We have a fairly large and complex solar/ HVAC solution for off-grid use.
Our solar consists of 2925W of residential panels, 22.8kWh of 48V LiFePO4 batteries, two 48V 3kVA inverters configured for split phase 240V, an autotransformer for balancing 120V loads across the inverters, another autotransformer for stepping up single phase input sources to split phase, and several other components. The end result is that we have constant 120V/240V power supplied to the entire rig, that is only turned off if a fault or maintenance occurs. We have a built-in Onan 5500W generator for backup use.
The HVAC consists of a LG 30K condenser and two 15K indoor units. Since they are roughly three times as efficient as the original AC units, and provide more usable cooling power, these were a huge improvement for us. We’re able to run air conditioning throughout most hot days, and the electric heat pump on cool days, without having to plug in or run the generator. The condenser unit is mounted in a custom enclosure that is welded to the frame of the trailer.
We originally started with just 1950W of panels and 11.4kWh battery, but in order to accommodate running the mini-splits around the clock, we had to remove the rooftop AC units and add panels in their place. With our current 2925W and 22.8kWh of battery, we very rarely need to use the generator.
There are two common variants of RV electrical, 30A and 50A service. 30A service is a single pole 30A 120V breaker, while 50A service is a double pole 240V breaker.
Diving a little deeper, 50A service consists of two legs (hots) that are each 120V to neutral, or 240V to one another, since they are 180° out of phase. Most RVs do not use the 240V potential, and just have a split main panel with each 120V leg distributed to half the panel. When booking RV parks, you usually specify the service you need, and book the appropriate site. A lot of older parks do not have any 50A sites, or have limited quantities of them.
We have run across a couple campgrounds that are miswired and do not provide 240V from L1 to L2, but those are outliers you just need to be aware of if you rely on 240V service. Similarly, a lot of generators, such as our on-board Onan QG5500LP, are incapable of providing native 240V service.
The mini-split unit we picked for our rig requires 240V, so unlike most RVs we actually require properly functioning 50A service that provides 240V leg to leg. Making matters even more complicated, is the fact that our generator only provides 120V service. To remedy both situations, we opted to add an Autotransformer that steps up 120V single phase to 240V split phase.
This consists of a separate inlet for single phase service, that feeds an input on an automatic transfer switch. The second input for the switch is fed by the paralleled outputs from our generator. The output of the transfer switch feeds the input of a Progressive EMS, which then feeds into a Victron Autotransformer. The Autotransformer takes the 120V single phase service and creates a new phase, 180° out, on a new leg. The output ends up being 240V split phase service, with L1, L2, N, and PE.
We’re using Victron Quattro inverter/chargers, which have two AC inputs, but you could easily enough use another transfer switch here. In our scenario, L1 goes to Quattro #1 AC2 hot, and L2 goes to Quattro #2 AC2 hot. The standard 50A inlet our RV came with feeds into a Progressive EMS as well, which feeds L1 into Quattro #1 AC1 hot, and L2 into Quattro #2 AC2 hot. The Quattros are programmed so that they expect 240V split phase service, which is now provided at both the AC1 and AC2 inputs on the stack, regardless of what the true input source is.
The output of the Quattros feeds a second Autotransformer (more on that later), which feeds a sub panel, which distributes the 240V output to the main panel, the air conditioner disconnect panel, and an auxiliary 50A outlet we added to let other campers plug into.
Now that we have a design that provides us with the needed 240V service at all times when plugged in, we need to look at the design for the Quattros when not plugged in, or programmed in hybrid mode.
The higher the voltage system, the lower the inverter and line losses are, and ultimately the more efficient the system is. This is especially true with high amperage draws. There are varying opinions, but the method I generally recommend when talking to people about RV solar is to figure out what your peak expected draw is, and choose your voltage according to a 150A target:
- 150A * 12V = 1800W
- 150A * 24V = 3600W
- 150A * 48V = 7200W
Since I expected at the time of my initial design to draw over 3600W for extended periods of time (no longer the case with the mini-splits, but I have no regrets going 48V), the 48V system made the most sense according to those guidelines. A 24V would meet those guidelines with our current HVAC upgrades.
