PV setup for an RV

[John Pitts]

We have some acerage that is off the beaten path. We have a 27 ft Dutchman 5th Wheel RV that we are going to put on the property to use as a campsite while we clear and build.

Whay we would like to do is run the RV off of PV. It has a small 12v baterry and system that is insufficient for running the AC and lights, etc.

At camp sites we hook into a 120v 30Amp hook-up and we do the same at home.

It seems the basic parameters for a PC system will be to deliver 120 Volsts at 30 amps. Am i getting started in the right direction or is my basic assumption off? Any builder know that witout a good foundation, everything else is a waste of time.

Thanks from an old guy learning new tricks

[Rob Beckers]

Hi John,

PV normally doesn't get sized to provide the maximum the circuit can deliver, it would get too expensive. In your case, 120 Volt @ 30A is a whopping 3600W. Not to mention that the sun only shines half the day. Instead the way to go about it is to find out what your actual energy use it (per day), by adding up all the appliances you normally use. Multiply their Wattage by the time in hours that they are running and you have their energy use in Watt-hours. Add all appliances together and you have total Watt-hours that PV would need to generate each day to meet the needs.

Once you have that it is possible to calculate the amount of PV needed to meet the demand, taking the number of sun-hours for your location into account (if you are off grid this usually goes by the worst-case, ie. mid-winter, unless you have another means of recharging the batteries). From the daily energy needs the battery size also follows, normally they are sized to allow 3 days of use to 50% discharge, for those winter days without sun. Again, this can be different if you have another means of recharging (ie. running the RV engine?).

While adding Watt-hours, also keep track of the Wattage of the appliances so you know what the maximum power in Watts is that the system would need to deliver (for example, when running the AC, TV, pump and a few lights at the same time). This determines the size of the inverter, since those come in different maximum continuous power (Watt) ratings.

-RoB-

[Julie Haugh]

John,

While you're adding up daily power usage, you also need to consider what percentage of days, as well as the number of days in a row, that you're going to use a given load.

The amount of PV and battery capacity needed to use a large AC load two days in a row (laundry on the weekends, for example), versus the same large AC load 3 days apart (laundry on Sunday and Thursday, for example) is very different.

My suggestion is to make a list of loads, by day of the week, and list loads and solar production, the same as you would a checkbook register.

It's okay to run a deficit, provided you have the battery capacity, from time to time. You don't want to design a system so it runs a deficit for more than a day in a row.

[John Pitts]

I've tried to gather the power requirements for the RV system. It has an inverter that takes part of the 120V input and runs the 12V systems. It appears that the lights and the water pump are all 12 volt while the frig and AC and Heater Blower are 120V. We don't have and have not been able to find an specs of the 5th wheel and its equipment. Still looking.
So my basic ideal was to put a Kill-a-Watt meter on the electrical hook-up but the only meters I've found are 15amp. I have the feeling that when the Ac is on and the frig compressor cycles with a few inside lights on that the draw is greater than 15 amps.

So you know of a metering source that will handle at least 20 amps and does not cost an arm and a leg?

[Julie Haugh]

John,

I don't see where you've specified the inverter manufacturer you plan to go with, but if you've chosen Outback, I'd suggest you hold off on what I'm about to suggest and go with the FlexNET-DC from Outback Power Systems. Otherwise, I'd suggest you contact Bogart Engineering and order a PentaMetric battery monitor. Be sure to mention you were recommended by Julie Haugh from greenHouse Computers -- they loaned me a PentaMetric for product development work and I need to earn my keep!

Either way, you'll need to purchase at least one 500A / 50mv shunt. You can get those from Maverick Solar -- http://www.mavericksolar.com. You'll also need a short length of 4/0 cable that has bolt-on lugs crimped on already. Maverick stocks those as well. Tell him Julie says "Hi" when you call or make your order.

Take the 4/0 cable and bolt it to whatever the present B- ("Negative") cable is bolted to, and bolt the other end of the 4/0 jumper to one of the lugs on the shunt. Then take the cable you just remove and bolt that to the other side of the shunt.

Now you have a way to measure current all the way from 0 to 500 amps. To do that, you'll need a digital multimeter with a scale that at least measures tenths of millivolts (100's of microvolts). Each 0.1mv is an amp of current flowing through the shunt.

Your cost -- not a lot of money for the shunt, and free for the DVM if you own one already. And if you don't already own one, you'll need to buy one anyway.

I don't know the proper etiquette here yet, but my company does solar designs for on and off grid, as well as consultation for anyone doing it themselves. Drop me a note if you need a hand -- jfh@greenhousepc.com (don't let the PC in there fool you -- we also do solar monitoring software and low power computing).

[Rob Beckers]

Welcome to Green Power Talk Julie!
Good renewable energy advise to posters' questions is always welcome, and as long as you don't continuously flog your own business I have no problem with the occasional mention. You can create a signature with a link to your business if you like, that way people can find you without the need to mention it too often in posts (ie. it'll already be at the bottom of every post you create). This isn't a black-and-white rule, the general idea is to avoid turning this forum into one with lots of gratuitous advertisements that will turn people off. That's all there's to it. I have no problem with links to sources where people can find certain products if they are appropriate.

John, do your large appliances (the A/C in particular) run off 12V, or can you currently only use them when you hook up to a 120V feed? I was under the impression that it is 120V only. Julie's idea works great if everything is currently coming from the 12V battery, so you can measure how much they use by measuring the voltage over the shunt when you switch them on. If it's 120V a quick way is to get a (cheap) current-clamp meter, and clamp it over a single conductor. Let me know if this is clear, and I'll explain further if needed. This will get you the continuous power use of your A/C and other 120V appliances (if you can find an accessible conductor to clamp onto).
Keep in mind that a compressor will draw about 3 to 4 times the continuous power during startup (unless it has some type of soft-start feature). The inverter you select will have to be able to supply this for a short time.

-RoB-