Give it to me straight: Pacific Northwest RV Solar Practicality

[amwbox]

I've been researching off-grid, boondocking style RV solar power systems for a few days now, and feel like I have a decent grasp of the fundamentals.

While I've found that yes, there is a massive decline in panel effectiveness under cloudy conditions, mostly it seems to be written of because eventually the sun does come out, and averaged over an arbitrary time from the power does add up. But, I don't think this is a useful consideration for RV boondocking.

Here in the Portland, OR area, the sun might not come out for as much as a month at a time for half the year. We are very pale, hairy, and microbrew addicted here. So what do I do? Do I have to massively overbuild the system? As in 600 watts worth of panels on top of a small camper, in the hopes that even with the tiny trickle of power from November-April I'll be able to skate through, which will mean I'll be massively overpowered for the sunny half of the year?

My rough plan is something like:

2 6 volt batteries in series, or 4 of them in series parallel if needed.
As much as 600 watts of panels, through a PWM controller, or a MPPT controller if using higher voltage panels, which frankly seem cheaper.

Loads: 27 inch TV set, possible entertainment devices like DVD players, game consoles, etc., a couple laptops, lights, ventilation fan, hopefully a drip coffee maker in the mornings. An electric mattress pad for the winter nights would be great, but I realize it might be a bridge too far.

My estimated max Amp hour usage would be about 100. That's worst case. I expect the average would be closer to half of that.

With the panels barely working in the gloomy winter, should I also connect to alternator for bulk amperage? (I realize alternators suck at charging, but it would be good for heavy lifting while I drive, as the solar can still trickle the rest of the day.)

Does anyone have any experience with severely sun-depleted winter conditions like we have in the Pacific Northwest?

Is solar even really practical for my climate on this basis?


Thanks so much in advance, I realize you must get a lot of newbie questions. Thanks for your patience.

[Mike90250]

You don't overbuild your PV, you accept the fact that you have to use a backup generator. There are some small quiet 1Kw inverter generators that would work with a decent battery charger to top up the batteries on a daily basis.

[amwbox]

You don't overbuild your PV, you accept the fact that you have to use a backup generator. There are some small quiet 1Kw inverter generators that would work with a decent battery charger to top up the batteries on a daily basis.

Generator is not really an option for a stealth boondocking setup. Charging from shore power is also not an option.

Does that mean that solar is truly impractical in this type of climate? As in, not worth the money, just stick with the alternator and accept that I'll be ruining batteries by perpetually undercharging them?

[inetdog]

Generator is not really an option for a stealth boondocking setup. Charging from shore power is also not an option.

Does that mean that solar is truly impractical in this type of climate? As in, not worth the money, just stick with the alternator and accept that I'll be ruining batteries by perpetually undercharging them?

As long as you are willing and able to use the vehicle alternator for emergency and deficit reduction charging, in the same way that someone with a cabin would use a generator, you have, IMHO, met Mike's criterion for a practical design.
But can you really get away with running the vehicle engine at a time and place where you cannot run a generator?

[Sunking]

2 6 volt batteries in series, or 4 of them in series parallel if needed. As much as 600 watts of panels, through a PWM controller, or a MPPT controller if using higher voltage panels, which frankly seem cheaper.

If you use PWM you are forced to use expensive battery panels and wire all the panels in parallel. If you go the PWM route you turn your 600 watt system into a 400 watt system. For much less money using MPPT controller and GT panels you actually get a 600 watt system.

[amwbox]

As long as you are willing and able to use the vehicle alternator for emergency and deficit reduction charging, in the same way that someone with a cabin would use a generator, you have, IMHO, met Mike's criterion for a practical design.
But can you really get away with running the vehicle engine at a time and place where you cannot run a generator?

Probably not. I guess a followup question would be to find out what sort of time it takes to replace the bulk of lost amperage from a 120 amp alternator. Probably a fair bit of driving, which could be done on a daily commute. The solar, even compromised by winter cloud cover, could still trickle charge at a low current rate...but at what I understand to be the same voltage as output by the controller?

I am correct that the voltage remains the same, even if the current is way lower due to cloud cover, right? So, even if its not much current, it still should be charging the batteries, correct, even if very slowly?

Its the higher voltage float charge that seems missing from what alternators can do.

What percentage of capacity should I expect during heavy cloudcover from the panels?

[amwbox]

If you use PWM you are forced to use expensive battery panels and wire all the panels in parallel. If you go the PWM route you turn your 600 watt system into a 400 watt system. For much less money using MPPT controller and GT panels you actually get a 600 watt system.

GT panels?

As I look through available panels, I notice that I can get 285 (thereabout) watt panels that run at 30+ volts. It seems like I could get a couple of those cheaper than the equivalent in smaller 100-140 watt panels that run at 18 volt. I'm not opposed to MPPT, indeed it seems like it could pay for itself by using higher voltage higher wattage panels.

Are fewer panels with higher wattage preferable to more panels with lower individual wattage? Is there some advantage to a larger number of smaller panels?

[inetdog]

Probably not. I guess a followup question would be to find out what sort of time it takes to replace the bulk of lost amperage from a 120 amp alternator. Probably a fair bit of driving, which could be done on a daily commute. The solar, even compromised by winter cloud cover, could still trickle charge at a low current rate...but at what I understand to be the same voltage as output by the controller?

I am correct that the voltage remains the same, even if the current is way lower due to cloud cover, right? So, even if its not much current, it still should be charging the batteries, correct, even if very slowly?

