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Author Topic: Is my math right? I'm confused...  (Read 455 times)

DebtFreeRVing

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Is my math right? I'm confused...
« on: July 29, 2020, 10:22:05 PM »
Hi everyone-

Can you help me understand something? If I'm trying to figure out my solar needs in order to run our gaming system off grid... is this correct?

Playstation = 13 amps
TV = 1 amp
Inverter = 3 amps
TOTAL = 17 amps

*If we gamed for 4 hrs/night, it would be 17 amps x 4 hrs = 68 amp hours needed?

**Also, to not drain the batteries over 50%, we'd need a solar kit that could handle a minimum of 136 amp hours...

Is this correct, or am I far off in my understanding of how this works?

Thanks in advance, everyone.

PJ Stough

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Re: Is my math right? I'm confused...
« Reply #1 on: July 29, 2020, 10:28:12 PM »
If you started with full batteries and only used 68 amp hours you would only need a system that could produce 68 amp hours during the day to replace the 68 you used.
PJ Stough   Iowa
2005 Winnebago Voyage 38J

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Lynx0849

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Re: Is my math right? I'm confused...
« Reply #2 on: July 29, 2020, 10:54:12 PM »
Just checking to be clear... your PlayStation and tv run on 12v?
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DebtFreeRVing

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Re: Is my math right? I'm confused...
« Reply #3 on: July 29, 2020, 10:59:06 PM »
They're both 120V (plugged in to the standard outlets)

Isaac-1

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Re: Is my math right? I'm confused...
« Reply #4 on: July 29, 2020, 10:59:13 PM »
Where are you getting those numbers from, note different models of Playstation, even different revisions have very different power draw requirements.  Also is this AC or DC amps, and at what voltage.

A quick google search shows that a current PS4 can draw up to 165 watts, and the PS4Pro can draw up to 310 watts, though actual draw will depend on the game being played

Most modern flat panel TV's in the size range found in RV's typically draw between 25-55 watts.

Inverters are around 90% efficient at converting DC to AC

If we Assume a standard PS and typical TV, your power draw will be around 200 watts per hour, call it 220 watts if running through an inverter to get AC power to feed the RV and PS4.  Maybe a bit less depending on the game being played.

Converting this to AH at 12VDC, and you will get about 220 / 12 = 18.33 AH  or about 73 AH for 4 hours playing time, so I get close to the same answer you have above.

Assuming you have a big enough battery bank, for sizing solar panels, you will need to put about 100 AH back into the battery bank per day once accounting for charge losses, just to cover your gaming use, not any other battery drain running lights, refrigerator controls, fans, etc.

A good rule of thumb for flat mounted solar panels, in mid America average conditions, is that you will get about 5 hours of peak output equivalence per day, in other words a 100 watt solar panel should provide about 500 watt hours per day into the battery bank. So to support your 4 hour per day gaming habit you will need about 180-200 watt worth of solar panels, mated to a good charge controller, preferably an MPPT charge controller, or about 200-225 watts worth of panels if you are going to use a cheaper PWM solar charge controller due to their lower efficiency.  Again this does not take into account any other things draining your batteries.

If you were to play in the daytime when the solar panels have direct sunlight, things get a bit better as you are not taking a hit on battery charging losses.
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DebtFreeRVing

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Re: Is my math right? I'm confused...
« Reply #5 on: July 29, 2020, 11:03:29 PM »
Thank you so much for your response! This helped elucidate things!

Ex-Calif

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  • Dan from Dayton...
Re: Is my math right? I'm confused...
« Reply #6 on: July 30, 2020, 09:57:19 AM »
Using Isaac' number of 73 amps you need solar to generate 73 amps during the day.  Conservative collection is about 6-8 hours - average 7. Efficiency of collection is like 80 percent. So...

73 / 7 = 10 amps of solar collection per hour

10 / .8 = 12.5 amps

12.5 X 12V = 150W of solar.

I personally would not drain the batteries 50% daily.  I shoot for 30% depletion so 240 amp hours is what I would want. One is that it is easier on the batteries and 2 if you have a rainy day you have some residual/backup power.
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Kevin Means

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Re: Is my math right? I'm confused...
« Reply #7 on: July 30, 2020, 10:34:58 AM »
There's nothing wrong with figuring your power needs like you've done - with data from product information documents, the internet etc. Those numbers will get you close, but if you want to get an accurate figure for how much power you consume in a 24 hour period, install a battery monitor. They're installed with a shunt, and they accurately count/display the amps going in or out of your battery bank at any given time.

A battery monitor takes all the guessing out of your battery bank's state of charge, and IMO should be the first device installed when trying to size a solar array to meet your needs - especially for those of us whose goal it is to replenish all the power we consume in a day.

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Lou Schneider

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Re: Is my math right? I'm confused...
« Reply #8 on: July 30, 2020, 10:48:46 AM »
X2 on the battery monitor - they can range from $300 down to about a tenth of that price on a couple of Chinese meters.   The important point is they are able to count both up and down measuring current in and out of the battery so you know the net amount of storage in the battery.

Other uses in the RV can consume a significant amount of battery power.  12 volt items include things like the water pump, furnace fan, exhaust fans and the control boards in the refrigerator and water heater.  Basically anything that operates without having to plug in or start the generator.  Light bulbs (unless they're LED) can use about 20 watts each.

I have this meter and it works well, about the same as the earlier $300 amp-hour meters:

https://www.amazon.com/Voltage-Current-Battery-Discharge-Bidirectional/dp/B01G17XPIE
« Last Edit: July 30, 2020, 10:51:28 AM by Lou Schneider »

solarman

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Re: Is my math right? I'm confused...
« Reply #9 on: July 30, 2020, 01:04:22 PM »
Hi everyone-

Can you help me understand something? If I'm trying to figure out my solar needs in order to run our gaming system off grid... is this correct?

Playstation = 13 amps
TV = 1 amp
Inverter = 3 amps
TOTAL = 17 amps

*If we gamed for 4 hrs/night, it would be 17 amps x 4 hrs = 68 amp hours needed?

**Also, to not drain the batteries over 50%, we'd need a solar kit that could handle a minimum of 136 amp hours...

Is this correct, or am I far off in my understanding of how this works?

Thanks in advance, everyone.


kind of...  as others have stated, you need a reasonably accurate measure of your total power consumption.
then read this: http://www.rvforum.net/SMF_forum/index.php/topic,113514.0.html

I have all the basic calculations done here.. just tailor it for your specific needs.

Note: The insolation link is now obsolete
here is a link to the nrel db i use.. https://nsrdb.nrel.gov/
« Last Edit: July 30, 2020, 01:15:04 PM by solarman »
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Isaac-1

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Re: Is my math right? I'm confused...
« Reply #10 on: July 30, 2020, 04:02:39 PM »
As I see it the biggest issue is the highly variable power draw of the PS4 since power draw is proportionate to compute power needed at a specific moment.
2002 Safari Trek 2830

DebtFreeRVing

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Re: Is my math right? I'm confused...
« Reply #11 on: August 06, 2020, 01:13:05 AM »
Thank you for all of your responses, links, and feedback. I appreciate it!