Can You Run a Residential Fridge on Solar: Thinking About Boondocking (Updated)

Solar. <sigh>

We want to have the ability to be off-grid, in the middle of nowhere if we want. I would like the only real working limitation to be connectivity, or if we’re on an actual vacation (like, not working that day) no limitations. Ultimately, we’re going to be somewhat limited by the availability of water, or the size of our holding tanks. That can be lessened by the use of gravity-filling fresh water (ferried in the Jeep), or emptying holding tanks using portable dump-tanks.

Electricity is the main issue, I think.  I’m going to try to figure out if its possible, with a reasonable RV Solar system, to run a residential refrigerator on solar power.  They use a heap of power, and would quickly drain the standard house batteries.

That could be lessened by the use of the generator, of course. But who wants to be in the middle of some beautiful, untouched wilderness, and rather than hearing the falcons calling overhead, you hear the rumble of your generator? I’m going to try to minimize the need to run the generator – though accepting that it’s probably going to be necessary occasionally.

So, I’m investigating solar. This is a (somewhat) complicated subject, it appears. There are a few variables which interact in various ways to decide whether you can get the power you want when you want it.

There are some basic principles though – like skipping the use of the big energy hogs like the AC and microwave.  But, one of the first things that will need to be determined is the basic composition of the installation.

Here’s my take:

You need to be able to bring in and store energy. You need to store it for when you’re not able to generate it. You need some wiggle room for when ‘bringing it in’ isn’t working out that well – cloudy, winter, etc.

That’s all well and good, and I think most people get that. You need solar panels and batteries. The issue is really ‘how much of what pieces of the puzzle do I need?’ and that’s where it gets complicated.

To figure that out, I took one of the more distinguishing features we have in choosing a new RV, and worked out some numbers. The issue is whether to get an RV with a Residential Refrigerator (RR), or a standard Absorption RV Refrigerator (ARR). The issue is that the ARR can run using propane, dramatically lowering the amount of power you need to bring in and store. The RR, on the other hand, is very electrically efficient, and generally has a lot more food capacity, so more desirable, and increasingly common in the RVs we’re looking at.  The downside of the RR is that it’s electric-only.  So, can you reasonably run a RR on solar?

There are going to be some numbers coming up, so hold on to your butts, if you’re a math-phobic.

(Totally ignoring batteries and the like for a second, just focusing on the input and output)

Many of the RRs we see need about 1.2 kilowatts-ish of juice a day. Don’t worry too much about exactly what a watt is, just remember that we need 1200 of them, per 24 hours.

A rule of thumb for getting solar energy, is to plan for about 5 hours of good sun per day, in the absolute best case.

Commonly available RV solar panels can generate about 100-160 watts. (some go a lot higher, but we’ll focus on the common and cheaper ones)

A watt over the course of an hour is a watt-hour. It’s a combined unit, like Miles Per Hour. Don’t get too wrapped up in what it actually means – it just juice delivered over time.

So, a single 100-watt panel could generate (in a perfect world) about 500 watt-hours per day. Moving the decimal point gives us .5 KWh per day. So, if we had three of them we’d have 1.5 KWh per day, which is greater than the 1.2 KWh we need. (.5 x 3 = 1.5)

One of the most important things to know, is: you will never achieve this result. We do not have perfect, lossless wiring, energy storage or conversion. So, we need to apply some mechanism to account for these inefficiencies. (there are additional inefficiencies we’ll account for later on, like not being able to use all of the capacity of our batteries.) Optimistically, I’m going to assume 80% efficiency. I think this is on the highly optimistic side, actually – though finding good numbers is hard – there are a lot of ways to make a system inefficient, like poor wiring, long wiring runs, etc.  We’ll just assume that most of the rest of the system is installed efficiently, in this hypothetical example.

Using the 80% number, that panel will really deliver 80 watts per hour, or 400 watt-hours (or .400 KWh) per day, best case. We need 1.2KWh, so now we’re only just squeaking by with 3 panels. (.375 * 3 = 1.2)

Ok, so that gives us 1.2 KW per day, when everything is perfect, and we’re only running the ‘fridge. I think this might work only if we left the RV alone for a few days to go on a jaunt through some town, or were staying at a friend’s house, or something, but it’s hardly going to work when we’re actually there, because the fridge isn’t the only thing we’re using. We have laptops and TVs too. The fridge door will be periodically opened. The RV itself has other systems running, which sip or slurp more juice too. The point is, just accounting for the largest item isn’t going to cut it – but the math should be similar. You’d just budget for more than just the fridge, probably more like twice that or more.

Yes we can!

Nevertheless, it looks like we’d be able to run a residential fridge on solar, but we’d need a minimum of 4 panels, or panels rated better than 100 watts. To be safe, I think I’d like 5, and higher output ones as well. That’s a pretty big installation, and is certainly not going to be cheap.

For perspective,  this one on Amazon  is $3700, and produces 480 watts.  That would work out to 1920 Watt-hours per day, which barely cuts the mustard.  The same company offers an add-on set of panels (for ~$500) producing 120 watts (480Wh/day) .   If used together, they’d be 2400Wh/day, which is better, and would probably be the minimum we could consider.  This is also the one reviewed/described by the Wynn’s in one of their videos.  (They have since upgraded to a much larger system)

If you want to figure out the number of panels for your rig, it’s (vastly simplified):

Total Watts wanted / (panel rating * 0.80), and rounding up.

