Questions about solar power

[MCDenny]

Now with the new fridge we enjoyed boondocking so much I'm researching solar panels to see how expensive and practical it would be to extend our off-grid time.

I've determined I'd like to get around 300 to 400 watt-hours of battery charge per day. We are in Florida at about 28 degrees N latitude and the 'full sun' period here is about 6 hours per day.

I would like to learn more about

1) I think a portable unit that folds in half for storage would be the best for us as we try to park the trailer in the shade. We could put the panels in the sun and adjust the position as the sun moves across the sky. Does this mean I can expect more juice than the insulation tables suggest? It seems like they are based on flat or at least stationary panels.

2) my first guess is that I need about 100 watts of capacity. I see kits on offer for between $200 to $600 for this much capacity but I don't understand what the difference is. For instance

this 90 Watt Portable Solar System - Professional Series | Off The Grid RV SolarOff The Grid RV Solar at $625

This 90 Watt Portable Solar System - Economy Series | Off The Grid RV SolarOff The Grid RV Solar at $425

And this 100WATTS Portable Folding Poly Solar Panel 100W 12V RV Boat Battery Charger Kit | eBay at $200

3) I consume between 500 and 600 watt hours per day and have enough battery for 4ish days with no recharging. I'd like to double or triple that. Does a 100 watt panel sound like about the right size?

Thanks for your help.

[Darwin Maring]

Check Harbor Freight. I saw kits there.

[blodn1]

RV Solar System Sizing by "Rules of Thumb"
Here's a good "rule of thumb" guide. The rest of the site is good, too.
The other solar resource, though really for permanent systems, is Handy Bob: 2014 SOLAR IDEAS, BATTERIES & OTHER THINGS « HandyBob's Blog

[Lorelei L]

The Harbor Freight sets are not adequate. Try to get a Kyocera, Renergy or other quality panels. The Renergy 100w panel, expandable with connectors, is $149 on amazon right now. Northern Arizona Wind and Sun really know their stuff and can tell you what you need. windsun.com

[FTTRV]

Costco has a solar charging kit.
Chuck

[David B.]

I had bought a 45 watt Harbor Freight 3 panel set years ago, and never used them. One HF polycrystalline 15 watt panel turned out to be about the same size of a quality 100 watt mono-crystaline panel. This is a big deal if you're only dealing with a 13' Scamp. 100 watt perm. mounted panel was plenty enough panel for a single group 27 battery to keep us fully charged for a couple 60 day straight vacations off grid. We had a Danfoss refrigerator, LED lights, sure-flow water pump, 75 watt inverter for recharging lap top, phones, etc. I am going to check into flexible stick-on panels for the roof of our soon to come Lil Snoozy.
Dave & Paula

[padlin00]

Here's a Solar Irradiance calculator for you. Pick your location and it gives you a figure, multiply the given figure by the size of your panel and you have your daily output in watts. Play with it a bit and you'll see what the best angle is for a given season.

http://www.solarelectricityhandbook....rradiance.html

[MCDenny]

Dave, That's a good data point. Thanks.

How sunny, shady, cloudy was it when you were going 60 days off the grid?

[MCDenny]

Originally Posted by padlin00

Here's a Solar Irradiance calculator for you. Pick your location and it gives you a figure, multiply the given figure by the size of your panel and you have your daily output in watts. Play with it a bit and you'll see what the best angle is for a given season.

http://www.solarelectricityhandbook....rradiance.html

Tried it but got a techy error message. I used my phone though. Does it work for you?

[calimobber]

The best sites I've found for solar are Solarblvd.com and ebay. Harbor F is very expensive. use dollar per watt to compare cost. So $200/100w is $2 per watt. you pay allot more for the foldable kit.

I have 60w on my vehicle, with 2 batteries. I can last a few days before I need to start the vehicle.

Cheers

HandyBob's Blog « Making off grid RV electrical systems work is always a good no BS read about rv solar.

