Q's on Calculating Power Consumption Requirements

[dave_in_delaware]

OK, I have a few really dumb (basic) questions related to calculating power consumption requirements for my RV's battery and solar system. I know I have to figure out exactly which devices I'll be using, how long I'll be using them for in a typical 24-hour period, and how much power they consume, in order to properly size my electrical system....

When a device (TV, computer, refrigerator, DVD player, pump, etc) or an AC adapter for a device has the power information printed on it like:

AC 110-240V~50/60Hz 0.7A
or
Input 120V AC 60Hz 22W, Output 12V DC 800mA
or
Pri: 100-240V~50/60Hz 0.6A, Sec: 12V == 0-2.0A (Set at 2.0A)
or
Amps (Max) = 7

Do these numbers mean for every hour in constant operation, so I'd have to multiply this number by 24? Or is it a different unit of time?

And do I use the input or output (pri or sec) values in my calculations?

And do I use Ohm's Law for the conversions? What final unit do I want everything to be in? Amp-hours? Watt-hours?


Sorry if these are stupid questions, but electronics and electricity were never something I got into before.

Thanks for any clarification!

[Mike90250]

Use the input power when given.

AC 110-240V~50/60Hz 0.7A for 6 hours
is thusly .7A x 6 h = 4.2ah

Convert amps or ah to watts (or watt hours) by multiplying by the voltage
Or convert it all to watts first, and then multiply by the voltage

There are some "energy estimator" web based caculators on the web.

So, the RV rooftop is not likely to be large enough to hold or orient enough PV. Thinking of detachable "sandwich boards" with one side being a panel, or an awning ?

You may be tempted to add 12V batteries in parallel, but that practice leads to huge recharge currents, and equally huge wires. Sometimes having a "different" (24 or 48) system voltage has advantages.

[Sunking]

AC 110-240V~50/60Hz 0.7A for 6 hours
is thusly .7A x 6 h = 4.2ah

Mike sorry to but in however 110-240V 50/60 Hz .7A is meaning less jibber jabber.

At 110 VAC x .7 amps = 77watts.
At 240 VAC x .7 amps = 168 watts

So 77 w x 6 h = 462 watt hours, or in 12 volt battery amp hours = 38.5 Amp Hours
168 w x 6 h = 1008 wh, or 84 AH @ 12 volts.

Dave FWIW work in watt hours, not battery amp hours.

Watt Hours = watts x hours

[Mike90250]

Mike sorry to but in however 110-240V 50/60 Hz .7A is meaning less jibber jabber.

At 110 VAC x .7 amps = 77watts.
At 240 VAC x .7 amps = 168 watts

Yeah, but that was the OP's data. I figured it to be a wide range wall wart supply, and at 120V, it used the max amps, at 240vac, it would use less.

[dave_in_delaware]

Use the input power when given.

So, the RV rooftop is not likely to be large enough to hold or orient enough PV. Thinking of detachable "sandwich boards" with one side being a panel, or an awning ?

You may be tempted to add 12V batteries in parallel, but that practice leads to huge recharge currents, and equally huge wires. Sometimes having a "different" (24 or 48) system voltage has advantages.

Yeah, input power makes sense. Which is what the device needs in order to operate.

However, if the device is something like a netbook or laptop, or an external hard drive, could you use the output power since that's what the device is actually using after it's converted to DC power? Could you then use a DC-to-DC converter to power these devices right off/through the batteries?

Actually, there is plenty of room on the roof of a Ford E-350 EB van. Many owners of these "store bought" RV's use two or three roof-mounted PV modules. I was planning on something like Kyocera (or equivalent) 135 watt panels, which are only 59.1 inches long x 26.3 inches wide.

I was planning on connecting the batteries in series and keeping them 12v. I'm not sure how many amp-hours I need yet, but I've been looking at a few AGM batteries from a local battery store. So far, my choices are: Group27 92 Amp-hour, Group31 116 amp-hour, and a Group4D 198 amp-hour battery. I'm assuming I'd need 2 of them regardless of which size it turns out I need based on the calculations.

Mike sorry to but in however 110-240V 50/60 Hz .7A is meaning less jibber jabber.

At 110 VAC x .7 amps = 77watts.
At 240 VAC x .7 amps = 168 watts

So 77 w x 6 h = 462 watt hours, or in 12 volt battery amp hours = 38.5 Amp Hours
168 w x 6 h = 1008 wh, or 84 AH @ 12 volts.