Its the higher voltage float charge that seems missing from what alternators can do.

What percentage of capacity should I expect during heavy cloudcover from the panels?

The alternator is really doing only float charging, initially at high current and then as the battery charges at lower current.

A deep cycle battery, rather than the cranking battery that starts the engine, will not want to be charged at more than about the C/10 rate. That means the current which would theoretically take the battery from zero capacity to full charge in 10 hours.
But if you discharge the battery only to 80% SOC, as recommended, it would take a little over an hour to get the battery to the point where the panels could probably finish the job.
If you have a 200AH battery, that would only be taking ~20A of charge current even though the alternator could deliver as much as 100A after you deduct the needs of the vehicle systems and the cranking battery.

If you absolutely have to charge the battery quickly, you could spend more money (and replace more often) on an AGM type battery and charge it as fast as C/4. If you are at 80% SOC to start, it would take only about 15 minutes at 80A to do the job of getting the battery up to a state where the panels could finish the job.

[amwbox]

The alternator is really doing only float charging, initially at high current and then as the battery charges at lower current.

A deep cycle battery, rather than the cranking battery that starts the engine, will not want to be charged at more than about the C/10 rate. That means the current which would theoretically take the battery from zero capacity to full charge in 10 hours.
But if you discharge the battery only to 80% SOC, as recommended, it would take a little over an hour to get the battery to the point where the panels could probably finish the job.
If you have a 200AH battery, that would only be taking ~20A of charge current even though the alternator could deliver as much as 100A after you deduct the needs of the vehicle systems and the cranking battery.

If you absolutely have to charge the battery quickly, you could spend more money (and replace more often) on an AGM type battery and charge it as fast as C/4. If you are at 80% SOC to start, it would take only about 15 minutes at 80A to do the job of getting the battery up to a state where the panels could finish the job.

...Could be doable. Would leave me about 40 amp hours of use each day, which is significant if limited to ventilation, LED lighting, and a basic laptop. How much of a job would solar panels be doing under these conditions? Is it only 10% of capacity? Less?

[inetdog]

...Could be doable. Would leave me about 40 amp hours of use each day, which is significant if limited to ventilation, LED lighting, and a basic laptop. How much of a job would solar panels be doing under these conditions? Is it only 10% of capacity? Less?

You would have to specify more input about your load in kWh, as well as the total panel power and the equivalent sun hours each day of the month you are designing for.
But with good weather and a good parking location, you could still have PV generating 100% of your load. You just need the additional charging sources to make up for the bad days (bad weather, parked in the shade, etc.)

[amwbox]

You would have to specify more input about your load in kWh, as well as the total panel power and the equivalent sun hours each day of the month you are designing for.
But with good weather and a good parking location, you could still have PV generating 100% of your load. You just need the additional charging sources to make up for the bad days (bad weather, parked in the shade, etc.)

In kWh my estimate is 580.

Estimating total panel power at between 285 and 570 watts, depending on whether or not I need to use 2 285 watt panels.

The problem I'm trying to work through is that here, literally half the year is rain. So my sunny days per month should be considered nil. I'm told, without any specifics, that solar panels still produce power in rainy conditions, just much, much less. I'm trying to figure out how much less.

During the sunny half the year, when I can realistically count on awesome weather for about 3 months at a time, I realize I'd have plenty of juice.

I'm just trying to figure out exactly what efficiency loss I'd experience in the rain, and would like to try to size my system on the basis of the rainy part of the year, as opposed to the sunny part. I'd much rather deal with the enviable problem of a summer surplus than the problem of a winter deficit.

Thanks btw, everyone who's responded.

[PNjunction]

I'm told, without any specifics, that solar panels still produce power in rainy conditions, just much, much less. I'm trying to figure out how much less.

This might help - in my experience with rain, yet with *some* sun peeking through, once you able to faintly detect any shadows, the panels will be producing about 10% of their rated output. Usually less. No shadow - forget it.

Have you actually heard how quiet a Honda EU-2000 or 3000 generator is? Perhaps if you had a demonstration, you might consider this stealth, unless of course mere visibility of it makes the crowd go wild.

I'm with Inetdog on the alternator. At this point under your limitations, your best bet would be to beef up your alternator to one that can handle HUGE amounts of continuous current, and invest in an AGM - which typically are C/4, but I'd go one step further with something like a Concord SunXtender or similar that can just gobble as much as you can throw at it as long as it is voltage regulated.

[russ]

In Spokane, WA or Bend, OR you can do well with solar

In Seattle, WA or Portland, OR it is a loser.

When it is raining in Portland (6 months out of the year) or Seattle (even worse) you will make next to nothing.

[Sunking]

Probably not. I guess a followup question would be to find out what sort of time it takes to replace the bulk of lost amperage from a 120 amp alternator.

Wake up. A 120 amp alternator can generate more energy in 30 minutes than a RV roof full of panels can generate in a week.

[Mike90250]

a) My 3,000 array puts out 200 - 300 watts in cloudy weather. So I would expect 600w of panels to give 50-60w of power, for 3 hours in winter, not much.

b) the little honda eu1000 will be quieter than any automobile, except a EV

c) There is now some production Lithium batteries for drop-in automotive replacement. 1 would cost about the same as 3 auto batteries , and if you gently cycle them 20%-80%, they should last years. Very different charging scheme than lead acid. There are some threads here about them if you feel lucky.

http://www.solarpaneltalk.com/showthread.php?9986-LiFeYPo4-questions