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How do I calculate the inverter / batteries / Solar panel thingy?


MoonTimber

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Using a UPS for power isn't too bad although the UPS batteries are usually rated for a very low number of charge/discharge cycles and charging will be very slow if they are gel batteries.

 

12 volt marine batteries are usually expensive, not the initial cost but the cost per watt of power they will deliver before they die, T-105 type golf cart batteries are usually a good bit cheaper per delivered watt and tolerate deeper discharge cycles so you need less of them to avoid early battery death from deep cycling them.

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What exactly IS the formula for calculating the battery power? I mean without worrying about loss. I ordered that inverter and that power usage meter. So if I plug the TV into the Kill-A-Watt meter, and it gives me back a number, what formula do I use to figure out how long my battery can keep it running before the battery is down to 50% capacity?

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You're kill-a-watt meter will give you a really good idea for all of your 120v items, but doesn't help much for your 12v systems. As a general rule of thumb, I generally calculate individual amp draws at 125% to allow a generous capacity buffer... but then again.. I'm one that doesn't like to take my batteries under 70% SOC (generally not under 80%) so that might be too conservative in your particular application.

 

As was mentioned earlier.. that is where the Trimetric meter realllllyyyy comes in handy. It will read (and log) actual real time power consumption/addition across the board, at the source, regardless of where the draw is coming from (120v, 12v, parasitic, etc). Just sayin... ;)

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It's probably easiest to just look at the amp reading from the kill-a-watt meter. So if your TV is pulling 1.5amps, then that equates to 1.5 amp hours for every hour that the TV is on. If you have a 100ah battery, then you can run your TV for 33.33 hours (100ah @ 50% for 50ah's available divided by 1.5ah draw).

 

 

Except that the kill-a-watt is giving you the amps at 120 volts. 1.5 amps at 120 volts is approximately 15 amps at 12 volts, so the TV would only run for 3.3 hours before the battery is at 50%.

2000 Volvo 770, 500HP/1650FP Cummins N14 and 10 Speed Autoshift 3.58 Rear 202" WB, 2002 Teton Aspen Royal 43 Foot, Burgman 650 Scooter

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Moon Dude (Fun questions on your part, you're really into this and trying to learn so we all enjoy helping)

Posted Today, 08:28 AM

 

QUESTION

 

"What exactly IS the formula for calculating the battery power?"

 

ANSWER

 

The amount of electrochemical energy a battery can store (more lead more acid more weight generally means more capacity) may be listed on it as AMP HOURS (typically 205 to 230 on a 6 volt golf cart battery) Often a traditional engine cranking battery may be listed as CCA or Cold Crank Amps instead of AH, but a deep cycle is often listed as AH) . Something like it can deliver X amps for X Hours. So if your battery has 110 AH of stored energy, it can deliver say 5 amps for 22 hours (5 x 22= 110) or 10 amps for 11 hours (10 x 11=110) etc.

 

NOTE a watt hour meter or your electrical device will give you Watts of use (like your 170 watt laptop) so now you have to convert that to Amps and then to Amp Hours............

 

Since you batteries have to deliver power to your inverter to obtain the necessary 120 VAC and since they operate at lets use 12 VDC nominal

 

To convert Watts to Amps you divide the Watts by 12...........If a device like your laptop uses 170 watts that would compute to 170/12 = 14.16 amps drawn out of your battery to power the

inverter for 120 VAC conversion (NOT accounting for losses!!!!)

 

Almost Finally all you have to do to find how long a battery will power your device is figure how many amps it draws at 12 volts from your battery (14.6 Amps) and Multiply that by the Time (say

1 hour). So that 170 watt laptop draws 14. 6 amps while running, so the Amp Hrs it uses is simply the amps x the time. 1 Hour = 14.6 AH, 2 hours = 29.2 AH and so on.

 

FINALLY to compute how long a battery can power the device, divide the battery AH (110) by the amps the computer requires (14.6 Amps) 110/14.6= 7.5 hours

 

NOTE 1 This does NOT allow for heat losses and inefficiency and I used 12 VDC although that will vary from 12.6 down to near 12

NOTE 2 In my examples above, I did NOT compute the total amp hours as I don't like to draw my batteries down that deep, I prefer to use only 1/2 the amounts listed above

NOTE 3 Quasi Deep Cycle RV/Marine batteries will not work as well as true deep cycle golf cart batteries if you really draw them down to the full rating time and time again. A battery

is only rated for x number of Life Cycles being a full draw down followed by a recharge

NOTE 4 The AH rating may be at a 1 amp discharge rate or a 20 amp rate and that makes a difference, I did NOT take that into account above THIS IS ONLY AN APPROXIMATION YALL

.