Dave FWIW work in watt hours, not battery amp hours.

Watt Hours = watts x hours

Sorry for the meaningless jibber jabber. The "samples" I posted were from devices I had in front of me at the time. Maybe it would be more beneficial to me if I posted some of the "stats" of devices I'll actually be using in the RV?

Refrigerator: top-load to help keep the cold IN when opening. I'm looking at an Engel 34 qt or 43 qt portable refrigerator. Their requirements are the same for both models: 2.5 amps at 12v. I'm also interested in the Dometic CoolMatic "coolers" but I'm not sure on the power info on those.

Fresh water pump: Probably something like a Shurflo RV Whisper King Multi-Fixture Fresh Water Pump, 12 VDC drawing 5.5 amps. This is a demand pump so it wouldn't be running all the time. Maybe figure 2 hours total to be safe?

Heating unit: I know an RV furnace can drain a battery overnight, so that's why I'm going with a hydronic heating system instead. I'll create a closed-loop from my water heater (propane) and run the hot water through a car's heater core using a circulating pump. There will be a fan behind the core to blow the heat out. Kind of like a baseboard heat system in a home. Google "Jim Phypers" to see what he did for his 24-foot Travel Trailer. Mine is a smaller scale version of what he did. The pump he used was a Hartell brushless 12v pump drawing 1.7 amps, and the fan was a 16" Dayton/Cliffton 24v fan running at 12v and drawing 1.1 amps. Both the pump and fan would operate continuously, but it was lower than the RV furnace's draw. I'll know specifics when I choose my exact components, but maybe I'll figure twice that of Jim's system to be safe?

Roof ventilation fan: I'm considering at least one Fantastic Fan in the roof for ventilation and air circulation. This unit operates at 12v and draws 3.0 amps on high.

Lighting: I've been considering LED lighting over incandescent and fluorescent since many RVers are converting to this more power sensitive kind of lighting. I haven't looked into any brands yet so i have no idea of any power usage here. I am planning on 1 or two ceiling lights for general van lighting, then a few task lights in specific areas.

Detectors: for fire/smoke, carbon monoxide, etc... I haven't looked into these yet, either. I might go with battery-powered units instead of hooking them to 12v so they're not draining the house batteries all the time.

Entertainment: Instead of having a separate TV, DVD player, radio, and computer for taking care of all entertainment, I'm going to wrap it all into one device and use my netbook. Using it in the RV was the reason I bought it anyway, and it can do everything I need it to do for entertainment purposes. So, its specs are (output from the AC adapter) 12V == 3.0A so if I use a DC-to-DC converter for this I only need to worry about the output number, right? The input is 100-240V ~ 50/60 Hz 1.0A

That's it as far as I know so far, other than the display units themselves for battery levels, solar charger levels, etc. Although I would think they're very small loads.

I have a "block diagram" of my systems that I created from studying the diagrams of other RV's. Would that help to include that here?


So it sounds like I need to calculate all of these things in watt-hours? Then convert to amp-hours to compare against the battery sizes?

Thanks for everyone's help so far.

[Sunking]

So it sounds like I need to calculate all of these things in watt-hours? Then convert to amp-hours to compare against the battery sizes?

Now you are on the right track. Battery Amp Hours is th elast step. Example let's say you do your home work and determine you need 1 Kwh of power per day..

Rule 1 is you never every want to discharge your battery more than 50% at any time.
Rule 2 minimum battery capacity with a generator is 3 days, 5 days without a generator.

So now to find the amp hours required without a generator is 5 days x 1000 wh / 12 volts = 416 AH. FWIW that is about 250 pounds worth.

Last with a RV keep this in mind. The vehicle alternator can generate more power in 30 minutes than what a 100 watt panel can do in 3 or 4 days. Have a heavy duty alternator installed and most RV's already have them and you can stretch that statement out to a week. You can use that power with very little money and equipment.

[dave_in_delaware]

OK, I am hoping I'm doing something wrong here. Can someone check my math and logic with me, please? I've set up an Excel spreadsheet and plugged all my info in on each device....