[Carol H]

If your anywhere near an urban area I would check around locally for shops that specialize in solar. We have a company here in BC that sells solar for 90 cents a watt.

[padlin00]

Originally Posted by MCDenny

Tried it but got a techy error message. I used my phone though. Does it work for you?

Yes, it works from my laptop.

[Mike Magee]

100W might do fine most of the time, but you could get a string of cloudy or rainy weather days. If I were doing it I would lean toward 150W of solar. 75W panels at Solarblvd.com are under $100 apiece, and they have controllers as well or you could get a "Solar30" with LCD readout from ebay for about $45 (Norm and Ginny knew someone who installed one of these, IIRC).

[David B.]

Originally Posted by MCDenny

Dave, That's a good data point. Thanks.

How sunny, shady, cloudy was it when you were going 60 days off the grid?

We were pretty much through the gamut of weather (sun/rain/hail/fog/cloudy), as both times we started in sunny AZ, and traveled up the west coast line, California, Oregon, Washington, on over to Sand Point Idaho, Colorado, etc.
Makes me want to get back on the road, although it would be tough leaving this beautiful weather we're having here right now.
Dave & Paula

[peterh]

We have two trailers, a Scamp 5er and a Surfside. We've had the Scamp for seven years now, and installed a 50w roof-mount panel on it shortly after we bought it. It wasn't quite enough for our needs, so we have since installed a second 55w panel. That does it pretty well for us. The panels cost us around $400, I think. Prices have come way down.

We bought the 100w panel we installed on our Surfside from SolarBlvd for $125. Haven't had the trailer out yet, as we're still in the middle of our refit, but I did test the panel in the sun and SolarBlvd has a good reputation.

How much panel do you need? Depends a lot on how much power you use and what size battery you have. For us, 50w was just below our threshold and 100w is major overkill. We probably need about 60w or 75w of panel to meet our needs.

Something we've included in our Surfside plans is a secondary, portable panel we can set out in the sun for times when our trailer is parked in the shade.

[Byron Kinnaman]

Originally Posted by MCDenny

Now with the new fridge we enjoyed boondocking so much I'm researching solar panels to see how expensive and practical it would be to extend our off-grid time.

I've determined I'd like to get around 300 to 400 watt-hours of battery charge per day. We are in Florida at about 28 degrees N latitude and the 'full sun' period here is about 6 hours per day.

I would like to learn more about

1) I think a portable unit that folds in half for storage would be the best for us as we try to park the trailer in the shade. We could put the panels in the sun and adjust the position as the sun moves across the sky. Does this mean I can expect more juice than the insulation tables suggest? It seems like they are based on flat or at least stationary panels.

2) my first guess is that I need about 100 watts of capacity. I see kits on offer for between $200 to $600 for this much capacity but I don't understand what the difference is. For instance

this 90 Watt Portable Solar System - Professional Series | Off The Grid RV SolarOff The Grid RV Solar at $625

This 90 Watt Portable Solar System - Economy Series | Off The Grid RV SolarOff The Grid RV Solar at $425

And this 100WATTS Portable Folding Poly Solar Panel 100W 12V RV Boat Battery Charger Kit | eBay at $200

3) I consume between 500 and 600 watt hours per day and have enough battery for 4ish days with no recharging. I'd like to double or triple that. Does a 100 watt panel sound like about the right size?

Thanks for your help.

First off a 100Watt panel will probably do everything you need. We use a 65 Watt and it's more than adequate to keep two separate batteries charged and only charging the house battery about every 3rd day or less depending one the weather. I would recommend that all lights be changed to LED to reduce battery consumption.
I would also recommend that for solar panel sizing and calculations you use amps and amp-hours rather watt hours. Watts are great way to calculate when using different voltages, i.e. a 1000 watt heater draws 8.3 amps at 120 volts and 83.3 amps at 12 Volts. Your battery is rated in amp hours, the battery is what you're concern about. If you've been talking to a solar "expert" he starts talking in watt-hours, don't walk away, run. He's probably doesn't know what he's talking about and trying to flim-flam you.