"I mean without worrying about loss. I ordered that inverter and that power usage meter. So if I plug the TV into the Kill-A-Watt meter, and it gives me back a number, what formula do I use to figure out how long my battery can keep it running before the battery is down to % capacity?"

 

ANSWER

 

See the above...........Convert Watts into Amps (at 12 battery volts) by dividing watts by 12 (110/12 = 14.6 amps per laptop).............If the battery has 110 AH of stored energy and since your pulling 14.6 amps while running, how long can it power it?????????? 110/14.6 = 7.53 hours BUT THATS A FULL DRAW AND DOESNT ALLOW FOR LOSSES you likely wont get that much time

 

 

I think you should have it now!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Your money your choice, WE REPORT YOU DECIDE LOL

 

John T

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Except that the kill-a-watt is giving you the amps at 120 volts. 1.5 amps at 120 volts is approximately 15 amps at 12 volts, so the TV would only run for 3.3 hours before the battery is at 50%.

 

LOL. No kidding! It must have been past my bedtime and had battery meter readings on the brain.. not the Kill-a-watt.

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My motto has always been, "Any solar is better than no solar and there is no such thing as "too much solar!"

 

So just buying one 50 watt panel and hooking it to your battery with clamps will get you something (as long as you keep track of battery voltage). We have powered our camping lights, laptops and entertainment radio on two 35-watt panels often... and spent 3 years on a sailboat using two 33-watt panels and a home-built wind generator.

 

How much does a 50-watt panel cost? Well, Amazon has this one for under $90.

 

http://www.amazon.com/RENOGY-Watts-Solar-Panel-Listed/dp/B00DCDQKII/ref=sr_1_2?ie=UTF8&qid=1425485508&sr=8-2&keywords=50+watt+solar+panel

 

It does not have a charge controller - and in general I do not recommend that you buy a "kit" that "includes everything" because they ding you on the controllers and cables - but it does have diodes that prevent the battery feeding the solar panels after the sun goes down (I had to use a separate diode on my sailboat). Because it has no controller you need to watch the battery voltage but since it's only 50-watts it isn't likely to do much damage to a good battery: less than 5 amps in full sun, after all.

 

And you can mazimize its effectiveness by aiming it at the sun as the day wears on. Or put it out in the sun while you park in the shade.

 

So, for under $100 you can get some idea of what solar will do for you.

 

Buying the golf cart batteries first is a good idea, though.

 

WDR

1993 Foretravel U225 with Pacbrake and 5.9 Cummins with Banks

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.

. . .

ANSWER

 

See the above...........Convert Watts into Amps (at 12 battery volts) by dividing watts by 12 (110/12 = 14.6 amps per laptop).............If the battery has 110 AH of stored energy and since your pulling 14.6 amps while running, how long can it power it?????????? 110/14.6 = 7.53 hours BUT THATS A FULL DRAW AND DOESNT ALLOW FOR LOSSES you likely wont get that much time

 

 

I think you should have it now!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Your money your choice, WE REPORT YOU DECIDE LOL

 

John T

 

 

That's a really great explanation. I'll have to go over it several times to make sure I've got it, but all the variables are there.

 

Just to clarify your calculation, if I plugged one laptop into one imaginary battery that had zero loss, with imaginary lossless wires and a lossless inverter, and I drained it completely dry, it would last for 7.53 hours. Right?

 

OK, now that we have that part, when you ballpark it, what percentage do you attribute to loss? 10%? (which would be about 45 minutes in this example)

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And by golly, I read it all. Four times! I don't understand what a shunt is.

A "shunt" is a very low resistance resistor. Any resistor "drops" voltage (converts it into heat). A very low resistance resistor doesn't turn much of that voltage into heat but it does turn "some" of it into heat. If we have a meter that can detect just how much of that voltage has been turned into heat (the difference between the voltage at one side of the resistor and the other side) then we can calculate (using Ohm's law) the current through that resistor.

