# How do I estimate the energy consumption of a ductless mini-split A/C unit?

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. Aug 26, 2017 at 8:12
• 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. Aug 26, 2017 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. Aug 26, 2017 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. Aug 26, 2017 at 16:51
• 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. Sep 28, 2017 at 23:03