RV Solar Power Pros and Cons, Part 1

You ask about the number of 100 watt panels you need for a small load. There is no easy answer. And realize that a “100 watt:’ panel is 100 watts only when at right angles to the sun, and might make 50-60% of that when laid flat on the roof of a RV. And there had better not be anything shading the panel! One dirty secret that no salesman will tell you is that 100% of the panel needs to be in full sun – a shadow from a air conditioner that covers 10% of the panel will zero out that panels generation.

To answer your question you have to know how much energy YOUR lifestyle uses. YOU need to do an energy usage log, then convert those numbers into battery size, then from there into battery charger size, then from there into solar panels needed.

Someone that lives in the New Mexico desert with an 8 hour summertime solar day for months at a time will need fewer panels than someone who lives in Montana in the wintertime with a week of 4 hour solar days followed by a week of cloudy days.

Air conditioning is the biggest energy hog. Refrigeration (i.e. fridge/freezer) is the second largest, simply because they are on and using energy a big percentage of the time. A microwave oven is the third, biggest energy hog but since it is drawing power for short periods of time that lowers it to #3 in total energy usage per day.

The biggest single thing you can do to reduce your energy usage is to improve the insulation of your RV and to close off any outside air leaks..
Example #1: – I have an mid-1980s Fleetwood class A. I had a built-in vacuum that exhausted outside. I did not realize it, but that was effectively a knee-high 2-and-a-half inch diameter hole from the bedroom wall to the outside bottom of the coach. Once I plugged that hole (with an old sock) my bedroom warmed up tremendously in the winter and cooled down in the summer.
Example #2) a friend had to replace the outer skin on his older RV and when he opened up the walls he found almost zero insulation. He installed new insulation and lots of it. He also replaced the old windows with new double pane windows. His RV had dual roof-mount air conditioners and dual propane furnaces – one of each for the bedroom and the other for the front. After the insulation refit he was using only the front unit and for about half the hours per day. His solar system is less than half of what he would have needed prior to the insulation retrofit..

Note that in many cases you can use your the energy you do use more efficiently. Many pieces of electronic equipment use AC powered, “wall warts” or battery chargers but frequently you can get 12 volt versions. I use a charger designed for a automobile cigarette lighter for my cellphone. My internet DSL modem and Linksys router both use a 12v DC wall wart. I have both connected to the house battery directly. I also have an automobile cigarette lighter charger for my electric razor. My CPAP (overnight breathing machine) came with a 120vAC-to-24vDC converter – it’s a 24 volt device internally. I made a 12 volt to 24 volt converter box using an industrial power converter module I purchased on ebay for $34. My RV TV was designed to run on 120v AC but internally runs on 40v DC. I modified another industrial converter module that originally made 48 volts from 12 volts and it now powers .the TV from 12 volts.
All of the above means that I no longer need to run my RV inverter at night… and the no-load idle power of the inverter was about 3-4 amps at 12 volts…. 36-48 amp-hours of battery were wasted every hour doing absolutely nothing.

A laptop uses DC power from an internal battery – and most are laptops are 17 to 19 volts internally. They come with a laptop charger that runs on 120v AC (home wall outlet power), but for an RV to have 120v AC means that the RV has to have an inverter to make that AC power from the 12 volt battery… and a laptop charger that uses 1 amp of 120 volts AC is going to use at least 10 amps at 12 volts (assuming perfect conversion efficiency and perfect conversion does not exist… there is always some conversion loss) .
So what do I do?
When I bought my last laptop I made a point of picking a model that had a 12 volt charger available – both Dell and Panasonic have them for customers that work out of their cars – insurance agents, construction sites, etc. Picking the right laptop is one more way that I can avoid running the RV inverter.

The energy usage of a TV set varies – the older glass picture tube units were big energy users. The plasma flat screens were less, but the newest flat screens are energy misers by comparison.

Again, the correct answer to your question is for YOU to do an energy audit – start by keeping an energy diary for at least a week, preferably 2 weeks, and ideally for a month..

Start with waking up: do you turn on the furnace for a half hour to take the chill off? (and note that the RV is chilly because you don’t have enough insulation to keep the heat in) Log it.
How long do you run the microwave for your morning coffee? Log it.
How long do you run the toaster for your breakfast english muffin? Log it.
How long do you run the air conditioner or fans? Log it.
How long do you run the TV? Log it.
How long do you run the Nintendo and TV? Log both.

And do the log as the number of watts (from the appliance nameplate) and the time in tenths of an hour. If the nameplate does not have a watts number then multiply the volts times the amps – for example, 120 volts times 1.5 amps is 180 watts.
Then multiply the watts by the time to come up with watt-hours.
Example 1: 180 watts for 1 minute (i.e. for 1/60 of an hour) uses 3 watt-hours of power.
Example 2: a microwave that is nameplated as 1200 watts for 8 minutes is 160 watt-hours.
(method of calculation is 8/60 = 0.1333. 1200 times 0.1333 = 160)

Then when you go through your months worth of log the highest total watts used at one time plus 25% will be your inverter size you need. The 25% covers conversion loss and gives you a bit of “headroom” on your system..

The total watt-hours for the day, week or month will determine the battery capacity you
need – and (the longer that you make the log the more accurate and useful your energy audit will be)

Then divide the total month of watt hours by 120v and that determines the amp-hours of energy you used.
Add 25% and write that number down.
That’s the number of amp-hours you will need to pump into your battery every 24 hours. Then realize that a solar panel has to be at right angles to sun for maximum energy generation, and one that’s flat on the roof of the RV is making maybe 60% of that, and usually less.. And realize that a “day” can be as short as 5-6 hours of sunlight depending where you live and even less if you have a series of cloudy days.

Lets say that your log shows 3200 amp-hours per month (and that’s a low number).
Figure a 30 day month.

That’s 106.6 amp hours per 24 hour period, and that divided by 5 solar hours is 21.3 amps every daylight hour. Add 25% to cover losses and a bit of headroom gives about 27. amps.

At 12 volts that’s 324 watts. You’d need four “100 watt” panels at right angles to the sun for a system that has almost zero excess power. If the panels are flat to the roof of the RV you had better have five. And it still won’t survive a cloudy week (you had better have a good Honda generator).

The secret to a good working solar system is to minimize the voltage drops and to maximize the amps into the batteries, which means lots of thick copper wire. ,
Look up the RV solar writeup by “handybob”. That guy knows his stuff.

For what it’s worth, I have 4 panels on tilt-able mounts and am budgeting for two more.
I don’t run the microwave, TV or run the air conditioner unless I’m plugged into shore power.