 

So then all we have to do is design a computer that will keep track of that current and display how much of it has passed through that line. Since, in our case, that "shunt" is in series with the cable that goes from the negative terminal of the house battery bank to ground, that would be ALL the current used in powering the house lights, radios, etc.

 

Make sense yet?

 

WDR

1993 Foretravel U225 with Pacbrake and 5.9 Cummins with Banks

1999 Jeep Wrangler, 4" lift and 33" tires

Raspberry Pi Coach Computer

Ham Radio

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You should be able to find a discharge level to lifetime in cycles chart for your battery. In general it will show you a lot of cycles if you use 20% of the capacity of the battery before recharging and not so many cycles if you use 50% of the capacity of the battery. Going below 50% can kill a battery in a week. You have to decide if it is worth adding more batteries to lengthen the lifetime or to just replace the smaller number of batteries more often. Most folks that aren't hitting a weight or space issue find it is better to add more batteries.

 

http://www.trojanbattery.com/tech-support/battery-maintenance/#Discharging (first item)

 

http://www.trojanbattery.com/pdf/datasheets/T105RE_TrojanRE_Data_Sheets.pdf (page 2)

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Sell a customer a Windows computer and you'll eat for a lifetime.

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QUESTION

 

"OK, now that we have that part, when you ballpark it, what percentage do you attribute to loss? 10%"

 

Id guess its actually less then 10% loss overall (wire and inverter and connections etc), but that's not too bad a number to use for estimation purposes. Those late model quality Inverters have efficiencies in the mid or more nineties if I recall. As I said before those battery AH ratings are for around 50% draw down but I don't like to draw mine down that deep. And don't forget then you need a good recharging system, preferably a so called "Smart" 3/4 Stage for best results and battery longevity, although the old style humming heat generating units will get the job "done". Id guess your rig charges the house battery while driving via the engines alternator and even if that's not a "Smart" charge it still pumps electrons back into the battery. Things can stlll "work" even without the late model fancy schmancy equipment we have been talking about.........

 

John T

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My motto has always been, "Any solar is better than no solar and there is no such thing as "too much solar!"

 

So just buying one 50 watt panel and hooking it to your battery with clamps will get you something (as long as you keep track of battery voltage). We have powered our camping lights, laptops and entertainment radio on two 35-watt panels often... and spent 3 years on a sailboat using two 33-watt panels and a home-built wind generator.

 

How much does a 50-watt panel cost? Well, Amazon has this one for under $90.

 

http://www.amazon.com/RENOGY-Watts-Solar-Panel-Listed/dp/B00DCDQKII/ref=sr_1_2?ie=UTF8&qid=1425485508&sr=8-2&keywords=50+watt+solar+panel

 

It does not have a charge controller - and in general I do not recommend that you buy a "kit" that "includes everything" because they ding you on the controllers and cables - but it does have diodes that prevent the battery feeding the solar panels after the sun goes down (I had to use a separate diode on my sailboat). Because it has no controller you need to watch the battery voltage but since it's only 50-watts it isn't likely to do much damage to a good battery: less than 5 amps in full sun, after all.

 

And you can mazimize its effectiveness by aiming it at the sun as the day wears on. Or put it out in the sun while you park in the shade.

 

So, for under $100 you can get some idea of what solar will do for you.

 

Buying the golf cart batteries first is a good idea, though.

 

WDR

 

Based on the calculations everyone has provided, I think that one 100W solar panel might actually do us a lot of good. It wouldn't be enough for us to boondock full time, but for our laptops and TV it might supply enough power that we don't have to always turn the generator on. The manual for my RV says that the engine alternator will charge the coach battery when we're driving, so if I have a solar panel putting a little juice into the batteries for a few hours in the morning and then we drive a few hours to our next campsite, everything might be charged up by the time we pull over.

 

I had not seen that 50W you just pulled up. That's cheap enough I could buy it this week. But I was looking at the 100W kit which would have all the connections I need:

 

Renogy 100Watt Kit $170

http://www.amazon.com/dp/B00B8L6EFA/ref=wl_it_dp_o_pC_nS_ttl?_encoding=UTF8&colid=23242NYAHPMOI&coliid=I3EUHMN3FUG756

 

But they also have just the panel for $150

http://www.amazon.com/RENOGY-Monocrystalline-Photovoltaic-Battery-Charging/dp/B009Z6CW7O/ref=pd_sim_lg_5?ie=UTF8&refRID=1MS341TEJPFMBE5Z0876

 

I can't buy the 100 Watt this week since I just ordered the other hardware, but I should be able to buy one of these kits in my next paycheck. (I budget paycheck to paycheck).