[MCDenny]

Originally Posted by Byron Kinnaman

First off a 100Watt panel will probably do everything you need. We use a 65 Watt and it's more than adequate to keep two separate batteries charged and only charging the house battery about every 3rd day or less depending one the weather. I would recommend that all lights be changed to LED to reduce battery consumption.
I would also recommend that for solar panel sizing and calculations you use amps and amp-hours rather watt hours. Watts are great way to calculate when using different voltages, i.e. a 1000 watt heater draws 8.3 amps at 120 volts and 83.3 amps at 12 Volts. Your battery is rated in amp hours, the battery is what you're concern about. If you've been talking to a solar "expert" he starts talking in watt-hours, don't walk away, run. He's probably doesn't know what he's talking about and trying to flim-flam you.

Thanks Byron,

All the lights are LED, phone and computer charging can be mostly done in the car so not much inverter use.

Since putting up this post a couple of days ago I've been doing my own research and learned I can expect the equivalent of five 'full' sun hours each day here in S Fl in January. I have a 220ah battery bank use about 40ah/day so can go 3ish days at 50% discharge. I'd like to at least double that so at a minimum my solar system would need to harvest 120ah over six days. In theory with perfect cloudless sky that would be 5 hrs x 6 days = 30 hrs. 4 amps per hour x 30 hrs = 120 ah.

4 a x 12 v = 48 watts. 100 watts is double my theoretical minimum . Is this an OK safety factor to allow for clouds and shade. We will have a portable, probably folding, panel so shading shouldn't be too bad.

A question about solar panel ratings: typically a 100 w panel is rated at 17 volts output. A controller would knock that down to 13.6v if the battery is nearly full and 14.5 if they are somewhat discharged. Figuring worst case do I derate the panel by 1-13.6/17 or 20% to allow for a PWM controller's "waste". If I got an MPPT controller could I count of the full panel output?

Thanks for your help.

[blodn1]

The higher the voltage of the panel, the more amps you'll actually get into the battery. Panels rated higher (>17V, therefore more costly) will be a better deal than an expensive MPPT for a small system.

[Byron Kinnaman]

Originally Posted by MCDenny

Thanks Byron,

All the lights are LED, phone and computer charging can be mostly done in the car so not much inverter use.

Since putting up this post a couple of days ago I've been doing my own research and learned I can expect the equivalent of five 'full' sun hours each day here in S Fl in January. I have a 220ah battery bank use about 40ah/day so can go 3ish days at 50% discharge. I'd like to at least double that so at a minimum my solar system would need to harvest 120ah over six days. In theory with perfect cloudless sky that would be 5 hrs x 6 days = 30 hrs. 4 amps per hour x 30 hrs = 120 ah.

4 a x 12 v = 48 watts. 100 watts is double my theoretical minimum . Is this an OK safety factor to allow for clouds and shade. We will have a portable, probably folding, panel so shading shouldn't be too bad.

A question about solar panel ratings: typically a 100 w panel is rated at 17 volts output. A controller would knock that down to 13.6v if the battery is nearly full and 14.5 if they are somewhat discharged. Figuring worst case do I derate the panel by 1-13.6/17 or 20% to allow for a PWM controller's "waste". If I got an MPPT controller could I count of the full panel output?

Thanks for your help.

Look at "short circuit" current for maximum current you can expect. It should listed in the specifications. Higher voltage does not necessarily equate to higher current. Actually it the opposite for equal wattage ratings.

[MCDenny]

But how does the controller reduce the panel's 17v to the 13.6v float charge the battery needs. Don't PWM units turn off the panel output( I know, 1000s of times per second) so the average voltage over time is 13.6? That would mean the panel is turned off 20% of the time the sun is shining.

And I thought MPPT controllers had some way of getting all the power and converting it to the lower voltage.

Please enlighten me. I'm just trying to learn this stuff.