Bob C Posted December 11, 2011 Share Posted December 11, 2011 I have a stock alternator in my 1988 22RE Toyhouse and would like to know what is the largest inverter I can install. The previous owner installed a 4 cubic ft refrigerator and it is 120 volt only. I am looking for a used gas/electric frig but until then I would like to run the 120 v unit while driving. I never trust my calculations with Ohm's Law so I am asking you "experts." I have only been here for about 5 months but have learned so much. I definitely had luck finding the MH that I bought with your help. I avoided several units that I may have bought without your knowledge. Quote Link to comment Share on other sites More sharing options...
Derek up North Posted December 11, 2011 Share Posted December 11, 2011 I don't know about the 'biggest', but many have gone as far as 1500w, which I believe would be more than enough for your fridge. Do you know how many watts the fridge needs? I'd start with that info and work backwards to find the smallest that'll work (plus a bit of safety margin). Quote Link to comment Share on other sites More sharing options...
WME Posted December 11, 2011 Share Posted December 11, 2011 There is roughly a 10 to 1 ratio using an inverter. IE a 750w inverter takes 75 amps @ 12v to power it at full output. So if you had a 750w inverter your stock alternator could not keep up under full load. The battery has to supply the remaining power. HOWEVER if you have a 750w inverter and place a 200w load on it, it will only draw 20 amps to supply the load. Remember also your Toyota takes power from the alternator to just run the engine, so you don't have rated amps available to use. And remember to turn off the inverter when you stop driving so that you don't kill the house battery while your out playing tourist for half a day. Full sine wave inverters are not as efficient as modified sine wave inverters when it comes to making 120v ac, but the equipment runs cooler and at full rated power with a full wave inverter. Especially microwaves, TV and computers like it better too. Start where Derek said and see how much power you need. Quote Link to comment Share on other sites More sharing options...
Bob C Posted December 11, 2011 Author Share Posted December 11, 2011 I am not sure about the frig in the MH because it is in storage. I have another frig, same size in the basement, the plate says current usage 1.4 A, 83 watts, 115 volts. I calculated this to be about 161 watts. I tried a 400 watt inverter that a friend let me try and it went to overload as soon as I plugged it in. The frig runs on house current really well with a very fast cool down. I know that start up requires more surge power that to run the unit. I was told that the starting surge is sometimes double the power it takes to continue running the frig. That does makes sense to me. I have looked this up on the net and according to several sites, the 400 should have been enough power. Quote Link to comment Share on other sites More sharing options...
waiter Posted December 11, 2011 Share Posted December 11, 2011 Bob Your calculations are correct. 400's are cheap enough, everyone should have one. Buy a 400 and try it. I would say your probably at the limit of the 400 as when the frig compressor starts, it'll be at least twice what the rating is. Basically, take your "amps" at 120 Vac, multiply by 10 to get an idea of the amps at 12 Vdc. So you'll be seeing abpout 15 amps. (no cigarette lighter plug, must connect straight to the battery with large cables, and keep the length as short as you can. Test it, with the frig, As WME says, the inexpensive inverters are square wave output, some appliances don't like this. John Mc 88 Dolphin 4 Auto Quote Link to comment Share on other sites More sharing options...
Maineah Posted December 12, 2011 Share Posted December 12, 2011 It takes around 220% more power just to start a motor then it does to run it. So you maybe hitting the max at 400 watts, plus starting a compressor turning too have a good look at the inverter often they like to sell you on the max load and the run current is less then the advertised 400 watts they will only stand the highest current for a second and you'll only get that with really good wiring, connections and a fully charged battery. 700 watt units are pretty cheap but you'll need to put it as close as you can to the battery and use some pretty stout cables. They are pretty liner so the 12 volt power consumption goes up with the load. Inverters by the way will kill a battery even when they are not doing any thing so when the MH is not in use disconnect it. Quote Link to comment Share on other sites More sharing options...
Bob C Posted December 12, 2011 Author Share Posted December 12, 2011 I was going to have the inverter turned on only while under way. I used a 12 volt refrigerator in our pop-up for many trips. I learned quickly that you turned it off unless the engine was running. Failure to do so resulted in the use of jumper cables. Only needed to happen once. I was going to install a relay to handle the switching of the inverter to allow it to be connected only while the engine was running. I tried an inexpensive 400 watt unit but it went to overload immediately. I think that the inverter I tried said 500 watt max. I will try a 750 watt unit when the RV comes out of storage. They are pretty reasonably priced and I want an inverter on board anyway. Is it ok to mount the inverter under the hood in the engine compartment to keep the 12 volt cable short? I will have to make sure that water does not get at the inverter. Quote Link to comment Share on other sites More sharing options...
Derek up North Posted December 12, 2011 Share Posted December 12, 2011 Is your coach battery located under the hood or are you talking about hooking the inverter to the starter battery? Quote Link to comment Share on other sites More sharing options...
