breaker panel.

Would a Magnum 4000 watt inverter be to much for a 2000 watt solar array ?

I also have Honda 6000 watt generator I could hook into if needed.

Thanks for your info.

BB.

In general, the inverter+battery bank AH (and voltage) set the maximum AC inverter (and maximum "cost effective" solar array).

For a 24 volt battery bank, basically a 500 Watt Inverter (and solar array) maximum per 100 AH @ 24 volt battery bank or:

• 900 AH * 500 Watts / 100 AH  (at 24 volts) = 4,500 Watt AC inverter (and max solar array)

So, a 4,000 Watt AC inverter would work. If I read your post correctly, you would replace the 2x 1,500 Watt inverters with the one 4,000 Watt inverter, correct? The Magnum inverter you are looking at is a 120/240 VAC split phase unit?

Now the other question is how much energy do you use in a 24 hour period? Guessing at some numbers (as an example).

First we size the solar array based on the size of your battery bank. 5%-13% rate of charge... 5% for a weekend/seasonal cabin, and 10%+ for a full time off grid home:

• 900 AH * 29.0 volts charging * 1/0.77 panel+controller derartings * 0.05 rate of charge = 1,695 Watt array minimum
• 900 AH * 29.0 volts charging * 1/0.77 panel+controller derartings * 0.10 rate of charge = 3,390 Watt array nominal
• 900 AH * 29.0 volts charging * 1/0.77 panel+controller derartings * 0.13 rate of charge = 4,406 Watt arry "typical cost effective" maximum

Then there is the typical 2 days of storage and 50% maximum discharge recommend draw from a battery bank:

900 AH * 24 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 maximum discharge = 4,590 WH per day load for 2 days

And there are the loads that you may actually use in summer:

• 3,300 WH per day for full size fridge, LED lighting, LED TV, laptop computer, clothes washer, water pump
• 1,200 Watt A/C * 50% duty cycle * 8 hours a day = 4,800 WH per day (assume only sunny days 6 months of year)
• 3,300 WH per day every day for 12 months, and +4,800 WH just during "summer" = 8,100 AH per "summer" day

Fixed solar array, facing south, somewhere around Houston Tx:
http://solarelectricityhandbook.com/solar-irradiance.html

Houston
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 60° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.53	3.87	4.65	4.98	5.20	5.20
Jul	Aug	Sep	Oct	Nov	Dec
5.40	5.24	5.19	4.75	3.93	3.51

Guessing you may have a fair amount of cloudy weather (marine layer) in the summer? So, a couple of calculations:

3,300 Watts in "winter", pick February as the "break even month" (use genset in bad weather, if needed to keep battery "happy" (recharge from 50% to 80%+ with genset during bad weather):

• 3,300 WH per day * 1/0.52 off grid system eff * 1/3.387 hours of sun (Feb) = 1,640 Watt array (FEB break even/non-AC usage)

And then there are the sunny/hot/humid days at 8,100 WH per day and may through September or 5.19 hours of sun minimum for break even (you may need to run genset during miserable weather, or simply turn of AC/dramatically cut AC runtime when you have cloud cover):

• 8,100 WH per day * 1/0.52 off grid system eff * 1/5.19 hours of sun (Summer) = 3,001 Watt array "summer break even"

So, if this is full time off grid and you want to "limit" your genset runtime (and you really want A/C), then you are looking at a 3,001 to 4,500 Watt solar array (larger array, less genset usage).

If you are going to go without A/C, then you could get down to a 3,390-4,406 Watt array and have a "near normal" off grid electrical system (full size fridge, lighting, washer, well pump, etc.).

I made lots of guesses about your location, needs, and energy expectations... Some other issues... for a 10%+ rate of charge, you are going to need a second charge controller (on 24 volt battery bank). 3,300 Watts can be "enough" for a very energy efficient/near normal electrical existence--And this may be more than you actually used (your battery bank is pretty good sized for a small home/cabin system).

