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Old 02-24-2018, 08:31 AM   #1
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Generator Sizing

If I want to run 2 rooftop AC's and still be able to charge the batteries, what size generator am I going to need?

I am thinking around 4000 watts....
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Old 02-24-2018, 09:24 AM   #2
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Kinda depends on what they draw. You will always need more than what the stated running load requires.
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Old 02-24-2018, 11:37 AM   #3
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Quote:
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If I want to run 2 rooftop AC's and still be able to charge the batteries, what size generator am I going to need?

I am thinking around 4000 watts....
Class As with 2 a/c run on 50amp shore power and(I think) a 13kw gen

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Old 02-24-2018, 11:52 AM   #4
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I had a 6kw Kubota in my last bus. It ran two 12.5k Btu roof airs just fine.

The roof airs that I ran needed about 1800 watts each while running and a little headroom for compressor startup. There are some newer high efficiency units that are closer to 1400watts.

As far as the battery charging, tell us what sort of charger you are running? Voltage and current?

What else will you be running from the generator?
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Old 02-25-2018, 09:15 AM   #5
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I am thinking 2 Dometic units at 1750W primarily while we drive. I am still not sure my OTR A/C will work, it blows cool so I am hopeful but just thinking long term.

Otherwise, we think we will be on shoe power primarily when using the AC's.
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Old 02-25-2018, 10:57 PM   #6
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Probably at least a 6kw unit. And stage the startups.

Bill
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Old 02-26-2018, 09:14 AM   #7
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Sounds funny but, could you install two smaller units, one for each leg? Each leg with a seperate breaker and just carrying 1 A/C.

I am really trying NOT to take up any under bus storage in my coach but I have 2-3 smaller areas that might fit the smaller genny's...

Remember I am converting an MCI Coach, not a schoolie.
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Old 02-26-2018, 10:02 AM   #8
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Quote:
Originally Posted by PNWorBUST72 View Post
Sounds funny but, could you install two smaller units, one for each leg? Each leg with a seperate breaker and just carrying 1 A/C.

I am really trying NOT to take up any under bus storage in my coach but I have 2-3 smaller areas that might fit the smaller genny's...

Remember I am converting an MCI Coach, not a schoolie.
The 6 kw that I installed was in an Eagle. Worked great. I don't think an 8kw would be outrageous but if ? I want to make the best choice, you should make a list of all of the loads that you will be running from your generator along with their power consumption (Watts) and total that up

That will give you a better idea of what size generator is needed.
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Old 02-26-2018, 10:17 AM   #9
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Well if I am mostly worried about being able to run the 2 A/C units, each at roughly 1700 watts, then two 2k generators would suffice no?

Now if I am boondocking, I would be close to my limits running both at the same time if I expect the generator to charge the 12v battery bank, which will be running my lights, outlets, small appliances.

Or I could run one AC and use the other charge the battery bank.

I want to use a separate quality 4 stage charger for the house batteries, and the 24v alt for the coach side. Keeping the battery banks separate...

Does that make sense?
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Old 02-26-2018, 10:45 AM   #10
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Quote:
Originally Posted by PNWorBUST72 View Post
Well if I am mostly worried about being able to run the 2 A/C units, each at roughly 1700 watts, then two 2k generators would suffice no?

Now if I am boondocking, I would be close to my limits running both at the same time if I expect the generator to charge the 12v battery bank, which will be running my lights, outlets, small appliances.

Or I could run one AC and use the other charge the battery bank.

I want to use a separate quality 4 stage charger for the house batteries, and the 24v alt for the coach side. Keeping the battery banks separate...

Does that make sense?
If the AC's are the ONLY load that you are going to be running then you could likely make it play. The Honda EU200i will run modern roof ac. Barely. It is actually rated at 1600 watts continuous, 2000watts surge.

I would suggest that you size the generator to not only run your AC's but also your house battery charger/converter, lights, tv, coffee maker etc.

I would also suggest that you have a modest amount of headroom.

My strong personal preference is for a diesel generator in a diesel powered rig. There are many benefits to diesel generators. The most obvious is that you don't have gasoline on board.

