Q's on Calculating Power Consumption Requirements

[dave_in_delaware]

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.

I believe most "RV types" calculate the same way you guys have told me, but they STOP at the amp-hours per day required. This is what I thought was the correct way of calculating needs.

For example: In my earlier understanding of the calculations (while using my current winter Ah number), I would need 180 Ah of battery power per day. Having two 198 AH batteries in series would total 396 total Ah, and the rule of not dropping below 50% would still be 198 AH I could take each day. 198 Ah - 180 Ah still leaves a surplus of 18 Ah in the batteries, so I'd be low, but I'd still be good-to-go.

Nowhere that I've seen or read about sizing an RV's battery/solar systems did it say to multiply the Amp-Hours by a factor of 5. THIS is where my surprise lies. So, all this time I've been thinking that I'd be good with 2 198Ah batteries and solar panels, but with everyone's experience here it's more like 6 batteries of the same size, which there's no way I have room for. AND, these calculations do NOT take into account that solar can't put back that same amount of Ah into the battery as it was removed. Even the "store-bought" RV's that I'm basing my designs on carry no more than 2 AGM batteries or 2 or 3 solar panels.

Which is why I'm not understanding HOW every RVer out there with a similar setup isn't going through batteries every year from overly deep cycling, or isn't draining their batteries every night while boondocking, or isn't complaining about not having enough power for their lights, their electronic entertainment, or more specifically, their heating needs in the winter and cooling needs in the summer. Which is where my biggest power issues are at this point of the design stage. And while some do carry generators and use them to replenish their battery power, some do not use a generator and claim that they can boondock for a week or so. Huh?

Anyway, sorry for the rant. I'm not questioning the knowledge of the members here. I'm questioning the "claims" by the RV manufacturers and RV owners that their setups are adequate for boondocking, but based on my calculations there is no way they can be....

In the meantime, I'll keep looking for a lower amp circ pump. Maybe I am looking at "industrial" types of pump when I don't need one that beefy? I'll also keep looking for lower power fans. Maybe a lower CFM rating on the fan will help yet still blow the heat into the RV enough? Further research will tell.

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.

Wow. So we're talking about 65% of nameplate, PLUS 5% controller loss, PLUS 20% battery loss for a solar panel's actual use? That's 90% loss. WTF? Luckily i'm not using an inverter, too, or else then I'd have 100% loss....

And lights waste more energy from the driver electronics as well? I can't seem to win here.

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.

I wasn't comparing the two components. I know an alternator can generate way more power than a solar panel can for putting juice back into a battery. I was saying that I'll use BOTH components in my design so I can get all the help I can.


Thanks again for all your replies. I'm not trying to sound like I'm doubting anyone's info here. I'm not. You guys know what you're doing. I don't. And apparently many other RVers don't when calculating the electrical system size requirements.

[Mike90250]

Sorry, I was not clear between Loss and Efficient

100w panel (nameplate)
80% of nameplate = 80W (20% loss)
65% of nameplate = 65W harvested (35% loss)

Batteries are 80% efficient (20% loss)

Controllers loose 5% (95% efficient)

So you are only down 60% from nameplate. If you can tilt your panels, you can gain some of that back.

[Sunking]

For example: In my earlier understanding of the calculations (while using my current winter Ah number), I would need 180 Ah of battery power per day. Having two 198 AH batteries in series would total 396 total Ah, and the rule of not dropping below 50% would still be 198 AH I could take each day. 198 Ah - 180 Ah still leaves a surplus of 18 Ah in the batteries, so I'd be low, but I'd still be good-to-go.

OK here is where you are gong off-track. If you put a 12 volt 198 battery in series with a another like battery, it is still a 198 AH battery. Difference now it becomes a 24 volt 198 AH battery. 24 volt @ 198 AH = 12 volt @ 396 AH = 4752 watt hours.

Like I stated earlier forget AH, work in watt hours. Then at the end you can convert to AH. You are making it harder and introducing errors along the way with all the conversions.

Process goes like this

1. Add up and total daily watt hours needed. For example let's say that is 1000 watt hours or 1Kwh.

2. Multiply the daily watt hours by 5, and divide by the battery voltage. So [1000 wh x 5] / 12 volts = 417 AH. Battery work is done move on.