 

I know the kit doesn't come with a great charge controller, but it seems pretty reasonable for something that will probably suffice for our limited boondocking plans. They charge $30 for that controller by itself.

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A "shunt" is a very low resistance resistor. Any resistor "drops" voltage (converts it into heat). A very low resistance resistor doesn't turn much of that voltage into heat but it does turn "some" of it into heat. If we have a meter that can detect just how much of that voltage has been turned into heat (the difference between the voltage at one side of the resistor and the other side) then we can calculate (using Ohm's law) the current through that resistor.

 

So then all we have to do is design a computer that will keep track of that current and display how much of it has passed through that line. Since, in our case, that "shunt" is in series with the cable that goes from the negative terminal of the house battery bank to ground, that would be ALL the current used in powering the house lights, radios, etc.

 

Make sense yet?

 

WDR

 

That's some really cool science!

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Yo Bill B FUN CHAT FOR US SPARKIES AT LEAST LOL

 

"Umm - JT the 10% is a good operational figure."

 

It works for me also as far as an estimate, the older equipment was less efficient with more heat losses while the later models are well up into the mid nineties as I recall

"They traded cheap for long charge times and battery life, both on your expense. Output 3-5 amps, magnetically regulated, so 15 to 20 X longer charge time and continually after that (boiling)."

 

 

Been there done that on the older almost fixed voltage converter/chargers. I had some that ran near a constant 13.6 (ie no Bulk charge up in the fourteens) and finally (took a longgggggggggg time) they over cooked the batteries grrrrrrrrrrrrrrrrrr

 

 

" A PD9280 - http://www.progressi...rters_9200.html - will do most of that in 1 hour and protect the batteries. They also sell conversions"

 

I have a PD 9200 Series with Charge Wizard and love it. It Bulk charges around 14.4+ then Absorption at 13.6 then Floats at 13.2. As I reported previously, even if I drive 6 hours with the engine alternator charging my house batteries and their voltage around 13.9 constantly, soon as I power up the PD it still jumps to 14+ very briefly then 13.6 then 13.2 almost like it didn't care I had the alternator on my batteries all day at 13.9 volts lol

 

John T

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Umm - JT the 10% is a good operational figure.

 

The old charger, not so much. They traded cheap for long charge times and battery life, both on your expense. Output 3-5 amps, magnetically regulated, so 15 to 20 X longer charge time and continually after that (boiling). So if we are on a genny, might as well run it day and night to your and neighbors enjoyment. A PD9280 - http://www.progressivedyn.com/power_converters_9200.html - will do most of that in 1 hour and protect the batteries. They also sell conversions.

 

Remember that the MH is a 1991, pretty close to 25 years old and battery charger and knowledge have changed greatly since then!

 

A good charger is really cheap compared to "propane"?

 

Once Moon gets around the charging and all, and finds the charger, we can get into that aspect of the upgrade and what they want to do. Energy audit is still key to all of our guesses though.

 

Another explaination - http://www.progressivedyn.com/battery_manage_101.html

 

 

Most of that went over my head. It might come into play down the road if I can make changes without "cutting wires".

 

I know I'd love to go RVing for the next 20 years, but I'm not sure about my wife and kids. This RV is a test. It only cost me $4500. I'm lower middle class, but that's not a ton of money for me to risk, especially since I'm confident I can sell it again later for close to what I paid. My wife is excited right now and the kids are begging to go on a trip, but that doesn't mean they will still want to do this a year from now. If my wife stays interested, then next year I'll take out a small loan and get something a little newer. Hopefully ten to fifteen years old with a better interior and better mechanics. I expect that I will be able to move most of the electronics I'm buying right now forward to the next RV. Basically, I'm trying to avoid putting money into this Motorhome that I can't easily unbolt.

 

If my wife loses interest but we decide to keep this motorhome, I might consider putting permanent upgrades into it. But that would be at least two years off. I expect my wife is going to want me to spend several hundred dollars on the interior, and I don't mind. Changes to the interior are changes that will help it to sell faster if we decide to upgrade to another RV. I think most people looking at buying something in this age range are more interested in clean linens than an efficient charging system. But for right now, changes to the engine compartment aren't in my plans.