The heart of the system is the Quattros, which have to be purchased in the appropriate voltage, so we picked up two 48V 3kVA models capable of 4.8kW sustained output and a peak combined output of 12kW.
As you read above, you might have noted above that each leg of the 50A service feeds half of the main panel. That places a practical sustained limit of 2.4kW on each side of the panel – so what happens if you have a 120V air conditioner running at 1.7kW and try to run the 2kW convection microwave as well? The inverter will eventually shut down due to overload. To avoid ever running into this situation, we decided to use another Autotransformer to load balance the 120V loads across both inverters. The way it is wired, if we have a 1.5kW load on one side of a the panel and a 500W load on the other side, both inverters will see a 1kW load. This means that we’re able to fully utilize the potential of the inverters and don’t need to size up to larger models to absorb any imbalances.
Along with ordering the proper voltage inverters, you have to order the proper batteries and protection mechanisms. You can run multiple, lower voltage batteries in series to have 48V, or go with native 48V batteries. We opted for native 48V batteries, made by SimpliPhi, that can simply be paralleled and added as needed to provide additional capacity. There are a few caveats that drove us to choose this over a series+parallel layout.
- When running LiFePO4 in series, if they aren’t perfectly balanced, the first battery to reach 100% SoC will cut charging to the whole string. This can result in being unable to use the last few percent of a few batteries. It’s not the end of the world, but we’re chasing maximum efficiency in our design.
- A lot of manufacturers don’t support/warranty them in series, due to the above and possibly other issues I’m not aware of
- When running them in series, you have to add them in sets of V*N, where V is the nominal voltage of the battery and N is the number of batteries required to meet the nominal voltage of the bank. If we were using 12V batteries, with a 48V bank, we would have to add batteries in sets of four.
A lot of lower voltage systems use simple standalone breakers or fuses to protect everything. In our case, most of those standalone breakers will not work with a 48V system and we didn’t want to use fuses. We opted for actual breaker boxes with high voltage DC breakers, much like you would find in a house (but DC instead of AC). Since these breakers are often used in residential, commercial, and industrial environments, it’s easy to find more reliable and higher quality units than you would in the standalone, lower voltage variants. That’s not to say that you can’t find high quality breakers for other system sizes, but it’s easier to at 48V and ends up being deployed differently since these are designed to be used inside breaker boxes.
And finally, RVs still use 12V systems so we need to be able to continue to support our 12V loads. We chose to keep a standard RV/marine battery and float charge it with two Orion 30A DC-DC converters. Since the DC-DC converters are running at a slightly higher voltage than the battery, loads will use the power supplied from the 48V bank and converted to 12V first, then any excess requirement will be fed from the 12V battery. This allows us to power high draw loads such as our generator starter, hydraulics, and brakes while keeping the 12V lead acid battery full and ensuring that our brakes are hardwired to a 12V battery should anything happen to our 48V system or converters.
There are 48VDC mini-split AC units, which we seriously considered before opting for our current unit. As much as I wanted one, I couldn’t find any review on them and refused to buy them based off of advertising alone. There are two common importers, so I looked up their hash tags on Instagram and sent DMs to actual owners requesting feedback. The overwhelming feedback from the handful of people who responded was to run, not walk, away from those units. With that decision being made, I started researching household units.
I initially planned on having two smaller outdoor units, but eventually decided to go with a single multi-zone unit in order to save both space and weight. The outdoor units alone would have added at least 70lbs to the rear of the trailer if we went with individual units for each zone.
As for the make and model, I spent a lot of time on https://www.ahridirectory.org comparing the efficiency ratings of various units and chose the most efficient unit I could find in the 30K BTU class. I also wanted to ensure it was a name brand and easy to find parts for, after hearing some of the horror stories associated with some of the 48VDC units.