3. To find panel wattage needed multiply daily wh by 2, then divide by the area's worse case Sun Hours assuming clear view of horizon and optimum tilt angle. So let's say in your are you have 2 Sun Hours. [2 x 1000 wh] / 2 hours = 1000 watts. Now here is where you get into trouble. If you do not have optimum panel tilt, and clear view of the Horizon, it is going to take a lot more panel wattage.

[Sunking]

So you are only down 60% from nameplate. If you can tilt your panels, you can gain some of that back.

Plus have full unobstructed view of the east-south-west horizon.

[dave_in_delaware]

Sorry, I was not clear between Loss and Efficient

100w panel (nameplate)
80% of nameplate = 80W (20% loss)
65% of nameplate = 65W harvested (35% loss)

Batteries are 80% efficient (20% loss)

Controllers loose 5% (95% efficient)

So you are only down 60% from nameplate. If you can tilt your panels, you can gain some of that back.

Oh, gotcha. Whew. You were scaring me earlier. I think I understand now.

I'm sure some MPPT controllers are more efficient than others? Or is 5% the standard across the board?

OK here is where you are gong off-track. If you put a 12 volt 198 battery in series with a another like battery, it is still a 198 AH battery. Difference now it becomes a 24 volt 198 AH battery. 24 volt @ 198 AH = 12 volt @ 396 AH = 4752 watt hours.

Like I stated earlier forget AH, work in watt hours. Then at the end you can convert to AH. You are making it harder and introducing errors along the way with all the conversions.

My apologies. I'm not trying to make things more difficult for myself. So either way (12v or 24v) we're talking about the same Wh value.

1. Add up and total daily watt hours needed. For example let's say that is 1000 watt hours or 1Kwh.

I did that for two worst-case scenarios (based on heating or cooling devices running):

Winter = 2110 Wh
Summer = 1434 Wh

2. Multiply the daily watt hours by 5, and divide by the battery voltage. So [1000 wh x 5] / 12 volts = 417 AH. Battery work is done move on.

Winter = [2110 Wh x 5] / 12 = 880 Ah
Summer = [1434 Wh x 5] / 12 = 598 Ah

3. To find panel wattage needed multiply daily wh by 2, then divide by the area's worse case Sun Hours assuming clear view of horizon and optimum tilt angle. So let's say in your are you have 2 Sun Hours. [2 x 1000 wh] / 2 hours = 1000 watts. Now here is where you get into trouble. If you do not have optimum panel tilt, and clear view of the Horizon, it is going to take a lot more panel wattage.

I'll have to find this info for my area and continue with the calculations....

It also sounds like I'll need to figure out a way of tilting the panels on the roof, and maybe even rotating them to follow the sun, instead of mounting them in a fixed horizontal configuration.

[russ]

Look at PV Watts to get the insolation for your area. There is a link on the forum page.

[Sunking]

I'm sure some MPPT controllers are more efficient than others? Or is 5% the standard across the board?

No sir. There are two types of controllers PWM and MPPT, and there is a lot of difference.

For ball park calculations we lump all inefficiencies right up front to account for them. PWM best case efficiency is 66%, and best case for MPPT is 96%.

If using a PWM controller when you figure out daily watt hours, is where we accounted when we multiply by 2 for 50% overall efficiency.

If using a MPPT controller we multiply by only 1.5

FWIW for you I assumed PWM in our examples.

I did that for two worst-case scenarios (based on heating or cooling devices running):

Winter = 2110 Wh
Summer = 1434 Wh

I'll have to find this info for my area and continue with the calculations....

OK this is where the news gets really bad for you. 2000 watt hours is a large number to make even for a home system. In your home state of Delaware December insolation ranges from 1.7 hours at 0% tilt, up to 2.6 hours @ 90% tilt. So to make 2110 wh requires anywhere from 800 watts to 1250 watts. I do not think you physically have that much room on an RV not to mention the expense involved. Winter camping is really putting a huge demand on things. For summer all you need is 250 watts in Delaware.

