Dick, >I am trying to design a house battery/inverter combination which will supply >the 120 VAC needs of Ninth Wave, a KK42. I have Sub Zero refrigeration and >propane range. I do not know how large the house battery should be in order >to have to charge only once a day. I don't even know if this is possible. I can't tell you how much you'll need in the way of batery capacity, but it's pretty straightforward to calculate it. You'll need to make a list of all the things you'll be running during the day WHILE THE GENSET IS OFF: anchor light, cabin lights, TV, microwave, coffee maker, toaster, and, of course, the Sub Zero. Then you have to determine how much power (in watts or amps and at what voltage) each uses when running. Then estimate how long each will be running. All that data needs to be translated into ampere-hours at 12 volts and THEN you multiply THAT total by four to get the desired capacity of your house batteries. Then the capacity of your battery charger can be calculated from that (along with the running time of your genset). Of course, if you don't have a genset, you can use a high-capacity alternator on yur main engine and run that instead. >How much battery capacity do you have and how often do you have to charge >when at anchor? Our house battery capacity is 440 ampere-hours and we recharge it every day with a 2 to 3-hour genset run. HOWEVER, we have a holding-plate refrigeration system that requires much more power to operate than your Sub Zero probably does (since ALL the cooling is done in that short period -- it runs on 110 VAC directly from the genset). The genset run is determined by the required cooling for the holding plates -- 2 hours in May and October, 3 hours in the July-August months. The difference is because the heat gain for the refrigerator/freezer box is higher in the hotter months. It's directly proportional to the temperature difference between the inside of the box and the outside temperature. Even 2 hours is usually plenty for our battery charging needs. We also compress other 110 VAC needs into the time the genset is running as well -- like making hot water, running the dryer, heavy TV watching, etc. Here's what our approximate energy budget looks like: Item Item Power@volts Time used AH Item Power@volts Time used AH consumed@12 VDC Anchor lite 20 Anchor lite 20 watts@12 VDC 10 hours* (20/12)*10=16.7 Cabin lites 40 Cabin lites 40 watts@12 VDC 2 hours (40/12)*2= 6.7 Microwave 750 Microwave 750 watts@110 VAC 0.1 hours (750/110)*12*0.1= 8.2 Toaster 1200 Toaster 1200 watts@110 VAC 0.2 hours (1200/110)*12*0.2=26.2 Coffee Maker 1200 Coffee Maker 1200 watts@110 VAC 0.1 hours (1200/110)*12*0.1=13.1 " " warm 1200 " " warm 1200 watts@110 VAC 0.2 hours** (1200/110)*12*0.2=26.2 TV 75 TV 75 watts@110 VAC 2 hours (75/110)*12*2=16.4 TOTAL 113.5 ampere-hours * On a photocell controller so it doesn't get left on all day accidentally ** Runs about two hours, but it has a 10% duty cycle on "keep-warm" I have NOT included an efficiency factor here for the 110 VAC appliances (to account for the power loss in the inverter), but it will be fairly large, maybe 85% to 90% -- Note that we do NOT run the refrigeration except when the genset is on (unless the main engine is on so we can run off the inverter while the alternator maintains the battery charge). Since your refrigerator is "on" all the time, you must include that with an appropriate duty cycle. For example, if it draws 3 example, if it draws 3 amps@110 VAC when running, then the 12 volt current will be (110/12)*3=27.5 amps. It would probably be closer to 30 amps including the loss in the inverter. If the Sub Zero runs directly on 12 volts, it might use 30 amps directly (I'm guessing at the Sub Zero numbers and I could be WAAY off -- check the instruction manual and/or name plate -- use my calculations as only an example.) Now assume a duty cycle, say 20% for a 21-hour day (don't include the time the genset is on and charging). This gives 30*0.2*21=126 ampere-hours. If you add this to the numbers above, you'll get a total of 240 ampere-hours. Multiplying by 4 gives a total capacity of 960 A-H, a little more than the equivalent of 4 8-D batteries (though I'd HIGHLY recommend using 8, 6-volt golf cart batteries to make up this bank -- MUCH cheaper, easier to haul in and out, etc.) >Also what type of alternator/charger do you use? The 110 VAC charger is a Professional Mariner-Promatic 50-3, Elite Series (50 amps, 3-outputs -- all 3 outputs are ony used during winter months when charger is connected to house, starting, & genset batteries -- in summer it's only connected to the house battery) and the alternator is a Balmar model 910-100 (100 amp) controlled with a Hehr Power Systems Aqualine smart regulator (connected through engine room switch to starting battery -- salon switch is used to parallel the house battery to the starting battery when we're underway so the alternator will charge both -- it's isolated otherwise). You can see the 50-amp charger is about adequate to recharge the 113 AH in 3 hours (the efficiency factor for charging is about 75%). OK, WHY multiply the AH budget by 4? That's because you NEVER want to take the batteries down to less than 50% of their capacity -- 75% is MUCH better for long life. Also, charging from 50% to 75% can be done quickly, whereas, charging from 75% TO 100% takes a LOT longer -- two reasons -- battery chemistry AND even the smart chargers taper the charging current down as they get close to full charge. Thus, you will want to see the battery charge fluctuate between say, 60% and 85% after a few days at anchor -- 65% and 90% would be even better -- 50% and 75% is about as low as you want to go. I monitor this VERY closely (I use a digital voltmeter -- others use E-meters) to make sure we never draw down below 50%. If we get close, we'll shut down systems, and adjust our schedule for charging. Two things are rationed at anchor, fresh water and ampere-hours! Hope this helps... I'd recommend reading Nigel Calder's section on all this in his, "Boatowners Mechanical and Electrical Manual." Be sure you have the Second Edition because this section in the First Edition is pretty lame. - Bryant