formulas for energy audit?

[dddire]

Getting ready to a long trip next summer with a few nights boon docking and thinking generator, battery bank, inverters etc and know I need to do an energy audit for my trailer. Trying to put together a spread sheet and realize I'm just far enough out of high school to find this a challenge.

What formulas do I use to find my 120 V, x wattage consumption, to 12dc amp hours needed in my battery bank?

https://docs.google.com/spreadsheet/ccc?key=0AnPcD7QoHLy2dDVuN0xhXzZPVFp2QkpwMEktMk9YX3c&usp=sharing

[Almot]

Old-Biscuit wrote:
RV Energy Consumption......LINK

I think this list is a little dated. A few examples:

Fans drawing 2.6A at 120V do exist, but people interested in conserving energy today are using 12V fans that draw 7-10 times less.

25" TV may draw 300W, yes, if this is a CRT. People in permanent cabins do have those, mostly because nobody would want to steal it - too old and too heavy. For comparison, 25" LCD TV will draw 30W.

[MEXICOWANDERER]

"Not really. High current loads do distort the amp-hour capacity, but then there are low-current loads as well, and those distort it in the different direction. And if we are not talking about recharging, i.e. battery losses when juice is going in and out, the estimate can be more accurate than 40%."

Any other caveats?

You have made a superb hangman's knot rope argument as to why kWh meters are far superior to amp hour meters.

Spreadsheets and wishful thinking are fun. But spreadsheets and three bucks will get you a cup of coffee at any Starbucks. Energy calculation is not like trying to compute mpg/cost of fuel/and miles driven. Having done this for a living since the advent of inverters I can tell you the very most conscientious estimate is going to be off a country mile unless a person lives like I do, and that style of living would make a Trappist Monk look like Al Gore's lifestyle.

People are not fire hydrants. Their life style evolves. They do not camp in the same place on the same days in the year, in an unvarying climate like we have down here.

I can guarantee you that choosing too small of capacity battery bank, inverter, and method of recharge is a hell of a lot more painful than choosing too large of capacity. I have had customers SCREAM "That's too big! What are you nuts? You're wasting my money" Only to come back in a year or so and contritely admit "Gawrsh I sure am glad you talked me into the bigger unit - we are at near capacity. I had no idea we would want xxxxxxxx later on".

[harold1946]

MEXICOWANDERER wrote:
The best spreadsheet on the face of the earth



And A Few Days Of Use.

Energy audits are a waste of time. I have never seen one performed that was within FORTY PERCENT of reality when applied to recreational vehicle camping, battery capacity and recharging ampere hour values.

X2

[Old-Biscuit]

RV Energy Consumption......LINK

[Almot]

westend wrote:
Try this downloadable spreadsheet Energy calculator

x2

A nice tool. Open Excel spreadsheet in the 1st link and click on the bottom tab that says "RV Load Calc". Make sure you don't count some items twice. Fans are in both DC and AC sections, and there can be items not mentioned there. Use your brains.

P watts = I amps * U volts. So 100W can be 8A*12V, or 0.8A*120V. Inverters have 10-15% loss and this spreadsheet accounts for that.

But, after all, the audit is just an estimate, even if you use actually measured values for amps and watts rather than those from top of you head or manufacturer's label.

Edit - PS:
There are 2 items where you can't trust the watts shown on the manufacturers label.
One is microwave - the label shows "cooking power", and the actual consumed power is about 1.5 times more.
Another one is fridge - a typical propane fridge will draw some 12V current even when not cycling, and the only way to find out this current is to measure.

MEXICOWANDERER wrote:
Energy audits are a waste of time. I have never seen one performed that was within FORTY PERCENT of reality when applied to recreational vehicle camping, battery capacity and recharging ampere hour values.

Not really. High current loads do distort the amp-hour capacity, but then there are low-current loads as well, and those distort it in the different direction. And if we are not talking about recharging, i.e. battery losses when juice is going in and out, the estimate can be more accurate than 40%.

But the values for W, I, U and time need to be as accurate as possible. If they have no idea how many times they are going to make a popcorn in microwave or how many hours to watch TV, then energy audit is a waste of time. Drycamping on a battery without any energy sources means limited use of most 120V devices, and this is what it is - they have to control their habits.

[MEXICOWANDERER]

The best spreadsheet on the face of the earth



And A Few Days Of Use.

Energy audits are a waste of time. I have never seen one performed that was within FORTY PERCENT of reality when applied to recreational vehicle camping, battery capacity and recharging ampere hour values.

[Peg_Leg]

Yeah, so far out of high school that the fingers don't bend as well when counting.

I remember the power formula this way.

Think PIE.

P - power - measured in watts
I - current - measured in amps
E - voltage - measured in volts

Draw a circle - draw a line across the middle, leaving a top half and a bottom half.

Now from the center down draw a line. You now have 3 pieces of the PIE.

Write the letters into the circle - P in the top half - I in the left quarter at the bottom - E in the right quarter.

To find any one, you need to know the other two values.

Place your thumb over the value you want to find.

Examples
Covering P will leave I & E side by side the formula IE is multiplication.
Current (amps) X Voltage = Power (watts).

Covering I (current or amps) would leave P/E (P over E) which is a division formula P divided by E.
Power (watts)/E (volts) = I (current or amps)

If you actually draw this on paper and try it, you'll see it works easier than you can explain it.

I like PIE and cake and cookies.

[SteveAE]

dddire,

An energy Audit is one way to do it (and I would recommend doing one to become familiar with the power usage of the various items).
However, I would suggest getting a battery monitor (Trimetric or similar) that measures how much power you actually use and go camping to see what the actual usage is. From this information (hopefully it is similar to the audit) you can determine how much energy you need to save, generate, and store.

Have fun working through it all,
Steve

[vermilye]

Your wattage is the same, whether you are powering the device at 120V AC or through an inverter powering it from 12V. Since wattage is amps X volts, the amperage is going to be about 10X at 12V than it will be at 120. (Actually a bit more since there will be inverter efficiency losses).

What you need to do is determine what you want to run & for how long so you can determine the number of amp/hours you plan to consume. For example, if you have a 240 watt device that you plan to run for 3 hours, you will use 240/12 or 20 amps at 12V. Over 3 hours that is 60 amp/hrs that you will need to put back in your batteries to make up for what you used. Again, you will actually need to put back more since there is efficiency losses, both with the inverter & charging the batteries.

You also need to determine the amp hour capacities of your batteries. True deep cycle batteries will have published amp/hr ratings. You do not want to draw down your batteries more than 50%, so if you are going to depend on batteries alone, you will need 2X the battery capacity of your calculated usage.

You also need to think about how you plan to put the energy back into your batteries. A generator through your converter, solar, recharging by your tow vehicle are all possibilities.

A useful couple of web pages dealing with batteries are Mark Nemeth's 12V Side of Life, part 1 & 2.

[gatorcq]

Go to this web site

http://www.batterystuff.com/kb/tools/solar-calculator-tutorial.html

[harold1946]

An energy audit is not going to give much useful information. Its normaly done for estimating what is needed in the way of a solar grid to charge a battery bank.
What is it you are wanting to acomplish?

[westend]

Try this downloadable spreadsheet Energy calculator