Thursday, September 12, 2013

Off the Grid with a new RV Inverter Hybrid

Short story:
THIS is out on the market now, and it looks pretty cool:
allowing your generator power to combine with your inverter power, to power bigger loads, 
suddenly making those smaller 2.8k RV generators feasible options for our space-challenged motorhomes (and i got a much better price quote at

Long story:
We invested in a big solar array (1,000 watts) on top of the Eleganza some years ago, 
and hardly ever run the generator anymore... except when we really need the air con!
(like at BurningMan 2 weeks ago)... Well, ashes to ashes, dust to dust (dust in the brushes?)
now the 35 year old Onan stopped putting out voltage, So... not wanting to throw good money after bad, Its time to put the original gas-hog Onan 6k out to pasture

However, as most GMC Motorhome owners know, there are no easy replacement generators to fill this same slot, Apparently nothing currently under production except for the Onan Microlite RV QG 2800 watt (about $2500 plus installation)... Larger models (about $3600 plus installation) require cutting into the battery compartment (and unfortunately we can't do that... Ours is occupied by 2 Trojan T-145s, linked to 4 more up front,  and there's really nowhere else to put them!)

So hmmm...  2800 watts for a generator isn't so practical, or exciting...
(and this is where things get more technical....)
Even though we only run one roof air, it'd be nice to have some headroom for the battery charger, hot water heater, appliances and studio gear.  4k or more would be nice, but applicably-sized (discontinued) Hondas and Generacs just aren't showing up, used.
After all, our Magnum inverter / charger is 2800 watt sine wave, and allows surges to 3600 watts, and we've been really happy with it.  With the 1000 watt solar array feeding the batteries, we can run the water heater off the Magnum, or even the air con for short blasts, without firing up the noisy thirsty Onan.  So it'd be a weird fit to have our new generator actually less powerful than our inverter!  (Yet why would we want to custom install some rather large generator, when we hardly ever use it?)

So i called Magnum to see if there was a possible mode setting that would allow charging the batteries while simultaneously running the inverter, as opposed to its default generator or shore power pass-through (with our model MS2812); so we wouldn't have to "power manage" and be so vigilant with our usage.  If the AC draw could be from the inverter rather than the generator, then the inverter's "surge" 3600 watt headroom would probably be enough to allow this system to work for our usage patterns. Otherwise, it'd be rather inefficient and silly to have to run the generator JUST to charge the 6 big batteries, or JUST to run air con, one operation at a time... 

So was there a mode or setting to allow this?  The answer was no... Not for the model MS2812.... nor any other previous Magnum model...   Its either pass-through / charging or inverter mode (and so the only work-around i could see would be to install a 2nd battery charger, which would only be used for those times that the generator and inverter were running simultaneously... not practical)

But then they told me about this new product, the new Magnum MSH3012M
This "hybrid" model will actually combine your shore (generator) power with any needed inverter power, to allow for larger loads.
So, that 2800 watt generator is now a great option, since the Magnum could also combine its 3000 watts of inverter power when needed... allowing for loads up to 5800 watts.  Since the battery charger is built into the inverter, I would imagine that this switching happens dynamically and proportionally, so that the batteries get charged when the AC load is under the generator's capacity of 2800 watts.  For example, if your 13,500 BTU air con (plus any other AC draw) is taking x number of watts, then any spare generator AC will charge the batteries with whatever remaining wattage is available.  Or, if you're running air con, water heater and some other appliances, far beyond the generator's 2800 watt capacity, then the inverter kicks and draws from the batteries a bit, to allow the larger loads.  

This is the theory as far as i understand it, but  before i'd commit the funds and swap out my inverter, i'd want to confirm the efficiency of this, and with respect to the generator (I could imagine that this type of inverter/charger could have smaller generators running at max capacity a lot of the time, and i'm not sure if this is good for the life expectancy of these smaller generators?)  This is the only potential downside that i see, other than the overall unexpected COST of a new generator plus inverter upgrade (i'm trying my best to look at this as a long-term investment :)

So this new Magnum is back-ordered for a couple more weeks anyway, so we've got some time to ponder it; but if this really is our best option, then we'd probably pull the trigger on the Onan 2800 watt in the next week or so....  
Anybody have some observations or experience in these directions? Any feedback appreciated, and thanks for reading!

SolarSonic / Los Angeles (currently in Marin County)


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