2

I'm designing a "solar RV" built from a cargo trailer with PV panels mounted on the roof. Currently, I'm listing the expected electrical loads so I can size the solar system and battery bank appropriately.

The largest load will be a ductless mini-split air conditioner, likely a Mitsubishi 6,000 BTU, 33.1 SEER unit. It's the smallest on the market, but it should be plenty since the RV will only be ~110-130 square feet. I expect to use it frequently on shore power, but I'd like to be able to run it at least part-time while off-grid.

I've searched extensively for real-world energy usage reports from people who own that model (or similar), but haven't found any, so the next step is to estimate based on the available specs. Wikipedia describes how to calculate energy consumption based on BTU, SEER, and hourly usage, but the latter confuses me - at least in terms of how it applies to mini-splits.

My understanding is that many inverter compressor models (including the Mitsubishi) are constantly running, and modulate the compressor load as needed to maintain the desired temperature.

Does that mean I'd simply calculate energy consumption based on 24 hours/day (a 100% duty cycle)? So:

6,000 BTU / 33.1 SEER * 24 hours/day = 4.35 kWh/day

Or, is it more complicated than that? It would seem so, as that would be equivalent to a unit with 1/3 the efficiency operating for 1/3 the time, which seems fairly typical in the real world:

6,000 BTU / 11 SEER * 8 hours/day = 4.36 kWh/day

Surely the mini-split must use significantly less power than such a unit?

  • Estimating aircon units is really hard, even when they're fixed-speed. You still need to know how much it's going to run, and a fixed cycle unit is unlikely to run for eight hours solid on anything but the hottest day. – Someone Somewhere Aug 26 '17 at 8:12
  • 1
    I question whether this Mitsubishi unit is designed for mobile service. It very likely has delicate parts which could not tolerate the vibration of frequent movement on roads. These mini-split ductless units are put on manufactured housing which is towed on the highway to the site of use, but these are towed once or twice in their service life. – Jim Stewart Aug 26 '17 at 10:31
  • @SomeoneSomewhere Is there a way to at least determine worst-case scenario, or maybe a "budget" for power consumption expectations based on an assumed duty cycle, like 25%? If this unit pulls 315 w during cooling, is calculating 25% duty cycle as simple as 315 w * 24 hours/day * 25% = 1.89 kWh? I don't need a very precise estimate, just more of a ballpark to understand what to aim for. – Bungle Aug 26 '17 at 16:47
  • @JimStewart I'm sure you're right - I've heard others voice the same. I plan to do more research to understand if there's a way to mitigate that, e.g., design a suspended mount to isolate some of the vibration. There are also manufacturers who design commercial mini-splits for high-vibration environments (buses, subway cars) that I'm looking into, though those are likely substantially more expensive, harder to service/repair, and possibly not sold retail. – Bungle Aug 26 '17 at 16:51
  • 1
    Don't forget about in rush currents needed to start the compressor and fans the draw is typically 3-5x on startup if your inverter is undersized you may be in the market for a new compressor and inverter. – Ed Beal Sep 28 '17 at 23:03
1

I doubt that any PV array which would fit on this camper could power this a/c unit. The power consumption of the a/c unit is far above what a small PV array could supply. Based on my recollection of the circuit requirements of room a/c units rated at 5000 BTU/h, when running, this unit will draw about 10 A at 120 V ac so 1.2 kW continuously.

EDIT

One sees campers with 5000 BTU/h a/c units on top. I have always assumed that these were only for use in campgrounds with electric plug in sites.

  • 2
    Rated input power on that unit, during cooling, is 315W. It depends massively on indoor and outdoor conditions and temperatures, setpoint etc. The 1.2kVA is due to things like defrost and power factor. – Someone Somewhere Aug 26 '17 at 10:12
  • @SomeoneSomewhere That was my understanding, too. Does that mean that it's generally safe to assume that 315W continuous is the maximum load - e.g., worst-case I'm looking at 315 Wh * 24 = 7.56 kWh/day if the A/C runs constantly at maximum power? Of course defrost cycle would raise that theoretical power consumption, but it's also not likely that it will run full-speed around the clock. – Bungle Aug 26 '17 at 16:30
  • @Jim Stewart Thanks for your input! This blog post (goo.gl/MsAJuy) is the best I've found to give context to solar RV A/C. Note that his RV is not well-insulated, is much larger and has two far less efficient A/C units. Mine would have 1600-2000 w of PV panels (4-5 LG NeON 2 400 w 72-cells), for what it's worth, so might yield up to 10 kWh/day under perfect conditions. Shore power would likely be the main way to run A/C, but knowing how long I can expect it to run off-grid will help me plan my battery requirements and my range of travel. – Bungle Aug 26 '17 at 16:41
  • Listed power consumption in heating is a little higher, at 545W. Depending on climate, it's also going to spend more time per day heating than cooling, and you'll have less PV. Estimating total daily usage is far more about how much space you're trying to heat/cool, and not much about the actual unit capacity - a bigger unit will just run less. Either way, make sure you have about 1.5kVA spare on the inverter for it. – Someone Somewhere Aug 27 '17 at 1:03
  • Power consumption is fine but the PV system would need to be sized for 3-5x for starting currents. Its not just about wattage while running. – Ed Beal Sep 28 '17 at 22:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.