Netbook
3.0a x 12v = 36.0 Wh, 06 hr/day = 216 Wh/day = 18 ah x 5 = 90 ah req'd

Refrigerator
2.5a x 12v = 30.0 Wh, 24 hr/day = 720 Wh/day = 60 ah x 5 = 300 ah req'd

Water Pump
5.5a x 12v = 66.0 Wh, 01 hr/day = 66 Wh/day = 5.5 ah x 5 = 27.5 ah req'd

Hydronic circ pump
1.1a x 12v = 13.2 Wh, 24 hr/day = 317 Wh/day = 26.4 ah x 5 = 132 ah req'd

Heater core fan
2.5a x 12v = 30.0 Wh, 24 hr/day = 720 Wh/day = 60 ah x 5 = 300 ah req'd

LED lighting
.38a x 12v = 4.56 Wh, 03 hr/day = 13.7 Wh/day = 1.1 ah x 5 = 5.7 ah req'd

LED task light
.19a x 12v = 2.28 Wh, 06 hr/day = 13.7 Wh/day = 1.1 ah x 5 = 5.7 ah req'd

LED task light (2nd)
.19a x 12v = 2.28 Wh, 06 hr/day = 13.7 Wh/day = 1.1 ah x 5 = 5.7 ah req'd

Carbon monoxide detector
.06a x 12v = 0.72 Wh, 24 hr/day = 17.3 Wh/day = 1.4 ah x 5 = 7.2 ah req'd

Propane detector
.05a x 12v = .55 Wh, 24 hr/day = 13.2 Wh/day = 1.1 ah x 5 = 5.5 ah req'd

So, I had done a "worst-case" scenario for the winter (using heater fan and circ pump) and for the summer (using Fantastic fan), and added up each "season"

Winter: 2,110 Wh/day = 176 ah x 5 = 880 ah req'd
Summer: 1434 Wh/day = 120 ah x 5 = 600 ah req'd

Um, WHAT!!? Did I do something wrong with my calculations? I've been figuring I'd need two 198 Ah AGM batteries to handle all my needs. But these calculations tell me I'd need to have 5 of these batteries to handle my load?? WTF?

How do other RVers with similar rigs and similar devices and hobbies run only one big AGM battery?

I know solar panels can put some of the used amps back into the batteries, but not THAT much, and especially not in the wintertime or rainy days.

Help!! Can someone go over my calcs? Please tell me I goofed up somewhere. I can upload the Excel chart if that'll help.

[Mike90250]

you say:
3.0a x 12v = 36.0 Wh, 06 hr/day = 216 Wh/day = 18 ah x 5 = 90 ah req'd

I say:
3.0a x 12v = 36.0W * 6 = 216 wh / 12v = 18ah * 5 = 90 ah req'd

So the math is the same, but the terms were a bit off,

You don't get watt hours till you multiply the 36 w by the 6 hours

And the 18ah *5 is for enough battery capacity to only deplete it 20% in one night.

Adds up quick. Do you need a trailer to haul them with ?

The other RV's are really working the batteries hard, and may only get 2 years life out of them. And if you run a genset 1x a day, you can get a quick bulk charge into them. Solar, if you had 400w for 3 hrs - that's 1,200 wh right back into them.

[Sunking]

Dave you got it right. Wait until you figure out how large a wattage of a panel you need and tally up the expense of it all.

[dave_in_delaware]

So the math is the same, but the terms were a bit off,

You don't get watt hours till you multiply the 36 w by the 6 hours

And the 18ah *5 is for enough battery capacity to only deplete it 20% in one night.

The other RV's are really working the batteries hard, and may only get 2 years life out of them. And if you run a genset 1x a day, you can get a quick bulk charge into them. Solar, if you had 400w for 3 hrs - that's 1,200 wh right back into them.

Oh, my bad. I added an "h" in there before it was time. I corrected it in my Excel chart. Thank you.

What I'm not understanding is that on the Sportsmobile's Price Sheets, they list their auxiliary AGM batteries as "two STD 27 @ 100A = 200 Amp" or "one large 40 AGM = 210 amp" but how are these owners even running their lights, refrigerators, TV's/DVD's, water pumps, etc if I need 3-6x that? I'm not planning on doing anything significantly different than they are. And they have one or two 135 watt (or maybe 185 watt) panels on their rooftops. But still... the numbers aren't adding up for me. I'm just not "getting" this.

If I only had two Kyocera 185 watt panels on my roof, what would be the Wh back to the batteries? I know there are factors like sun intensity, partial shading, panel inefficiencies, controller inefficiencies, etc to factor in. I'm assuming a solar panel probably loses 50% of it's rating?