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Watts = volts X amps = volts X amps

 

100 watts = 120 volts X 1 amps = 12 volts X 10 amps

 

 

That's good paraphrasing and will make my spreadsheet easier to build!

 

Also, you said:

 

"I mentioned LED lights a ways back. They would be sipping from the bucket. The regular lights in there now are using 10 X the power (as heat) to do the same thing. You don't have to change them all, just the ones you use the most often, which will help NOT use up your bucket. (An incandescent light use 1.5 amps- drinker vs a LED that gives the same light output for 0.15 amps- sipper). Don't change the light in a storage compartment, but do change the one you use at the table or for reading."

 

I was thinking about this when I first bought the thing, but I don't know where to find LEDs that fit. Do they sell them at Autozone? Amazon? What do you look for?

 

Most of the motorhome's running lights are dead and need to be replaced. There are at least five interior lights that I think should get upgraded to LED.

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One thing is 140 watts for a laptop is a little heavy. That is probably the max when your battery is dead and is charging. One of the first steps, using a Kil-o-watt meter to get a more real set of numbers. That has to be the first step. My laptop PS is 2A at 125 vac -- 250 watts. Meter shows 55 watts running and 5.4 watts in sleep.

 

Older one, 17" screen (NOT LED, fluorescent backlight, LCD), dual hard drives, 16 GB memory, etc. Just saying

 

Then, from that, I would size the inverter / battery combo.

 

Also, look at lighting and other loads. LED lights use 10% or less than incandescent lights. Energy saved is energy that doesn't have to be made or stored.

 

My Kill-A-Watt monitor arrived in the mail last night. What a fun little tool!

 

According to the Kill-A-Watt, my 170W power supply on my laptop pulls a mere 72W on startup. Most of the time it's only using between 30 and 45 Watts. My wife's laptop seems to run between 15 and 25W!

I have an older 24" LCD TV that we rarely use. I was planning to use it in the RV so I measured it. It seems to use just under 60W. Is that a lot for a TV or is that pretty average?

 

1 Laptop at 45W + 1 laptop at 25W + 1 Television at 60W = 130W.

 

OK, so if I have Bill B's paraphrased formula right, the numbers I'm looking at are:

 

Watts = volts X amps = volts X amps

 

100 watts = 120 volts X 1 amps = 12 volts X 10 amps

 

It just occurred to me that while that formula looked simple I didn't understand how to use it. Does this next part look right?

 

OldJohnT said:

"To convert Watts to Amps you divide the Watts by 12...........If a device like your laptop uses 170 watts that would compute to 170/12 = 14.16 amps drawn out of your battery to power the inverter for 120 VAC conversion (NOT accounting for losses!!!!)"

 

So that means I divide my 130W by 12, right? 130/12 = 10.83 Amps.

 

If I have two batteries that provide a total of 202Ah, I divide 202Ah by 10.83 to get the maximum "Ruin-your-battery" power draw, 202/10.9 = 18.65 hrs.

Is that right?

 

Then I need to limit the time I can draw the power so I don't ruin the battery. I don't know what a conservative draw is, so I'll go with 40%.

 

40% of 18.65hrs = 18.65 * .4 = 7.46hrs.

 

Then, we subtract another 10% for all the power lost in the inverter and wiring. 7.46 - (7.46 * .10) = 7.46 - .746 = 6.71 hours (correct?)

 

If I take don't count any other electronics in the RV (Interior lights, the fridge circuits, etc.) we can run those two laptops and the TV all at the same time for just over 6 and a half hours without ruining the batteries.

 

Now I want to reverse all of this. If I buy the 100W solar panel kit on Amazon, will it be sufficient for our needs?

 

We probably won't exceed 4 hours of laptop time and three hours of TV in a given day. But I'll round that up to 4 hours for everything to keep it simple. So I think that's 4 * 10.83 Amps = 43.32Ah

 

The solar panel will create 100/12 amps per hour? Is that 8.3 amps/ hr?

 

To recharge the missing 43.32Ah will take a 100W solar panel 43.32 / 8.3 = 5.2 hours.

 

Obviously that's not going to be 100% accurate. The panel won't always produce 100 Watts, and energy will get lost in the wiring, but shouldn't that be somewhat close? Won't that be enough to recharge the energy lost from the laptops and TV in less than a day?