When I installed the Kubota generator in my Eagle I plumbed the fuel pickup into the main fuel tank with the pickup about 7/8 of the way down the tank. That way I could not run the bus out of fuel running the generator.

Regarding two generators and budget. If you don't want to spend$$$ on a diesel generator and are hot on the twin generator idea, take a look at the Champion 3100's. You can connect two of them in parallel. Then run one for light loads and fire the second one up when needed.

Just to throw a little chaos into the thread.... Have you considered Mini split AC's? They are much more efficient. More Btu's for less watts.
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Old 02-26-2018, 11:26 AM   #11
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I hear ya about the diesel generator, but starting at like 3-4k, thats out of my league right now. I might just need to wait.

I am not quite ready to make decisions on these things but I need to get the planning started.

Our floor plan, at this early stage is sort of segmented so I am not a single zone for the mini split would work. Plus it would take an entire bay under the bus.
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Old 02-26-2018, 02:36 PM   #12
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if you mainly want the Genny for A/C for driving why not just run the bus A/C for driving rather than rip it out?

engine driven comporessors are good at what they do - cool the bus when its on the road.. then when you sare on shore power you use the rooftoppers
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Old 02-26-2018, 05:32 PM   #13
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Turns out the OTR AC isnt going to last, too many leaks.

Looks like I am stuck with rooftops and genny's...
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Old 02-26-2018, 05:50 PM   #14
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Originally Posted by PNWorBUST72 View Post
Turns out the OTR AC isnt going to last, too many leaks.

Looks like I am stuck with rooftops and genny's...
what year bus? is it an R22 system or an R134a? if its 134a the leaks are usually at the fittings or just cut off the lines and crimp new fittings on... ifits R22 then thats a tough one to save in this day and age..
-Christopher
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Old 02-26-2018, 08:13 PM   #15
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1978 MCI8 - R22 I think. The AC guy said it leaks from a dozen spots...
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Old 02-27-2018, 10:25 AM   #16
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Fixing leaks is cheap compared to building a new system. I'm with Cadillac on using the over-the -road system when over-the -road.
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Old 03-05-2018, 01:15 AM   #17
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Remember, it may be severe weather so, where could you put it to keep it safe and be able to refuel it easily? If you're using power cords, will they be dry and as far out of the way as possible?
Small generator set (less than 5500 watts) can only do so much. If you overload the unit and damage it, you won't have any power at all. So be smart! Portable units are made up to almost 20k watts but they're not for people on budgets.
The bottom line is to add up the load (watts) you plan to connect and size your generator at least 25% higher. When motors like the compressor in the refrigerator start up there is a sudden draw of power higher than the normal running load and you want to compensate for that.
Small appliances will have the wattage marked on them. Lighting circuits will be less than 1200 watts each. Water heater is 4500 watts alone.
Also,you'll truly get what you pay for...unless you buy from one of those guys selling from the back of their pick-up truck (don't do that). Don't wait till the last minute or you'll be stuck with whatever is left in the stores.
Oh, and the "trick" with connecting the generator to the dryer outlet to backfeed the house? DON'T YOU DARE!! It will result in damage and/or injury...
Hope that helped a little...sorry it's so wordy
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Old 03-05-2018, 09:36 AM   #18
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Look up auxiliary power units for semi trucks. They're diesel powered, quiet running, can provide you with a 110 power source, 12v power source, an ac compressor, and heat if so desired.

I bought one from a truck scrap yard and am in the process of converting it to my needs right now.
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Old 03-05-2018, 10:37 AM   #19
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Look up auxiliary power units for semi trucks. They're diesel powered, quiet running, can provide you with a 110 power source, 12v power source, an ac compressor, and heat if so desired.

I bought one from a truck scrap yard and am in the process of converting it to my needs right now.
Can you post make/model of one that would have the above capabilities?? Sounds interesting.
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Old 03-05-2018, 01:02 PM   #20
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With these coaches, the alternators are massive 240A beasts. Some of the busnut folks are suggesting a dual alternator system and just let the engine charge the batteries while I drive.

The math is something like each alternator is like 6400w or something. I am exploring that idea for the interim but I will need some generator for backup.
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