[Mike90250]

I'm sure some MPPT controllers are more efficient than others? Or is 5% the standard across the board?

Controllers, genuine ones, have very little internal loss, but the knockoff clones sometimes are worse than a piece of wire.

MPPT controllers "match" the PV panel power to the battery, with an active circuit, like a DC-DC transformer. Very little loss, and good coupling efficiency

PWM controllers are basically a voltage controlled "knife switch" that connects and disconnects the PV to the battery - again very little loss INSIDE the controller, but you have no "matching" from the PV to the battery, so you loose system efficiency as the array voltage increases beyond the battery voltage. An 18V Pmax array has little loss charging a 12V battery @ 15V, but a 28V Pmax array will loose almost 50% of it's power, charging a 12V battery @ 15V.

It's a hard concept to grasp but the voltage difference between the 15v charge at the battery and the V Pmax spec on the panel, is "lost" and gone forever with PWM
28v - 15V = 13v @ 2A =26 watts "lost" from a 56w panel.
(assuming it's a 2A, 28V panel just for instructional purposes.)

[dave_in_delaware]

Look at PV Watts to get the insolation for your area. There is a link on the forum page.

I had already seen that link before and bookmarked it! Thanks!

For ball park calculations we lump all inefficiencies right up front to account for them. PWM best case efficiency is 66%, and best case for MPPT is 96%.

If using a PWM controller when you figure out daily watt hours, is where we accounted when we multiply by 2 for 50% overall efficiency.

If using a MPPT controller we multiply by only 1.5

FWIW for you I assumed PWM in our examples.

I'd be multiplying by 1.5 then, as I've only seen MPPT controllers available for the systems I'm interested in. Actually, I never heard of PWM Controllers until I found this forum.

OK this is where the news gets really bad for you. 2000 watt hours is a large number to make even for a home system. In your home state of Delaware December insolation ranges from 1.7 hours at 0% tilt, up to 2.6 hours @ 90% tilt. So to make 2110 wh requires anywhere from 800 watts to 1250 watts. I do not think you physically have that much room on an RV not to mention the expense involved. Winter camping is really putting a huge demand on things. For summer all you need is 250 watts in Delaware.

I'm a little confused on one of your insolation numbers. I found my area's insolation on the PVWATTS site and I came up with 1.7 hours at 0 degree tilt (same as you), but for the 90 degree tilt, am I still looking at December's number? If so, it's 2.81 hours (higher than your 2.6 number). Or is it just the lowest number in the chart? In that case, it's 2.53 hours (for June, a little lower than you got). And how'd you calculate the summer panel wattage? I'm not getting anything near that 250 number. I guess I'm using the wrong numbers from the insolation tables??

0 degree tilt table:
1** 2.06
2** 3.00
3** 3.90
4** 4.88
5** 5.61
6** 6.34
7** 6.03
8** 5.53
9** 4.34
10* 3.36
11* 2.20
12* 1.68

90 degree tilt table:
1** 3.46
2** 4.01
3** 3.44
4** 3.01
5** 2.61
6** 2.53
7** 2.58
8** 3.14
9** 3.32
10* 3.87
11* 3.23
12* 2.81

... It's a hard concept to grasp but the voltage difference between the 15v charge at the battery and the V Pmax spec on the panel, is "lost" and gone forever with PWM

If PWM is so bad and inefficient, why even consider it over a MPPT? Can it do something that MPPT can't?

Thanks again, guys! I'm trying to learn a lot here.

[Sunking]

I'm a little confused on one of your insolation numbers. I found my area's insolation on the PVWATTS site and I came up with 1.7 hours at 0 degree tilt (same as you), but for the 90 degree tilt, am I still looking at December's number? If so, it's 2.81 hours (higher than your 2.6 number). Or is it just the lowest number in the chart? In that case, it's 2.53 hours (for June, a little lower than you got). And how'd you calculate the summer panel wattage? I'm not getting anything near that 250 number. I guess I'm using the wrong numbers from the insolation tables??

Dave I am just demonstrating the extremes between June July 6 Sun Hours compared to December 2 or so hours. Winter demand is what is killing you because of the higher power usage and much shorter days.