 

I'm ready for school again!

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60 watts is a little high for a 24" TV. (Figure about a 66 watt draw from your inverter.)

 

For comparison, I'm looking at this 58" Samsung: www.walmart.com/ip/Samsung-58-1080p-60Hz-LED-Smart-HDTV-UN58H5202AFXZA/39405699
It ranks at the top of the energy star efficiency chart, drawing about 43 watts of power. The 12v draw will be about 47-48 watts depending on the efficiency of your inverter.

 

A typical 12v, DC 24" TV uses about 45 watts, like this one: http://www.roadtrucker.com/12-volt-lcd-tv-dvd-widescreen/naxa-12-volt-led-tv-dvd-24.htm?gclid=CPSLi7iMlcQCFUQ6gQodRAEAZw

The advantage is you won't need an inverter, saving you about 10% additional power draw.

 

Chip

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I'm looking at this 58" Samsung
You're putting a 58" TV in your A-frame? :P

 

 

Yeah, I'm going to remove the rear roof half and bolt it up instead. J/K

 

No, I'm thinking about getting it for a 30ft TT I plan of FTing in in a few years. I want to add a large solar/battery system to it, but since it will be my home I want a large TV but with high energy efficiency for boondocking.

 

Chip

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If you are going to get a new tv look for one with LED screen illumination, that is usually lower power than the old backlights and cooler too.

 

Another way of seeing the formulas:

 

Volts X Amps = Watts

 

Watts / Volts = Amps

 

Watts / Amps = Volts

 

Watts stay (roughly not counting efficiency and resistance losses) constant either at 120 or 12 volts as the current changes.

 

120 volts X 2 Amps = 240 Watts

 

12 volts X 20 Amps = 240 Watts

 

But kick in another 10% for the losses so you actually will have about 264 Watts of DC power use for 240 watts of AC use.

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Sell a customer a Windows computer and you'll eat for a lifetime.

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60 watts is a little high for a 24" TV. (Figure about a 66 watt draw from your inverter.)

 

For comparison, I'm looking at this 58" Samsung: www.walmart.com/ip/Samsung-58-1080p-60Hz-LED-Smart-HDTV-UN58H5202AFXZA/39405699

It ranks at the top of the energy star efficiency chart, drawing about 43 watts of power. The 12v draw will be about 47-48 watts depending on the efficiency of your inverter.

 

A typical 12v, DC 24" TV uses about 45 watts, like this one: http://www.roadtrucker.com/12-volt-lcd-tv-dvd-widescreen/naxa-12-volt-led-tv-dvd-24.htm?gclid=CPSLi7iMlcQCFUQ6gQodRAEAZw

The advantage is you won't need an inverter, saving you about 10% additional power draw.

 

Chip

 

That's good to know. We already have this TV collecting dust so we're going to use it for now, but when TV's starting going on sale in November I'll see if I can track down a 24" to 32" that uses less energy. I can't imagine anything larger in our motorhome, there just isn't space for it.

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If you are going to get a new tv look for one with LED screen illumination, that is usually lower power than the old backlights and cooler too.

 

Another way of seeing the formulas:

 

Volts X Amps = Watts

 

Watts / Volts = Amps

 

Watts / Amps = Volts

 

Watts stay (roughly not counting efficiency and resistance losses) constant either at 120 or 12 volts as the current changes.

 

120 volts X 2 Amps = 240 Watts

 

12 volts X 20 Amps = 240 Watts

 

But kick in another 10% for the losses so you actually will have about 264 Watts of DC power use for 240 watts of AC use.

 

 

OK, so did I do it right?

 

1 Laptop at 45W + 1 laptop at 25W + 1 Television at 60W = 130W.

Your formula: 130W / 12 (Volts) = 10.83 Amps

My two batteries provide 202Amps. Amps are Amps per Hour or Ah (Right?).

202Ah / 10.83 = 18.65 hrs.

To avoid ruining the batteries, we only use 40% of the available power. 40% of 18.65hrs is 7.46 hrs. (18.65 * 0.40 = 7.46)

Then, we subtract another 10% for all the power lost. (10% is 7.46 * .10, which equals 0.746)

7.46 - 0.746 = 6.71 hours

An estimated 6 and a half hours without ruining the batteries for two laptops and one TV.

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