# Basic Questions - Magnum MS4024 plus a math problem

Registered Users Posts: 5 ✭✭
edited September 2018 #1
Do I need a separate charge controller if I use a Magnum MS 4024?

Is the Magnum MS4024 all I need? (minus the wiring)  I feel like the surge watts on this unit can handle a small 6000 btu standard window AC unit.

If I have:
8 each:  12 volt 200 ah batteries (each battery holds 2,400 watts of power essentially x 8 = 19,200 watts of storage possible)
watts = volts x amps... 12 x 200 = 2,400 watts of holding capacity. x 8 = 19,200

If I want to use no more than half this power per day then I would need to use less than 9,500 watts per day.
If I'm running a 20 amp 120 volt circuit then I'm using a max of 2,400 watts per hour...
That gives me a minimum capacity of 4 hours per day...
... since almost no circuit runs at full rate all the time (compressors turn on and off based on need)
I could probably go longer before I used half the stored power.

I would need enough solar panels to pump 9,500 watts per day at least back into the batteries
150 watt per hour panels x 5.5 hours of sunlight per day (Texas) = 825 watts per panel
... I need at least 12 panels (825x12=9,900 watts per day)
So that means 12 panels (in 6 banks of 2 panels each to make 24 volt input to the Magnum MS4024)

So, what I'm trying to do is a size the minimum required specifications on this thing.

Then I want figure out what the maximum charge rate is, so I know what the maximum # of solar panels and batteries this unit will support.

OR, am I going at this all wrong.

Minimum Requirements: 120 volt circuit that will run a window AC unit (6000 btu) plus a lamp and laptop for at least 4 hours a day.
I'm trying to account for surge amps and I want an inverter that has a great service life.  Based on specs the window unit only pulls 600 watts (non-surge) when it's running per hour.

Then it's a question of seeing how many hours of minimum requirements I can get out of the system by adding on batteries and/or solar panels over time.

Thanks.  I may just be over-complicating this. I don't know.

Hi Tony,

Sorry, real life is taking up some of my time... I will go through your questions here to get a level set.
8 each:  12 volt 200 ah batteries (each battery holds 2,400 watts of power essentially x 8 = 19,200 watts of storage possible)
watts = volts x amps... 12 x 200 = 2,400 watts of holding capacity. x 8 = 19,200
To clarify units... Watts is a "rate" like miles per hour. Watt*Hours is an amount like miles driven (30 MPH * 2 hours = 60 miles driven).

AH and Watt*Hours are related... Energy = Amps * Hours * Volts = Watt*Hours. AH is used a lot, but works best if you are talking about one voltage (like a 12 volt automotive or boat application). When you work with a XX volt DC bus and a 120/240 VAC power system--It is better to work with Watts and Watt*Hours--We don't have to keep talking about what voltage @ AH is being discussed.
• 10 amps * 2 hour = 20 AH @ 12 volt battery bus
• 20 AH * 12 volt (DC) = 240 Watt*Hours
• 1 amps * 2 hour = 2 AH @ 120 VAC inverter load
• 2 AH * 120 VAC = 240 Watt*Hours
So--AH is just a partial definition of our power systems. When we use Watts and Watt*Hours, we only need the "one number" that can be used throughout the system design. Just converter to Amps and Amp*Hours (size of wiring, size of battery bank/storage) when we need to (at known working voltage).

Your use of 12 volt storage batteries... Generally, good batteries for a "starter system" (small to medium size system) is to use 6 volt @ 200 AH "golf cart" batteries. They are relatively cheap and rugged. Most of us murder our first battery bank or two (over/under charging, forget to do maintenance, somebody takes a battery bank dead by leaving all the lights on/using a hair dryer, etc.).

Note that 2x 6 volt @ 200 AH batteries are aboiut the same size and weight as 1x 12 volt @ 200 AH battery (i.e., the configuration of the lead and cells still does not change the amount of energy stored).

When you start placing batteries in series--I like 6 volt batteries. It is easy to check the individual battery voltages (i.e., 2x 6 volt @ 200 AH batteries in series, vs 2x 12 volt @ 100 AH batteries in parallel--One of the 12 volt batteries could be dead, but you cannot easily see that because the voltage across the two parallel batteries is the same).

Also, when you start paralleling battery strings--I like to suggest that 1-3 parallel strings is a good range... More than 3 parallels strings, you have lots of wiring/fuses/breakers to worry about (more costs, more complexity) and more cells to check (each 24 volt string has 12 cells to check... 4x parallel strings is 48 cells to check/water/measure specific gravity). Generally use lower voltage (6 or 4 volt batteries, or 2 volt cells) with higher AH rating to reduce parallel strings (12 volt @ 100 AH, 6 volt @ 200 AH, 4 volt at 300 AH, 2 volt @ 600 AH all store the same amount of energy... 12 cells in series--The 2 volt cells will give you a 24 volt @ 600 AH string--Vs 2x 12 volt @ 100 AH x 6 parallel stings for the same bank with 12 cells to check vs 62 cell bank).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
If I want to use no more than half this power per day then I would need to use less than 9,500 watts per day.
If I'm running a 20 amp 120 volt circuit then I'm using a max of 2,400 watts per hour...
That gives me a minimum capacity of 4 hours per day...
I believe you are talking about 9,500 Watt*Hours per day... And 2,400 Watts maximum 120 VAC load for a 20 amp circuit (not Watts per hour).

