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Old 02-16-2020, 03:55 PM   #1
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Understanding Inverter wattage display

The inverter display in the coach reads a usage of 155 W currently. My total available wattage at 50% of the (4) 6v batteries is 400 W. How do I determine how long the 150 watt draw will last. Is that a 155 W per minute per hour? Do I subtract it or what?
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Old 02-16-2020, 04:07 PM   #2
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You are mixing up watts, watt hours, amps and amp hours.


So your coach is drawing 150 watts. That is the instantaneous power draw and will go up or down as you turn loads on and off. 150 watts is 150/12V = 12.5 amps. Your 4 GC batteries wired in series/parallel have about 440 total amp hours of capacity. But for best life you shouldn't discharge them more than 50% so the practical capacity is 220 amp hours.


If your power draw were constant for a day then you would be half discharged after 220/12.5= 17.6 hours starting with full batteries.


So start your generator and recharge the batteries after about 18 hours at that rate.


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Old 02-23-2020, 07:02 AM   #3
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Thank you and take a look at this for me.

I have (4) Duracell Ultra 6V Golf Cart Battery and there spec say (215AH at 20 hour rate). There are wired in Parallel, so my understanding is I've got 215x4=860AH and 50% of that would be 430AH. So using the formula would it be 430/12.5=34.40 or about 35 hours?
Now all I had running was the TV at 150 watts and I understand (like you said) that will most likely go up as other lights, and small usages increase. But the formula is the same correct?
Draw / 12.5 = hours for use before recharge.
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Old 02-23-2020, 08:28 AM   #4
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Originally Posted by kimenker View Post
I have (4) Duracell Ultra 6V Golf Cart Battery and there spec say (215AH at 20 hour rate). There are wired in Parallel, so my understanding is I've got 215x4=860AH and 50% of that would be 430AH. So using the formula would it be 430/12.5=34.40 or about 35 hours?
Now all I had running was the TV at 150 watts and I understand (like you said) that will most likely go up as other lights, and small usages increase. But the formula is the same correct?
Draw / 12.5 = hours for use before recharge.
Nope. They are wired in series/parallel. Either that or you have a 6v RV. Two pairs of 6v wired in series and then the two pairs are the wired in parallel. This makes 12v and 430 AH.

So you have 430AH and 50% is 215 AH.
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Old 02-25-2020, 10:54 AM   #5
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With 2 batteries wired in series the voltage doubles, but the amp hours don't. So a pair of 6V batteries put out 12V at about 220AH.

Assuming you have a flat screen LED TV it should only pull 1-2 amps(12-24 watts) of 12V DC through the inverter.

Pulling 155 watts is really high. Something other than the TV is on. Unless you have an old picture tube TV.

If you are going to dry camp or boondock you want to find the source of your phantom loads and work to get down to 1 to 2 amps with everything turned off.

Also be sure to turn off your inverter. An inverter in standby(not powering any 120V devices) will pull from one to three or four amps depending on size & make of inverter. Note that just 2 amps is 48amps in 24 hours or almost 25% of your total usable battery capacity.
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Old 02-25-2020, 11:25 AM   #6
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There could be lots of reasons why the OP is reporting using 155 watts. If he has a DC compressor fridge, he will be drawing about 40 watts to power that. Also I think your TV draw is low. I measured 3-4 amps on my small 20", 3-4 more if the DVD player were running and 3-4 if the satellite dish were running.

It all adds up fast if we try to maintain our home lifestyle while camping.

See the Dry Camping thread for ways to manage lots of current draw- https://www.winnieowners.com/forums/...er-354344.html


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Old 02-25-2020, 12:24 PM   #7
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OP seems to be confusing watts and amps.

If his inverter is saying 155 watts that's 12.9 amps. Depending on his inverter it will use 3 to 6 amps just to convert 12v to AC. Even a propane fridge will use 4 or 5 amps. And, if his fridge is on Auto when the inverter is on it probably switched from Propane to AC. IF that's the case that's another 6 to 8 amps.