As to PWM or MPPT comes down to economics. For smaller wattage systems of about 200 to 300 watts or less, it is less expensive to use a PWM controller and a higher wattage panel, than a smaller panel wattage with a very expensive MPPT controller.

[dave_in_delaware]

Dave I am just demonstrating the extremes between June July 6 Sun Hours compared to December 2 or so hours. Winter demand is what is killing you because of the higher power usage and much shorter days.

Oh, OK. I thought you were getting the numbers from the insolation table, and I was somehow getting different numbers. No problem.

I'm currently researching different circ pump manufacturers to find one with a lower power draw. One company I'm looking into has a variable voltage AND variable flow rate pump, so I'm hoping that will improve my numbers.

Another place I'm looking into for saving wattage is the heater core fan. If it's running 24/7 for heating the RV, it'll have to have a much lower amperage than I'm getting now. I don't want to go too low though, then the air flow (CFM) numbers will be too low to get the heat into the RV.

I'm realizing that these RV systems are a careful balance between power draw, size, and their own specs to get optimum performance.

As to PWM or MPPT comes down to economics. For smaller wattage systems of about 200 to 300 watts or less, it is less expensive to use a PWM controller and a higher wattage panel, than a smaller panel wattage with a very expensive MPPT controller.

I'll have to finish my calculations to see which size of panels I'll need, and which controller I'll be using. Although that might be determined by what my local supplier/installer offers.


Once I "finalize" my calculations, I'd like to post some of the results here to make sure I'm doing this stuff correctly. I really appreciate everyone's assistance so far. I'm trying to grasp all of these concepts and make sense of them.

[Sunking]

I'll have to finish my calculations to see which size of panels I'll need, and which controller I'll be using. Although that might be determined by what my local supplier/installer offers.

Dave sounds like you are doing this right by taking your time and doing the proper homework. Don't screw it up if local suppliers do not have what you need. You can get whatever you want on the web and there are a lot of good dealers out there. It only takes 2 or 3 days for delivery.

FWIW do not take this wrong. Just buy you two of the largest panels you can fit on top of the RV, a good MPPT controller, two high quality AGM batteries, panel volt meter, 1 100 amp battery isolator relay, wire it to your alternator, and call it done. Lastly just realize you are going to have to use the engine alternator from time to time.

[dave_in_delaware]

Dave sounds like you are doing this right by taking your time and doing the proper homework. Don't screw it up if local suppliers do not have what you need. You can get whatever you want on the web and there are a lot of good dealers out there. It only takes 2 or 3 days for delivery.

Oh, I'm definitely taking my time. I've been thinking about this RV project for a while, and designing the "RV fit-out" part off and on for the past three years. I don't have the base van yet, nor do I think i'll be affording it anytime soon. So, I'm doing my homework now since I have time.

There are two specific local companies I want to check out. One is a battery place and they carry all kinds of AGM batteries, chargers, etc. The other place is a solar panel installation company for converting homes to off-grid power. The two companies have worked together to set up a few RV-type vehicles, so at least I know there's that sort of "one-stop-shop" kind of possibility just a 5 minute drive away.

FWIW do not take this wrong. Just buy you two of the largest panels you can fit on top of the RV, a good MPPT controller, two high quality AGM batteries, panel volt meter, 1 100 amp battery isolator relay, wire it to your alternator, and call it done. Lastly just realize you are going to have to use the engine alternator from time to time.

FWIW, that sounds exactly like what Sportsmobile does with their RV builds. But like I said, based on what you guys have been telling me on this forum, I need much more than that to handle the daily loads I'm thinking about.

[Mike90250]

forget the heater, put on a sweater. or snuggle.

[dave_in_delaware]

forget the heater, put on a sweater. or snuggle.

Oh, I plan on snuggling with the wife when we're winter camping. I'm planning on having sub-zero sleeping bags and blankets for winter, too.

But I thought it would be nice if we could warm the RV up while we're outside hiking or mountain biking. I at least plan on using the RV as a "base" while winter trail riding. I don't plan on being one of those "snow birds" that head south when the weather gets cold. I want to use the RV every season in Delaware and neighboring States.