So we can work that into a description of your needs... Using various rules of thumbs (because they work, and save pages of typing), your needs are:
• 9,500 WH per day @ 120 (or 120/240 VAC)
• 2,400 Watt maximum continuous load (note that most good inverters support 2x rated power for surge load--seconds to a few minutes)
First recommendation, 9,500 Watt*Hour per day--That is a fairly large system. For a backup system--I would suggest ~3.3 kWH (3,300 WH) per day is a nice starting system. Enough to run a full size energy star refrigerator, LED lighting, LED TV, Laptop computer, clothes washing machine, small water/well pump, cell phone charger (you have to be very energy efficient). Gives you a "near normal" electrical life (energy usage is a highly personal set of choices--What works for me may not work for you and your family--Just highly recommend conservation--Conserving energy is much less cheaper than generating the energy).

And the maximum continuous AC inverter power... More or less, the most power from a 12 volt battery bank is ~1,200 to ~1,800 Watts... For a 24 volt battery bank, ~2,400 to 3,600 Watts max. And over ~2,400 Watts, should really look at 48 VDC battery bank (200 Amps * 12 volts = 2,400 Watts--200 amps is a lot of current and you have to have ~0.5 volt drop in the DC wiring to the inverter--Lots of very expensive copper).

So--Size the battery bank first. For me, that is the "Heart" of your energy system. A good battery bank design will make the rest of your work much easier.

For a full time off grid system, I suggest 2-3 days of "no sun" conditions (it is difficult to recharge a lead acid battery bank from 50% to >90% state of charge every day--The sun is not "up in the sky" enough hours in a day to full recharge a lead acid battery bank--25% discharge per day is easier to get full the next day's sun). With 50% maximum discharge for longer battery life. For many reasons, a 2 day / 50% discharge bank is a good cost effective optimum to start with. Assuming flooded cell lead acid batteries (relatively cheap and forgiving) for design. A battery bank that can supply 9,500 WH per day (assume 48 volts---This is a "large" battery bank):
• 9,500 Watt*Hour per day * 2 days * 1/0.50 maximum discharge * 1/0.85 AC inverter eff * 1/48 volts = 931 AH @ 48 volt battery bank
For a 931 AH @ 48 volt battery bank, a maximum AC inverter (and maximum "cost effective" solar array) would be ~9,310 Watts. You could justify a nice AC inverter of 1/2 that value (~4,500 Watts). Remember that AC inverters (and battery bank) will support ~2x surge current (well pumps, power tools).

Charging a lead acid battery bank--5% rate of charge is good for weekend/sunny weather/seasonal/weekend cabin or Recreational Vehicle. For full time off grid 10% to 13% rate of charge is better (you don't have to monitor the loads vs weather every day).
• 931 AH battery bank * 59 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge =And 3,567 Watt array minimum
• 931 AH battery bank * 59 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 7,134 Watt array nomial
• 931 AH battery bank * 59 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 9,274 Watt array "cost effective" maximum
And then there is sizing the array for the amount of energy you are going to actually use (hours of sun per day)... Note that realistically, your day to day baseline usage should be around 65% of "predicted" capacity... To use 100% of capacity every day--Not really possible with an off grid system (some folks do use an electric water heater to use excess capacity once the battery bank is full). Assume fixed array near Fort Worth Tx:

### Fort WorthAverage Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 57° angle from vertical:
(For best year-round performance)

 Jan Feb Mar Apr May Jun 3.97 4.48 5.11 5.54 5.44 5.76 Jul Aug Sep Oct Nov Dec 6.04 5.65 5.54 4.95 4.16 3.78
Assume that the bottom 3 months will need a genset (or less energy usage, less fans, less A/C, etc.):
• 9,500 WH per day * 1/0.52 off grid system eff * 1/4.48 hours of sun (Feb "break even month") = 4,078 Watt array "break even" February
Before we even talked about hardware--We now have a set of specifications where we can now figure out what will support your needs.

Remember that flooded cell lead acid batteries will last ~5-7 hours in hot climates (more expensive batteries will last longer), your electronics will last 10+ years (AC inverter, solar charge controllers, etc.), and the solar array 20+ years (less if you have large hail).

This is a big system for "prepping"... A system about 1/3rd the size is easier to justify and build.