If his inverter was saying it was 155 AMPs well then that would be super odd unless he was running the air conditioner, microwave and a hair dryer.

So, it must be 155 WATTS as he mentioned and that's an easy 12 amp draw that makes perfect sense. Watts divided by volts = amps.
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Old 02-25-2020, 01:19 PM   #8
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Originally Posted by creativepart View Post
OP seems to be confusing watts and amps.

If his inverter is saying 155 watts that's 12.9 amps. Depending on his inverter it will use 3 to 6 amps just to convert 12v to AC. Even a propane fridge will use 4 or 5 amps. And, if his fridge is on Auto when the inverter is on it probably switched from Propane to AC. IF that's the case that's another 6 to 8 amps.

If his inverter was saying it was 155 AMPs well then that would be super odd unless he was running the air conditioner, microwave and a hair dryer.

So, it must be 155 WATTS as he mentioned and that's an easy 12 amp draw that makes perfect sense. Watts divided by volts = amps.
My Trimetric battery monitor shows that the control board on a gas elect fridge pulls 0.1 amp and when it pick the propane valve that is another 0.1 amp for a total of 0.2 amps of 12V DC.
When I forget to take my fridge off of auto or AC only and turn the inverter on, I see a jump of about 20-30 amps on the Trimetric when the fridge starts to cool. The fridge AC heater element pulls 2-5 amps of 120V AC power.
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Old 02-25-2020, 02:44 PM   #9
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All1florida is correct above. I have measured similar values on a absorption fridge.

Also to creativepart's post above, there is no way an inverter takes 3-6 amps with no load if that is what he is saying. Maybe an amp or less at no load and additional losses of 5-10% depending on load.

Yes the OP has to be talking about 155 watts not amps. The standard inverter is 1,000 watts, so it wouldn't take 155A DC even at full output.

As I pointed out above, he has several moderate loads that add up to 155 watts. He needs to go through the coach, turning off breakers and watching the amp display until he finds what is using the power. Then turn it off or plan to run the generator every 12 hours or so as long as he makes the upgrades discussed in the referenced Dry Camping thread above. Otherwise he will be running the generator all day.

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Old 02-25-2020, 04:00 PM   #10
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Also to creativepart's post above, there is no way an inverter takes 3-6 amps with no load if that is what he is saying. Maybe an amp or less at no load and additional losses of 5-10% depending on load.
David
I just checked my Magnum MS2012 PSW inverter specs. They spec that the no-load inverter idle is 2 amps. I know that other less efficient inverters use more at idle which is why I ranged it 3 to 6.

But if his inverter is on then who knows what incidental loads may have powered up - like phone chargers, MiFi devices, routers, cell boosters, etc. We have no idea what all could be powered up.

My point was that 155 watts showing on his inverter didn't necessarily indicate anything unusual.
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Old 02-26-2020, 06:20 PM   #11
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Simplifying Units To Basically Understand How Long Your Batter Bank Will Last

I think the OP is reading Watts of power based on the AC consumption of power as displayed on his panel. However, this is very misleading information for reasons I will explain below.

However, when you switch your display to select the rate in which you want to charge your house batteries, whether you are selecting or reading Watts or Amps on display... then you are talking DC-Amps of charging power to the 4 batteries and you need to divide that by 4 (assuming you have 4 house batteries). And there is a spec for your batteries on how fast you can recharge them. For example, you can charge AGM batteries faster by driving more DC-Amps back into the battery vs standard lead-acid batteries with venting caps.

Note: TIP... Did you know you can add four ounces of Mineral Oil per 6v cell? And the reason you would do this is, because that will slow down the evaporation rate of water. (3 cells per battery x4 batteries is 12oz total will do it.)

==== MORE INFORMATION ON POWER ===

IMO, you should stop thinking about how much power (Watts) you are using... and start thinking in terms of DC-Amp-hours.