Bob C Posted December 12, 2011 Author Share Posted December 12, 2011 starter battery Quote Link to comment Share on other sites More sharing options...
waiter Posted December 12, 2011 Share Posted December 12, 2011 Yes, you can do that (hook it up to the starter battery) but this would limit its use to when the truck is running. We use our inverter for an electric coffee pot and microwave oven when boondocking. If you mount it under the hood, you can connect it to the truck battery thru an isolator (same kind of isolator used for the coach battery) In fact, the small wire thats used to energize the coach isolator, just jumper off of that to energize your Frig isolator. ( it will energize only when the truck is running). John Mc 88 Dolphin 4 Auto Quote Link to comment Share on other sites More sharing options...
Maineah Posted December 12, 2011 Share Posted December 12, 2011 I was going to have the inverter turned on only while under way. I used a 12 volt refrigerator in our pop-up for many trips. I learned quickly that you turned it off unless the engine was running. Failure to do so resulted in the use of jumper cables. Only needed to happen once. I was going to install a relay to handle the switching of the inverter to allow it to be connected only while the engine was running. I tried an inexpensive 400 watt unit but it went to overload immediately. I think that the inverter I tried said 500 watt max. I will try a 750 watt unit when the RV comes out of storage. They are pretty reasonably priced and I want an inverter on board anyway. Is it ok to mount the inverter under the hood in the engine compartment to keep the 12 volt cable short? I will have to make sure that water does not get at the inverter. You can but you'll need to be very careful about your 120 volt wiring maybe even run it through a GFI right at the inverter before it goes any where else. Quote Link to comment Share on other sites More sharing options...
Derek up North Posted December 12, 2011 Share Posted December 12, 2011 I'd be a bit concerned about how an inverter would survive the 'harsh' environment under the hood. An isolator could be added (as John suggests) at or near the coach battery to allow the inverter to be kept clean, cool and dry. Quote Link to comment Share on other sites More sharing options...
Bob C Posted December 12, 2011 Author Share Posted December 12, 2011 I also thought about putting the inverter next to the coach battery. I was concerned about the fan noise on the inverter. My hearing is terrible but the frequency range that I hear pretty good seems to be the same as the small fans used on the inverters. I know that it would be better for the inverter to be mounted inside. Maybe I will mount it behind the refrigerator and run some heavy wire from the battery, including a circuit breaker right by the battery. That way I would not be hearing the fan noise as much. Quote Link to comment Share on other sites More sharing options...
waiter Posted December 12, 2011 Share Posted December 12, 2011 I mounted mine on top of the battery box. I don't hear the fan running, in fact, I don't think its come on yet. Before you decide on mounting, try it to see if it will work in your application. Keep the battery cables short, the shorter the better. (I just seen a 750 watt for $49 at Walmart) My 1500 watt Inverter Install JOhn Mc 88 Dolphin 4 Auto Quote Link to comment Share on other sites More sharing options...
dayoff53 Posted December 13, 2011 Share Posted December 13, 2011 I have a stock alternator in my 1988 22RE Toyhouse and would like to know what is the largest inverter I can install. I doubt the alternator will be a problem - I don't think the fridge will use more than the alternator puts out. The question is how much can your batteries handle and for how long. I put a small inverter in mine just to run my LED TV and blu-ray. I also added a 2nd extra heavy duty deep cycle battery. My brother put 2 commercial 6-volt batteries in his, along with the original 1 12-volt, and put in a 1500 watt inverter. He runs his microwave off his with no problems. Quote Link to comment Share on other sites More sharing options...
bufbooth Posted December 13, 2011 Share Posted December 13, 2011 I picked up power inverter at Meijer earlier in the year on clearence for $30, it is a Schumacker brand, model PID760 and in large print was advertised on the package as 760 watts. After I got it home and read the fine print, it stated 760 continuous watts with a 1520 Watts Peak. After reading this thread on how the average power inverter is measured/marketed, does that mean I really have a 1500 Watts power invertor? It has two 120V outlets and a digital display that measures Voltage in, Voltage out, and Power out. Anyone ever have any experience with the Schumacker brand? Thanks, Dennis B. Quote Link to comment Share on other sites More sharing options...
Bob C Posted December 13, 2011 Author Share Posted December 13, 2011 Your 760 watt will run a load of 760 watts but has some reserve for the start-up of the appliance. Quote Link to comment Share on other sites More sharing options...
Maineah Posted December 13, 2011 Share Posted December 13, 2011 Yeah it will stand 1500 watts on start up but only for a second then it's no more then 760 watts not a bad deal for a modified sine wave inverter. Make sure you have really good connections and at least a #4 wire fine stranded wire. Quote Link to comment Share on other sites More sharing options...
Maineah Posted December 13, 2011 Share Posted December 13, 2011 Yeah it will stand 1500 watts on start up but only for a second then it's no more then 760 watts not a bad deal for a modified sine wave inverter. Make sure you have really good connections and at least a #4 fine stranded wire. Quote Link to comment Share on other sites More sharing options...
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