What size pannel for my off road trailer

Shaun B

Hi, I have a off road trailer, i have a 90l fridge (i think 3amp) 3x lights 15w each used for max 4 hours daily, and an 600w inverter (very seldom used). with a 105amp deep cycle battery, i have a 45w panel which i was told would do the job but 1.5 days latter guess what No power = warm beer, unhappy wife.

What size panel would i need to keep me going.

BB.

Re: What size pannel for my off road trailer

Welcome to the forum Shawn!

What is the refrigerator power? 3 amps * 12 VDC, or 3 amps * 230 VAC @ 50 Hz or what? I am guessing it is 3 amps @ 12 VDC (you don't use your inverter much)--But then the next question is this 24x7, or 20 minutes out of every hour--Or what?

Also, what is the battery bank (voltage, Amp*Hour rating, flooded cell or AGM/Sealed, etc.)?

A 45 watt panel with about 5 hours of sun per day (are you in South Africa?) will give you around:

• 45 watts * 0.77 panel+charge controller derating * 1/14.5 volts charging = 2.4 Amp*Hours (at 12 volts) per day

Obviously not very much power--Your 3x15 watt lamps use more than 3 amps * 4 hours = ~12+ AH per day just by themselves.

Conservation will be a big help... Those 15 watt (filament?) lamps can probably be replaced by LED lamps that draw around 1-4 watts each. That would be a big help.

The refrigerator, need to understand its 24 hour power consumption. If it is 3 amps * 24 hours -- That is 72 AH per day just by itself:

• 72 AH * 1/0.61 DC system efficiency * 14.5 volts charging * 1/5 hours a day of sun = 342 Watt panel minimum

So, just to keep up with a 3amp * 24 hour per day load, you would be looking at over 342 watts of solar panel--probably about 10x what you have on the trailer today. You may be limited by your roof space as to how big a solar array you can install--Also, I took a guess on the amount of sun per day you have... In many parts of the world, 4-6 hours per day for 9 months of the year, but under 3 hours per day during winter.

Are you living off the grid for weeks at a time in the trailer, or is this just a weekend use setup?

Your Thoughts?

-Bill

Shaun B

Re: What size pannel for my off road trailer

Hi Bill

Thank you very much for your reply, I dident know that solar was so complex,

Yes i do live in South Africa so we do get a lot of sun in summer Sunrise is 5:30 - sunset is 19:00, my charging light will start flashing at about 6:30 and stop at 18:00( if that helps at all).

The Fridge is 12v, but you are right it wont run 24/7 and i think you right when you say 20min out of 60min, and the battery is a sealed unit deep cycle Deltec.

I will change lights to solar, i dident know that they drew so much power.

I also have a Kipor 1.1kva generator, but when you camp in a camp site you normally cannot use them, thats the reason for solar as we want to go away in March for 10 days off the grid, the battery however charges off the car while we are driving which also helps.

Thanks

BB.

Re: What size pannel for my off road trailer

How many Amp*Hours is the battery? somewhere between 80-220 AH is typical for a smallish trailer.

Regarding "hours of sun"--We use that a lot in North America as the hours of "noon time" equivalent sun which is about 1,000 Watt*Hours per sq meter.

For example, here is a quick survey of South Africa (PDF download) where they seem to indicate that you could be seeing as much sun as 6-9+ kWH per sq. Meter--which would be ~6-9+ hours of noon time sun per day--depending on season (and, of course, location). It looks like you could use 6 hours of sun per day for your calculations.

Conservation is critical for off grid solar... And it is really hard to justify a full solar array+battery bank that could support 9+ months off grid for a trailer that is only used 2-10 days at a time.

So, you need to look at your needs--Say you only camp (no generator) for two days at a time. And, your battery will last longer if you don't discharge it more than 50%--But for a travel trailer, perhaps you could justify discharging even deeper as long as it is recharged over the next day or two (generator/driving) as the battery is only used on a few trips a year.

• So, just to do some math. Guess 3 amps at 20 minutes per hour, or ~8 hour per day * 3 amps = 24 AH per day.
• For lighting, say 1 amp (~12 watts) 4 hours per day = 4 AH per day.
• Total power usage ~28 AH @ 12 volts per day.

Assume 50% battery usage and a 100 AH battery. The typical recommendation would be 5-13% rate of charge for a battery bank for good battery life/quick recharging. Choose 10% rate of charge here (good sized panel to battery ratio). The solar array for a 100 AH battery bank would be:

• 100 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 188 Watts of Solar

A 188 Watt panel would supply (assuming 6 hours of sun minimum):

• 188 watts * 0.61 DC system efficiency * 6 hours of sun * 1/12 volt battery bank = 57 AH @ 12 volts per day

That is quite a bit of power based on the above estimates (always pays to be conservative).

And if you have a couple days of no sun--That would be ~56 AH of battery discharge out of a 100 AH battery bank--Not the end of the world (discharge to 44% state of charge). And it would take about 2 days to recharge the battery bank (~28 AH to loads, 57 AH for loads+charging from solar panels).

Anyway--That is how I would pencil the system out based on our guesses so far.

You could use a smaller solar array and still get enough sun--We recommend 5% minimum rate of charge for the battery bank (for several reasons)--So, you could just about "break even" on a 94 Watt solar array (with guesstimate of loads).

Note,it is better to only plan on running around 66 to 75% of predicted solar array/battery system output... Some days are sunnier than others, and some days you may have extra loads (recharging a laptop computer, recharge camera/cell phone batteries, etc.).

But for a travel trailer, your costs, area for panels+batteries, etc. force trade offs. You can make up less solar array/battery storage with a genset and good quality AC battery charger.

Make sure you test the backup charging system (generator, tow vehicle charging, etc.) first. You may find that the towing system does not charge the trailer battery very well... For quick charging you probably want around 14.4 volts at the storage battery (sealed battery charging voltage). Many times, the vehicle alternator is set to charge around 13.8 to 14.2 volts, and by the time you add voltage drop for the wiring from the engine compartment to the trailer battery--You might find that you get nowhere near that voltage at the trailer battery (probably 5-10 amps maximum). That will not do a great job at charging the RV battery from the vehicle charging system unless you do "something else" (second alternator with higher output voltage, use a DC to DC battery charger, use an AC inverter and AC battery charger off of the engine battery, etc.).

Anyway--Hope you guys have a wonderful time.

-Bill