5K BTU Generator Questions

bigskypc50 · November 2017

Greetings everyone! Have a question I need to ask the vast array of knowledge that is here. I want to start and run 4 5K BTU windows units on something like the harbor freight 3500W Inverter generator, now it's my understanding that when they are running they are only using 600W a pop 2400W all together. However with the start up demands at lets say X2 the runing wattage thats about 5000W, something an open frame gen can run but they suck the fuel.

Now I was looking at soft start kits and I have no desire to take apart a small window AC and install cap, however I was wondering if I toke a bunch of cheap motor run caps and built a "box" that sat between the gen and AC's on the main power line, Would that be the same as a soft start and less the blow on the Gen? How many mfd's would I need?

MichaelK · November 2017

2X startup is unrealistly optimistic. I'd go with 3+X. Couple that with my personal experience of "Harbor Freight=Trash", you are setting yourself up for failure. Frankly, re-inventing the wheel to try to get high-wattage appliances to run on low-buget equipment is a bad idea that you will come to regret.

Perhaps you should take a step back and first explain why you want to run a bunch of air conditioning units on a generator. Are you setting up some kind of summer camp thing, or is this for emergency grid-down use?

From my own experience powering stuff with a generator, you want at least double the wattage you expect to be running, and 3X is better, Besides just startup, you have to contend with a dirty air filter, a fouled spark plug, and overly optimistic advertised specs to begin with.

bigskypc50 · November 2017

It's a school bus tiny house conversion and I will be parking and living off grid and being it's parked in the sunlight it will heat up like a tin can, in the desert southwest were it hits 95-105 degrees in the summer. The power company wants 8K to connect to the grid. In summer months I suspect it will cost 300-400 a month to keep one of those open frame generators going with gas 10 hours a day 30 days a month. I have no other choice that I know of. Way to big of a load for me to afford a solar system to power it.

The home standby gens are nice but need just as much fuel in terms of propane, and for 2 grand they only last 3600 hours, from what genrac said

Estragon · November 2017

I'd start with the colour of the roof. White or silver paint is cheap. From there, insulation and ventilation. In the dry southwest, maybe evaporative cooling after that. After all that, I'd think about ways to store "coolness" with thermal mass. After all that, I'd think about how to run an air conditioner.

BB. · November 2017

Conservation is a good place to start. Shades/screens/bushes/trees/etc. to keep summer sun off of the home will be a big help. Awnings over the windows, etc. It probably would be very difficult and expensive to keep the bus cool in sunny summer weather--And with off grid solar, even more expensive.

I know this is a solar forum--But we try to be practical and cost effective. $8,000 to run utility power to your lot--You would be very lucky to have even a smaller system (that could possibly run 1-AC unit) that would be near that amount (~20 year of up front costs, plus maintenance/battery replacement(s) every 5-8 years/inverter&charge controllers every 10+ years/etc.). Generally, the amortized costs work out to ~$0.50 per kWH (very cost effective installation) to ~$1.00-$2.00+ per kWH for typical off grid solar system--Vs $0.10-$0.30 per kWH for utility power.

In general, it is almost impossible to build an off grid-battery based solar+battery power system that is cheaper than utility power--Unless you have high up-front costs to bring power to the property, or have high per month connection fees and low power usage (i.e., seasonal cabins with $40-$100 per month connection fees and a handful of KWH per month in summer).

Also, if you ever have to sell the property--Having utility power is usually a big selling point. Off grid solar, in many cases, does not add to the price of the company, and for some folks, can reduce the value of the property.

Plus, if you plane to retire on the property--At some point, you may not want/be capable of doing your own power system work...

-Bill

wellbuilt · November 2017

4 a/c units seems like a lot .
I would try for 2 larger units, maybejust one blowing from the end thru the bus .
If you run a white canvase tarp over the bus it will help keep it cool .
I've run my house off a champion 4750 watt genarator when I'm there , and it will run for 10/11 hours on 3 gallons of fuel , the champion units run good and give ok power .
$ 8000 doesn't sound so bad once you rap your mind around the number ,
the power Company wanted 40 g from me , and I had to run under ground 350' to the house .
My father in law lived in a old mobile home and in may before the leaves where on the trees it was hot in there , one small 5000btu unit would cool 8x8x12 , with leaves on the trees it would cool the whole place .
Maybe a out side screened deck with roof would help .

bigskypc50 · November 2017

Thanks guys for the feedback, I was thinking of building an oversized carport to park the bus under. But I am struggling with what way to point the bus, I am thinking I want the long side (the side with the most mass) facing the be facing the northern sky and the other side would be on the southern sky but the sun would be high enough to be covered by the car port. Then just the back end would catch the lower after noon sun. I am saying 4 AC's because I had one going in bus parked in full sun (75-80 degrees and it was not livable) with no windows covered and it hardly would do anything. I need to stick to 5K size they fit in the windows.

How far would reflectix on most of the windows with blackout curtains and carport get me staying cool in the Southwest 100 degree heat?

BB. · November 2017

Buses are not well insulated--And are poor choices to leave in the sun during the summer. I am not sure that north/south vs east/west makes much of difference vs lots of shading to keep sun off the bus. If you point the bus north/south, you can possibly use the low winter sun to heat in the morning/evening time (design the overhead shade to block summer sun but leave lower angle winter sun in to heat in morning).