And regarding larger battery bank (future expansion/energy needs)--And that you are looking at a new AC inverter--You are getting to the point where a 48 VDC @ ~450 AH battery bank make make sense. Over ~800 AH battery bank, "extra" charge controller, heavier 24 VDC battery bus cabiling. If you are looking at a new inverter anyway, then a 48 VDC battery conversion may make economic sense (your present battery bank age, configuration, etc. is obviously another set of questions).

-Bill

Dave Angelini

If you time the power use correctly you will be fine. The better quality mini splits use less power and run best on 240V. Example LG makes 5 models of 3/4 and 1 ton units. The more expensive have a higher SEER or EER and wil use less amps.

Why not look at a 4KW Outback radian and it could communicate much better with the flexmax and give meaningful data to you?

Good Luck!

billybob9

Dave is right and timing is everything. The ultimate would be having a timer that would completely shut off after each cycle. So if you had 4 fridges and timed them to go on once an hour for 15 minutes and only one was working at a time, this do it. So does anyone sell a timer ( like a 12vdc sprinkler system ) that can do this. It would be 4 ac outlets and a timer ?

BB.

When you start trying to get into the details of overlapping operations--You really need to know how your device(s) function.

For example, a standard 120 VAC compressor that may take 120 Watts on a refrigerator can run from 25 to 50% duty cycle (obviously, duty cycle goes up in hot water, "warm" food&drink in fridge, etc.). Setting a 25% (15 minute per hour) duty cycle may leave you with a "warming fridge" in some conditions.

You may work well with 50% duty cycle and 15 minute delays between 4x refrigerators (as an example). The standard compressors run at 120 Watts, but may take >600 Watts when starting. So, by offsetting the start cycle(s), you don't hit >4x600 Watts with all turning on at the same time).

Other issues can include, if you have frost free models. The simple units seem to have a 12 hour timer that starts a ~1 hour cycle with a 600 Watt heater to defrost the evaporator inside the fridge (nothing like seeing a glowing read Calrod in the back of a refrigerator--Or getting burned on one if you have the back ducting remove).

And some electronic timer refrigerators have been known to start a defrost cycle every time the AC power is "restored"--Putting on a timer (or search mode inverter), and you may end up with little cooling and lots of defrosting.

One retired poster here did a test on his frost free refrigerator and found that with the defrost time disabled, after about 24 hours (for his fridge), the icing started to block airflow and cause the duty cycle to go up and "waste" more energy than he saved with a disabled defrost cycle+heater.

Not saying that this stuff cannot be done, but the "more complex" the loads, attempts to make things play well together on an off grid power system may not end well.

-Bill

billybob9

The fridge 600W starting is why I said each cycle would need to shut off completely. Even my sprinkler system has a slight delay. The 4 fridge was just an example and 3 would be more in the ballpark giving you 20 minute per cycle. This system will only work if all fridges are pre-cooled to their shut off temperature. So you are very right as this would be a little tricky but on the good side would utilize power that would just go to waste. These kind of things absolutely take up your time and need full attention so In my book it would go into " If I had to ".  Thanks for your post..

Billy

Thank you all for your answers.

So, a 4,000 Watt AC inverter would work. I would replace the 2x 1,500 Watt inverters with the one 4,000 Watt inverter, a 120/240 VAC split phase unit. And the 2000 watt solar panels should take care my small energy needs?

As long as I do not exceed my 3000 to 4500 watts at one time

Dave Angelini

You should be fine. Good Luck!

BB.

In my post #2, I made a bunch of guesses about your location and energy needs (based on your present system)... The A/C system is a pretty big wild card here (size of system, hours of use, possible starting surges, etc.).

Have you picked a cooling unit (like a 240 VAC mini-split A/C, possibly heat pump when some cold weather blows by)?

Staying with 24 volt battery bank, or looking at 48 VDC?

Something like this inverter?

https://www.solar-electric.com/maenms4040wa1.html

How sensitive are you to running the genset when high usage (cloudy/humid weather or such)?