Dave you got it right. Wait until you figure out how large a wattage of a panel you need and tally up the expense of it all.

Again, based on what I want to do, and what I know other owners with VERY similar setups are doing with 2 AGM batteries and 2 solar panels, I'm just not understanding how they're successfully running everything off of their systems, and boondocking for days and weeks at a time without plugging into shore power. Unless they run a generator, or drive 6 hours every day to use an alternator... well, I just don't understand.

I'll plan on putting in a larger alternator to help charge up the house battery, and a battery isolator. I'm NOT planning on a generator or 110v AC input from a campground so my only charging sources will be an alternator while driving and solar.

[Mike90250]

They are running their batteries into 90% discharged. You can see the numbers yourself. Or they lie.

Use a cataltic heater, no fan.

Consider a small honda eu1000 generator (or a clone) more power and cheaper than running your alternator to recharge.

Your 2, 185w panels, 50% is a generous guess, since they will hardly ever be oriented properly to produce full power. (unless you park slanted on a hill)

[dave_in_delaware]

I looked into the fanless catalytic heater, and for a while there I was convinced they were the way to go. No electric used = good, but using up oxygen in RV = bad, especially while you're sleeping. I'd rather not worry about cracking the windows enough in the wintertime to make sure I actually wake up in the morning.

I'm also not using a typical RV forced-air furnace. They're power hungry and noisy and use up your batteries overnight.

Which is why I've seriously looked into this small simple hydronic heating system based on Jim Phypers' ideas. I figured it wouldn't take nearly as much energy (propane, 12v) to keep my little RV warm compared to his 24' travel trailer. Maybe my numbers are wrong on the power consumption of the fan and circ pump, I don't know.

It looks like I might have to consider a small generator, even though I haven't been planning on one nor did I really want to use one at all while boondocking. Or maybe I should plan on a 110v system and plug in every few days to recharge the batteries?

One thing I'll do is try to tweak my calculations to see if I can save power anywhere. The power-hungry devices are definitely the refrigerator and the hydronic heating system's pump and fan. Maybe I can find lower-power drain models?

50% loss is generous for solar panels? What is a more accurate number for a solar panel? 35%? 25%? 10%? Much lower and they'll be pretty much pointless for the cost....


I'd like to thank everyone for their input and suggestions so far! You've opened my eyes to what these RV systems REALLY need in a REAL LIFE scenario. I'm not sure how other RVers calculate and handle their electrical needs without a generator or shore power. But I know now that their numbers don't seem to really add up. Maybe because they only calculate amp-hours and don't take into account the 5 days without a generator factor? I haven't read where owners are replacing their AGM batteries every 2 years because they keep discharging them beyond the suggested 50%... and i've been planning on keeping mine around 80% or above.

[russ]

Many RV types probably calculate their power like many do with wind turbines - 'it outta be'

The 50% for a battery based system is considering losses through panels, chargers, inverters, batteries themselves etc.

Your hydronic pump may be oversize for what you need? Consider oversizing the tubing to the extent possible to reduce pressure drop.

Russ

[Mike90250]

50% loss is generous for solar panels? What is a more accurate number for a solar panel? 35%? 25%? 10%? Much lower and they'll be pretty much pointless for the cost....

What Russ said.

PV=80% of nameplate, in an aimed/tited array, flat on roof, closer to 65% of nameplate.
5% charge controller loss
20% battery loss
10% loss if you have a good, high efficency inverter.

Lights - a 1 w LED actually burns close to 2 or 3 watts when you measure the driver circuit. Same with CFL's, they rate the bulb, not including the driver electronics for it.

[Sunking]

Dave you cannot compare the vehicle alternator to solar panels, well you can and you will see they do not compare. A heavy duty RV alternator rating are 100 to 130 amps. In terms of wattage is 1400 to 1820 watts continuous. The alternator can generate 1400 watt hours in 1 hour in park with engine idling. Assuming perfect conditions with say 2 130 watt panels in the southern US on a bright cloudless summer day would take two full days. Put those same panels in real conditions in winter could take 2 weeks or more.

That is the problem with solar, hype is everywhere, but reality is dictated by the law of physics, and you cannot break or violate the law of physics no matter what a politician or green mafia says.