If you have cold winters, letting the sun in the windows (when the sun is lower on the horizon) can really lower your heating bills... But, again a bus with poor insulation (no double pane windows, etc.) is going to be cold/hard to heat.

We had a hot west facing bedroom (stucco 1 story "ranch style" home, no wall insulation, 50 year old windows). We replaced (all) the windows (double pane, low E glass, vinyl frame) and that ended the baking summer afternoons in the west side of the house (had reasonable drapes, but they never made the room cooler).

For me, the stucco walls without insulation was working pretty well. It was the old single pane windows that were killing us (hot summers, cold winters in the bedrooms).

-Bill

bigskypc50 · November 2017

Ya I assumed the windows are biggest culprit of the heat gain. During the conversion I put 1 1/2" ridge foam on the floors and walls below the windows, then roof is painted white and it still has the one inch or so factory fiberglass bat in the celling. I am assuming Reflectix over the windows will give me a well enough insulation layer that it should be like double pane windows, provided there is an small air gap?

MarianoKi · November 2017

Great suggestions in this thread. Thanks for the info, I was wondering about the same thing essentially.

BB. · November 2017

Here is a quick / simple article on preventing heat flow through windows:

https://www.energy.gov/energysaver/energy-efficient-window-treatments

Note that draperies can reduce heat flow (on average) by 10%--That is not really a lot.

Here is a bit more information on insulation:

https://www.energy.gov/energysaver/insulation
https://www.greatdayimprovements.com/insulation-r-value-chart.aspx (nice chart at bottom of page)

And insulation--You need thickness. R values for Fiberglass and foam board are (roughly):

https://www.energy.gov/energysaver/types-insulation

Standard fiberglass blankets and batts have a thermal resistance or R-value between R-2.9 and R-3.8 per inch of thickness. High-performance (medium-density and high-density) fiberglass blankets and batts have R-values between R-3.7 and R-4.3 per inch of thickness. See the table below for an overview of these characteristics.

Foam insulation R-values range from R-4 to R-6.5 per inch of thickness, which is up to 2 times greater than most other insulating materials of the same thickness.

If you want your "attic" insulation to be R30 to R60 and the wall insulation to be R22 to R38 -- That is a foot more more of typical foam board insulation--Obviously not a very practical insulation for the inside of a bus conversion (loss of interior space to insulation).

The R value for a typical double pane window is around ~R2-4... Not near the insulation value of a typical wall:

https://www.allweatherwindows.com/the-pros/architect/glass-performance-chart/

Regular single pane windows are around R1, and good Storm Windows are around R2:

http://www.finehomebuilding.com/1994/11/01/insulating-older-windows

A well-sealed inner storm window should give an R-value of 2. You also should remove the window casing and sill and make sure the exposed areas are properly insulated because they’re often overlooked.

However, unless you want to save the original windows for historical reasons or otherwise, the best course of action, even though it may be drastic and expensive, is to replace your old windows with new windows using high-performance (HP) insulating glass. Most HP glass is double-pane glazing with special coatings on the glass, and the space between panes is filled with argon gas to achieve an average R-value of 4. This may not sound like much, but it is 400% better than your single-pane windows (typically rated at R-1) and twice as good as standard double pane insulating glass at R-2. Once installed, an HP window saves energy 24 hours a day, year-round, without your having to do anything.

And awnings/overhangs/shading is better than shutters/etc. for preventing solar gain:

You should not use movable window insulation to reduce summer heat gain. With a dark-color insulating shutter or shade closed during the day, especially on the south or west side of the house, you have created a solar collector, which stores and radiates heat. Shading the summer sun is best done with roof overhangs, exterior awnings or deciduous trees.

And given that your bus and window hardware is probably aluminum--That can conduct heat in/out directly through the aluminum. In the chart below, note that R value = 1/U value (U-2=R-0.5; U-0.5=R-2):

http://www.finehomebuilding.com/1998/02/01/understanding-energy-efficient-windows

Frame materials also affect energy performance

There is a great difference in the insulative value of common frame materials. Here are some typical U-values for common frame materials.

Frame material	U-value
Aluminum (no thermal break)	1.9-2.2
Aluminum (with thermal break)	1.0
Aluminum-clad wood/reinforced vinyl	0.4-0.6
Wood and vinyl	0.3 – 0.5
Insulated vinyl/insulated fiberglass	0.2 – 0.3

A bus with "some insulation" risks being a heating/cooling nightmare--Just not a very cost effective structure to heat and cool (especially trying to do it with off grid solar electric).

Checking with a bus conversion forum may give you some more practical answers--For example:

http://www.skoolie.net/forums/f51/windows-curtains-and-insulation-20187.html

I've installed UV and solar blocking film on my windows, Reflectix panels over them and curtains over all that. Comparatively, the curtains are there mostly for appearances. The biggest improvement came from the Reflectix.

Very heavy floor length drapes would probably help in the winter if you ceiling and floor are properly insulated.

There is a lot of glass area in the front of my bus. I've been thinking of getting some moving blankets from Harbour Freight to close off the driver's compartment from the rest of the bus during times of serious cold. It might be warmer just to stay home.

-Bill

jonr · November 2017

I's use a mini-split instead of window air conditioners. More efficient and the inverter models don't have high startup loads. Adding insulation to the interior side of windows usually causes condensation problems (in cold weather).

A solar powered and air conditioned space is quite feasible - but I wouldn't start with a bus.

5K BTU Generator Questions

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Frame materials also affect energy performance

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