A 2,000 Watt array is a bit on the large size for an FM 80 Outback controller (80 amps max--2 kWatt array would tend towards 90 Amp max on cool/clear days with 77% array derating):

• 2,000 Watt array * 0.77 panel+controller derating * 1/29.0 volts charging = 53 Amps "suggested minimum" MPPT controller

2 kWatt, no problem at all... (1/0.77 and * 29 were wrong in equation--- * 0.77 and * 1/29 volts it should be).

48 VDC bus would allow you a much larger solar array on the one FM80 MPPT charger.

-Bill

Dave Angelini

Xantrex use to make a pretty nice timer for 24 and 12 volt systems. I can look it up if the OP comes back. Used on boats and mobile.

I disagree that a 2KW array is "a bit large for an 80A mppt"  Billy probably does not want to replace batteries and rewire at this time. My guess...Who gets 2KW out of nameplate solar and the mppt will current limit a possible power spike.

Estragon

I'm not sure about that controller size arithmetic either.  Why "*1/0.77" for wire/cc derating (ie increasing expected wattage output)?  Should that read "*0.77" (decreasing output to ~NOCT)?

For CC capacity sizing purposes, it seems to me using nominal 24v (a bit under 50% SOC) as the divisor to get max current makes more sense.  At 29v, the CC is likely in absorb and tapering current.  At 24v, it's full on charging.

Using this, I get 2000 x 0.77 ÷ 24 = ~64a

BB.

Sorry, you guys are correct... My 1/0.77 should have been * 0.77 -- Will fix.

And * 29 volts should have been * 1/29 volts...

-Bill "the world is upside down" B.

Dave Angelini

I am not sure if the world is upside down either. I do know the market is up!     Have a good Friday!

MichaelK

I would have to disagree with the above comments if you're trying to run your 1hp well pump.  In the design phase of my system, I contacted Magnum and spoke with one of their technical engineers about running my 1hp Grunfos pump.  At 240VAC, it consumes about 10 amps while running, but has a startup surge of about 37-38 amps (that 9120 surge watts).  The Magnum engineer didn't feel that their 48V unit would be able to handle the starting surge.  If you look at the technical specification of their MS4048 unit, it has a 5 second surge of 8500 watts. The MS4024 unit is even lower, with only 5800 surge watts.  Now, assuming you only wanted to run your well pump for an hour around noon, the total solar output would still not equal the draw on the system, ~2400 watts, so, no, not the well pump.

I believe that you could parallel two MS4024 units together, but that would entail another set of panels, and another set of batteries.  In my own case, I went with a 48V system with Schneiders XW+6848 inverter, powered by 4500 watts of panels.  I can run the well pump from 8am till 4pm with totally full batteries at the end of the day. 

I think a very good rule of thumb for designing your system is to determine what your biggest load will be in watts, then multiply that by two for the watts of panels you need.  In my case, my load was 2400 watts, and I installed 4500 watts of panels.  It worked.  BTW, I determined my starting/running amps with a digital clamp meter that has "in-rush" capability.  I have two, a rather expensive Fluke, and a cheaper Chinese model made by UniT.  I've tested them side by side, and the UniT is accurate to 1% of the Fluke.  I got it off of Ebay

https://www.ebay.com/itm/Uni-T-UT216C-600A-True-RMS-Digital-Clamp-Meter-AC-DC-Multimeter-NCV-VFC-tester/323819337307?epid=710119969&hash=item4b6522f65b:g:oMcAAOSwhOVXc-Bm

Dave Angelini

His well pump could be a grundfos with zero surge and a 1.2KW load. I still think a 4KW Radian could do a standard 1hp pump but you are right Michael, it would be on the edge and need a full battery. I was assuming he is running this now?

Maybe he will come back......

Billy

Thanks again Guys,  Not looking at more battery bank although I see where it would be better with 48 volts!!

Only time I would need pump would be weekend and I would have Generator for that.

The main thing for my area would be an AC of some sort!! Didn't think of 240v AC!

I do like the Magnum inverter so I could have 120/240 and tie Generator in.

I know my outback 80 will work on 48 volts so don't have to replace but the 24v inverter is big ticket for my budget hate to have to upgrade from that!!