confused about inverter vs battery size

moonbow · February 2023

Hi everyone, I have a completely off -grid cabin that has been running smoothly on a suresine 300Watt inverter tied to 4 AGM 12V 155Ah batteries. I added an addition to my place and will therefore be adding more lights, and range hood and more outlets. My cabin is wired for AC. I have a friend that has a Royal Power 12V 3000Watt Inverter/Charger for sale at a very reasonable price. It, like the SureSine Inverter, is meant to be hardwired into an AC panel. In anticipation of enlarging my inverter I have now doubled my battery bank to now be eight 12 Volt 155Ah batteries.
I have been reading that an inverter of this size is too much for a 12V system. Am I misunderstanding this? I am not sure why a reputable company would manufacture a 12V 3000 Watt inverter/charger if its not a safe idea. Now, I will not really be pulling 3000 watts ever but I figure this inverter lets me or a future owner expand the system if ever needed. Can someone help me here?Thanks

BB. · February 2023

I am the one that suggest avoiding high power AC inverters on 12 volt battery banks... If nothing else, the heavy current from a 3 kWatt 12 volt inverter draws:

3,000 Watts * 1/0.85 AC inverter eff * 1/10.5 battery cutoff = 340 Amps worst case @ 3kWatt

You need a lot of copper to supply that amount of current with a (suggested) maximum 0.5 volt drop. For example, 11.0 volts @ 340 amps @ 10 feet (one way run) of cable:

https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=12&necconduit=pvc&necpf=0.99&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=11.0&phase=dc&noofconductor=1&distance=10&distanceunit=feet&amperes=340&x=0&y=0&ctype=nec

Result

Voltage drop: 0.42
Voltage drop percentage: 3.82%
Voltage at the end: 10.58

And 2x that for surge current (usually inverters are rated for 2x rated current for a few seconds to start motors, etc.).

NEC rates 4/0 AWG for a maximum of 260 Amps (depending on insulation, ambient temperatures):
https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

And Marine/Boat wiring can be rated to 445 Amps:

https://www.westmarine.com/west-advisor/Marine-Wire-Size-And-Ampacity.html

You can parallel 4/0 cables, keep distances short, and simply not "expect" a full 3kWatt (6kW surge) from the inverter unless you use the heavy copper cable(s).

Also, the minimum suggested battery bank size for running 12 VDC inverters on Lead Acid batteries (AGM do have lower internal resistance, so they do supply more current than FLA... (note that Li Ion batteries can supply more current with even less voltage drop than AGM--So these rules would be "different" for Li Ion)

For 12 volt battery banks, suggest a minimum of 100 AH per 250 Watt loads:
3,000 Watts / 250 watts per 100 AH = 1,200 AH battery bank (Lead Acid) suggested minimum

Your 8x 155 AH @ 12 volts = 1,240 AH--So you have that covered.

Also, you want to wire the battery bank up so that each battery will properly share current. This website has some good diagrams and explanations:

http://www.smartgauge.co.uk/batt_con.html

-Bill

BB. · February 2023

I should add, make sure you have enough solar panels and don't expect too much daily energy usage from battery bank...

Rate of charge, at least 5% for weekend/summer usage, 10-13% (or more) for full time off grid.

And for Lead Acid batteries, suggest using 25% of battery capacity daily normal usage (50% for 2 days of "no sun"):

1,240 AH * 12 volts * 0.25 usage * 0.85 inverter eff = 3,162 Watt*Hours average (no sun) daily load

10% rate of charge for your battery bank from solar:

1,240 AH * 1/0.77 panel+controller deratings * 0.10 rate of charge 5* 1/14.5 volts charging = 2,335 Watt array Nominal for your bank

And for other new readers, I suggest that 1,200-1,800 Watts is a "comfortable load" on a 12 volt FLA battery bank.

You may wish to revisit your present and future loads... Even with that large battery bank, running the inverter at 3,000 Watts will use up 1/4 of your battery bank capacity in just 1 hour...

And look at the inverter's Tare Load (how much energy it draws just "turned on"). The SureSine is probably around 1/2 an Amp. Larger inverters draw more current (although, newer inverters seem to be wasting less Tare current these days with their updated designs--And the Royal inverter specs look very good).

https://www.royalpower.com/p/3000-watt-transfer-switch-power-inverter-charger-12-volt-dc-to-110-volt-ac

No-load draw

< 0.8 amp (12V)

moonbow · February 2023

Wow, there is a lot to digest there. Thank you for all that. I am going to have to read and reread it a few times to wrap my head around all of it. What I can easily see is that the Inverter I have been offered is overkill for my purposes. What I am still curious about though is this...besides the latent draw of this large inverter, which, according to their website and your research (thanks for that) is < 0.8 amp, if I only pull up to a 1000 watts at any given time, am I still operating an inefficient poorly designed system or does it only qualify for the "inefficient poorly designed system" if I start to maximize the available 3k wattage that the inverter affords?
Your line "I suggest that 1,200-1,800 Watts is a "comfortable load" on a 12 volt FLA battery bank" stands out because I will be operating in the low end of this range primarily. Really, the fact that this inverter was offered at such an affordable price is what attracted me to it. Seems like I should pass on it and focus on something in the 1k to 1.5k range as my loads don't really justify such a powerful inverter. Can I ask why companies make such an inverter? From what I am coming to understand, 12V 3k watt inverters are not a very practical configuration. Thanks in advance.

moonbow · February 2023

oh, thanks for the link to the more efficient battery wiring diagrams. interesting!!!

BB. · February 2023

If the specifications are correct... It does not seem that there would be a downside to running the 3kWatt inverter down around 1,200 Watts given the low tare losses...

The downside is if you aim for 10% rate of charge, you will need two or so Solar charge controllers (larger units typically top out around 50-80-100 amps).

If you picked a 24 or 48 volt 80 amp controller:

14.5 volts charging * 80 amps * 1/0.77 panel+controller drating = 1,506 array "max cost effective" for 12 volt bank
29.0 volts charging * 80 amps * 1/0.77 panel+controller drating = 3,013 array "max cost effective" for 24 volt bank
58.0 volts charging * 80 amps * 1/0.77 panel+controller drating = 6,026 array "max cost effective" for 48 volt bank

One of the issues with "growing" the system, the "other equipment" costs (more copper for wiring low voltage battery bank, more copper for inverter & charge controllers, more charge controllers for larger battery banks, etc..

If you want an AC battery charger (backup genset, etc.)--This inverter only has a 20 amp battery charger.... Barely enough current to float the battery bank.

For a "nice" AC battery charger (10% or greater), you are looking at 125 Amps @ 12 volts or larger... Finding high current 12 VDC chargers is not always easy... Our host (Northern Arizona Wind & Sun)--There larger 12 volt AC chargers are around 50 amps or so max output.

Just to give you an idea what a higher end 3 kWatt 12 VDC inverter with a 120 Amp charger output:

https://www.solar-electric.com/victron-energy-multiplus-inverter-12-3000-120-ul-1741.html

All the "ancillary equipment" (another solar charge controller, additional AC 12 volt backup chargers, heavier cabling, etc.)--It all adds up.

And the suggestion to do several paper designs and see what works best for you before buying the hardware.

No "right or wrong" answers... Looking at your requirements and designing a system that meets those needs (including costs).

-Bill

confused about inverter vs battery size

Comments

Result

Categories