That means an AC appliance (aka "AC-load") will need to be converted to DC-Amps and then multiplied they the number of hours used... to determine how long your house batteries will last, because they are rated in DC-Amp-Hours.

So you need to know how to "normalize" your units. And by that I mean you need to think in terms of DC-Amps like this when running an AC load.

Why: 120Vac/10Vdc = 10 ...As is AC current needs to be multiplied by 10 to arrive at DC-Amps.

Example of a residential refrigerator: Your 1-3 AC-Amp residential refrigerator will draw 10-30 DC-Amps from your battery bank. I.e., your residential refrigerator nominally runs at 1A-ac, but when the compressor first starts up it will pull 3A-ac or 30A-dc for a few seconds.

Example of a 1A-dc Fan: Your DC appliances are rated in DC so just accept this value of 1A when figuring out your DC-Amp-hours.

If you are confused by the term "Amp-hours" this is simply the amount of time you run your appliance. For example: If you run a DC Fan that consumes 1A-dc for 4 hours, then it follows 1x4=4 Amp-hours. ...And if your residential refrigerator runs 11 hours/day then your 1.5Amps-AC (on average) x11 hours x 10 = 165 Amps(DC)-hours.

(Compare this to 200AH of battery storage if you want to treat your battery bank "nice." See blow on how we come up with 200AH.)

So now you know why your 4-GC2-6V House Batteries will only last 1 day, when you have a residential refrigerator and run other appliances like lights, computer, tv, etc.

So... assuming you decide to follow the 40-50% rule, which basically means: If you treat your batteries nice, by only running them down to 40-50% levels, then your 420 Amp-hour bank (when fully charged) will only provide you 252 DC-Amp-Hours of service. (420x60%=252)

And to repeat what other people have said, it is true that 4-GC2-6V batteries (rated at 210AH each) when wired in parallel, means you have 210x4 divided by 2 = 420 DC-Amp-hours of stored energy at best!

But the story does not end there, because your inverter is only 85-90% efficient. So that means you need to clip 15-20% off the 252 AH and you get ~200AH, which is more realistic.

==> So know you know why you are running your generator 3-4 hours/day to recharge your house batteries.

==> And your first day on battery power will probably be your best day, because each subsequent day you will not be able to recharge your batteries up to full potential, because batteries need to "come to rest" in order to minimize internal resistance and recharge fully.

(Never-mind what your panel says! It does not read very accurate unless your batteries have been a rest for 10+ hours and that rarely happens when you are boondocking.)

That is internal resistance? Well if you touch your batteries you will find them very hot. And that tells you your internal resistance is "pushing back" on the charge current coming from your charger... which for most of us is built into our Inverter. Hence the name "Inverter-Charger".

...And if you have 400W of Solar Power on your roof, then you have to think about energy conversion and efficiency losses there, which is another discussion, but basically this means you can trim 1 hour off your generator use if you have a residential refrigerator installed.

On the other hand, if you have a gas-absorption refrigerator then your cooling is derived from "entropy;" and you use propane fuel as your source of energy, so your battery bank is spared from powering your gas-absorption refrigerator. ...But even so, you will probably need to run your generator every other day. (...And no I'm not talking about you "Survivalists who can go 1 week between generator starts."

Alternatively, if you don't want to follow the 40-50% rule, you can just run the S*^t out of them and get more use between generator starts, but I'm guessing you will be replacing your $500 battery bank every year... and your battery cells will warp, which will reduce your stored energy over time.

This is why Walmart/Sams Club will no longer guarantee batteries installed in RVs. I.e., many RV owners were not treating their batteries very nice and they would just bring them back at the end of their camping season under warranty.

Anyway, I hope this helps you understand your "Panel" and amp-hour use. Remember, you need to think in terms of DC-Amp-Hours and not worry so much about Watts shown on your panel, other than more watts displayed means more power consumed. (Da!)
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