It depends on which all in one inverter you are thinking of. Many of the cheaper ones have quite a bit of difficulty with imbalanced loads. Especially with surge loads on only one leg. Sometimes an inverter that can handle 9,000 watts continuous with 15,000 watt surge on 240 volts, may fault out with a 3,000 watt surge on 120 volts. This was a common issue with many of the earlier units, even from the better brands. The current units from the better brands can usually handle imbalanced surge and imbalanced loads far better... but with this caveat:
You only get half the rated capacity on one leg. Like an old FX setup without the X240, or a Radian setup -- not like a XW setup that could give you 3/4 of full rated power on a single leg, or a X240 FX setup that could give full rated power on a single leg. So if you buy a 12,000 watt rated all in one inverter... you only get 6,000 watts on the one leg you are using, so you are essentially paying for twice the inverter (and incurring the tare load of twice the inverter) for what you actually get. If you can get one of the few all in ones that is designed for only 120 volt output, or that can be configured for only 120 volt output, it can solve these issues. Another issue with an all in one, is high tare load. If you are upgrading to an all in one, you will also need to expand the PV array, and possibly the batteries, to account for this. Tare on the SW4024 was around 25 watts, and it had a lower power search mode that may or may not work modern appliances (in my experience, 80% of modern appliances are no longer compatible with search mode). Most all in ones are at least 60 watts tare for the smaller ones, and 90 to 100 watts for the larger ones. You'll need to account for that in the load calcs, which on a older system can be significant. If you're doing a 8+kW array and much heavier loads that is typical of a lot of new off grid setups, then the % of the total load spent for inverter tare loss is actually no more than it was in most old systems... but directly retrofitting an all in one to a old system with a 2kW array can lead to big problems in the winter in my experience. Most all in ones will also need the array to be reconfigured to work with the all in one MPPT inputs. While most of them can operate under 100 volts MPPT, to directly connect an older array configured for a 150 volt charge controller, it does not work well in reality. The efficiency of their MPPT's is a bit less if the array is under 130 to 150 volts, plus the amperage limit of most AIO MPPT's is only 20 to 35 amps depending on the model... which means an older array may be too high of amperage for a single MPPT channel as well. Usually this can reconfigured in the array combiner box, connecting strings in series instead of parallel, without having to actually go out into the array wiring. Also good to remember that the all in ones get very unhappy (often blowing out the FET board in the inverter) if the array negatives are grounded, or if multiple arrays share a negative. This was common on older arrays, because on the older 150 volt charge controllers, all negatives were common, and grounded in some fashion. So check for that when re-using older arrays. If they just want an inverter update, without also doing battery and array upsizing, then the Victron Multiplus II inverters are a great option. Very low tare loss (even less than the old SW for some comparable models), better waveform, and they are all 120 volts -- which is annoying when doing a 120/240 volt system where you need two inverters minimum, but great when retrofitting a 120 volt system. Victron also makes nice MPPT charge controllers (none of which meet code unfortunately... no arc fault protection for the array), and monitoring equipment, and it's all compatible with lithium batteries (in open loop at the very least, and often even in closed loop). -- Zeke Yewdall PV Engineer NABCEP #031508-89 [email protected] 303-523-3592
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