Posted by Steve Dashew (April 24, 2008)
AC Power Management
With AC power consumption aboard at a personal record for us, we are experimenting with different management schemes. To begin with, we now have all four air conditioning units running. That is a total of 54,000 BTUs of capacity. In addition, the wash cycle is in full swing as this is being written. As the washing machine heats its own water, and the drier is a full sized unit, they really consume the Watts. In addition, it is breakfast time and the microwave is periodically using its 1000 or so Watts.
The Victron inverters have software which allows you to set, control, and watch them from a PC. We normally don’t use this, but as we are in a learning mode it is very helpful. The amp capacities shown are for one of three inverters, so to get the total values, multiply by three.
Our 8kW genset has a max load of 34.7 amps, and it is happiest at 28 to 30 amps. The Victron inverters will pick up any load in excess of a value we set on the “shore power limiter” (virtual knob upper right). In this case we have it dialed in to 10 amps per inverter, or 30 total.
With all the stuff running we periodically exceed the genset capacity, so the inverters pick this up with the batteries being maintained by the alternators on the engines.
What’s cool is – it seems to work!
Now an existential question. With high loads like these would we be better off using just inverters to create the electricity, or the genset? From the perspective of fuel burn, we are guessing that the genset is going to be more efficient, as long as it is loaded at 75% or more. If we use inverters running off batteries supplied by alternators, there are a series of conversion factors involved (alternators start out with AC current which is rectified to DC; the inverters are at best 90% efficient; and there are losses in the DC power as it travels from engine room to batteries to inverters). But for our normal light AC loads, which would only take half or less of the genset’s power, the straight inverter set up is probably better.
Now for a real world bit of data. The four air conditioners, drier, and occasional microwave are averaging about 28 amps at 230 volts, with periodic jumps to 32 amps.