You can play with the numbers a bit, just remember that the amount you get will always be less than that. For us, I think the result of using that 3-panel system linked above would be that we need to run the generator maybe once a day to get the batteries back up to snuff, or to make up the difference on cloudy days.

Ok, we have the incoming and outgoing, what about the storage?

We should be pulling in juice faster than we’re using it – if we weren’t, this wouldn’t work at all. We have to ‘earn’ 24 hours of power in 5 hours. So, the solar panels need to stuff that juice somewhere. It goes into the house batteries. (as an aside, the wiring from the panels to the batteries is very very important. It needs to be super thick cable, and as short as possible. This helps keep the inefficiencies low.  This, and other helpful design tips are covered well by the ‘Jack and Danielle Mayer’ link at the bottom of this post.)

‘House’ batteries are different from the ones in a car or the ones used for starting an engine. Car batteries deliver a lot of juice rapidly (2-30 seconds) to crank an engine. House (deep cycle) deliver ‘slower flows’ of juice, but spread over a much longer amount of time – potentially over many days. So, just remember that these are different batteries, and sometimes quite expensive.

Batteries are usually labeled with an ‘Amp-hour’ rating. This, like the watt-hour above, is a combined unit, indicating the delivery of energy over time. One amp for one hour is an amp hour. A good house battery might be rated at 100 amp-hours. So, It could deliver a 1 amp load for 100 hours. Or, 2 amps for 50 hours, etc.

Converting between watt-hours and amp hours is pretty simple:

watt-hours / voltage = amp-hours

Using my Amazon-linked example, we need to store at least 1920 watts a day, at 12 volts. 1920/ 12 = 160. So, we need 160 Amp-hours of battery. But wait! Most RV batteries are lead acid, which really don’t like being taken from 100% to 0%. This actually causes damage, and doing this will degrade the battery over time. The most common recommendation I could find is to assume 50% usage. So, we’d actually need 320 amp-hours of capacity in the batteries. This one here  is rated at 100 amp-hours, so we’d need 4 of them. Again, remember – this is really a minimum. This would really only work on the RR, and not a lot else. I think we’ll shoot for the 6 house battery setup.  (Again, there are many more options – using Lithium-Iron-Phosphate batteries, 6V golf cart batteries, etc – I’m not covering those in this post)

There are other complexities to worry about – like ensuring that the solar system chosen has a sane power management system – you can’t charge the batteries too fast, or overcharge, and that all the batteries are at the same charge level, etc.

BUT WAIT – THERE’S MORE!

Your RR can’t actually run directly off the juice from the batteries. It needs something called AC – like from the wall outlets in your sticks-and-bricks house, and maybe in your coach too. This ultimately comes either from the campground connection – if you’re on a hookup, from your generator – if its running, or through your inverter – if running from batteries. It’s the last one we are concerned about here. You need to have an inverter capable of handing the load of the RR, and whatever else you want to run at the same time. There are two main types, Pure Sine Wave (PSW) and Modified Sine Wave (MSW). The Wynn’s have a (as all of them are!) great video on the differences here [link]. You probably would prefer a PSW, especially of you’re running electronics as well. PSWs are more expensive, but won’t damage your electronic gear, and MSWs might.  That kit linked above (this one) includes a 3000 watt PSW.

The other option for inverters is the wattage rating. Most (or at least many) are 2000 watt. This is probably fine, unless you’re running a lot of gear. Spend some time totaling all of the things you want to run (at the same time!) and make sure you have an inverter that’s a smidge larger than that at a minimum. Remembering that some appliances are hideously energy-hungry, like blowdriers, microwaves, electric heaters, etc – not using these when on solar will help a lot to lower your overall power requirements.

In sum:

• Total your power usage.
• Find #panels:
  • Total Watts wanted / (panel rating * 0.75), and rounding up.

• Watt→Amp conversion (and vice versa):
  • watt-hours / voltage = amp-hours

• Assume inefficiencies – 80-85% efficient is a good start, but is probably way too optimistic.
• Use super thick cables – 10 gauge at a minimum, preferably 6+
• Use short cables (I’ve seen recommendations to keep the total length to less than 25 feet).
• Double your power storage requirements to account for lead-acid limitations.
• And most of all – understand that you will always get less than the perfect power production.

I’ve only touched on some important points in this subject. There is a lot to worry about, but this should get you able to think about what size of system you’ll need, and if solar is reasonable with what your rig uses. The actual installation, and much of the finer details are outside of my current understanding. When we actually have ours all set up, or when we actually are a bit closer to the installation, I’ll revisit this subject.

To get more info from people who actually know what they’re talking about:

• Jack and Danielle Mayer’s page here
• The IRV2 ‘Going Green’ forum here – There are some RV Solar installers in that forum, and a lot of other really knowledgeable folks too.  Hell, the whole site is chockablock with good RVing info
• Some been-around-a-while installers like AM Solar have ‘education’ sections which are super informative
• Several Gone With the Wynn’s Videos and posts: here, here and here.

EDIT:

Since this ‘went to press,’ the Wynns have presented an experiment in which they run their air conditioner solely from Solar.  Note that this is during the winter (More difficult to get as much juice compared to Summer), and they are running a single AC unit, but- most importantly – they have a pretty heavyweight solar setup.  See their article for more.

Also, there have been a few complimentary articles from others: YarrVee has theirs here,