View Full Version : Help needed to calculate electricity draw and recommend alternatives

Raymond Fries
01-31-2014, 4:27 PM
Is threre a device I can rent somewhere that I can measure the electric draw from one of those 220v heating and A/C units for a room. One of those was installed in the garden room we had built and our energy costs have more than doubled. There are also eight 300w grow lights that I am sure draw the juice. I checked out one of those Kill a watt devices from the library to check all of the 110v stuff. Looking to maybe make a change to save electricity. Maybe solar or a mini split would help.

Any help on these would be greatly appreciated.


Steve Rozmiarek
01-31-2014, 4:38 PM
Raymond, unless you didn't use much power before, I can't see how those items would have doubled your power. A lot of options out there to measure electrical usage, I would personally use a simple clamp style ammeter and a few calculations.

David Masters
01-31-2014, 5:23 PM
I pay almost as much heating my garage shop part time, as I do for my entire 3,000 sq. ft. home. Those 220v "hotel room" units are terribly inefficient. I've been supplementing my heat with kerosene heaters to try to keep the cost down. I'm going to have to find an alternative as well, so be sure to share information if you find an alternative.

Thought I better edit my post to point out that my garage shop is well insulated.

David Weaver
01-31-2014, 5:34 PM
I don't know what the 220V slide in units use, and don't know if there's a 220v kill-a-watt, but when I looked at the choice between a mini split or a slide-in, the slide-ins had an eer of 9 and a SEER of around 13 or so. The mini split is 14.5 and 20+, respectively, and I would imagine that quite a bit of the difference is the ability of the split systems to work at lower temperatures before the pre-heating coils come on.

For the 15k btu mini split that I got, if it's 45 degrees out, it makes about 20k btu of heat according to the book at about 1kw of consumption. It doesn't stop working until it's -13 outside, and the slide in units we looked at had various points, but one equivalent in size to the split stopped working around 13 degrees, which would suggest that it's working a lot less efficiently at typical winter temperatures. When it's zero degrees, the split's literature says it's 100% efficient. I don't know what that means, but I'd assume that it's making significantly less heat on 2.5 kw at zero degrees than it makes at 45 on 1 kw.

Your grow lights literally use more than 2 kw of energy, and it's likely that combined with the slide-in unit, you may be consuming 4 to 5 kw/hour when it's very cold outside. at 15 cents a kw hour, that would be 60 to 75 cents an hour of consumption when it's very cold. That adds up quickly.

Pat Barry
01-31-2014, 6:09 PM
I think all that is necessary to get a pretty good estimation is just to know the wattage rating on the 220V heater - AC unit. Next, how many hours per day are they all running. Once you got that its easy to figure the kwh.

Lee Schierer
01-31-2014, 7:24 PM
There should be a label on the 220 volt heater that tells how many watts it is. If it is 1500 watts, then it uses 1.5 KW per hour that it runs. For the A/C you can convert BTU's to kilowatts.

Jim Matthews
01-31-2014, 11:08 PM
I used a Fluke "amp clamp" (http://www.newark.com/fluke/fluke-324/clamp-meter-digital-true-rms-4digit/dp/88T2548?mckv=spnaa0bWn|pcrid|33870076701|plid|&CMP=KNC-GPLA)which uses induction to measure current.

I believe they have range adjustments to help make sense of the reading.
It should work, but I'm dredging the pond to remember if this would work.

Brian Elfert
02-01-2014, 7:35 AM
Just those lights alone would use 2.4 KW an hour to run. Locally it would cost about 25 cents an hour to run the lights at 11 cents a KW.

Dan Hintz
02-01-2014, 8:15 AM
Your grow lights literally use more than 2 kw of energy, and it's likely that combined with the slide-in unit, you may be consuming 4 to 5 kw/hour when it's very cold outside. at 15 cents a kw hour, that would be 60 to 75 cents an hour of consumption when it's very cold. That adds up quickly.

That's what I was thinking. You have a huge load in the lights alone. That energy has to go somewhere, but at least in the winter it helps heat your room.

Raymond Fries
02-01-2014, 10:18 AM
Good tip for the watts calculation. Here is what I came up using our kw rate of .0863:

I stand corrected. The lights are actually 400 watt.

Lights – 3.2 * .0863 = $0.27616/hr
Heater & A/C unit – 4.8 * .0863 = $0.41424/hr
Not looked at the A/C yet

The heater runs about 12 hrs per night. This calculates to about $139 per month.
The lights run about 12 hrs per day. This calculates to about $92 per month. So this comes up to about $230 per month for just that room. Our electric bill used to be about $175. It was $382 last month.

When it is extremely cold, we have to run the heater as well as the lights to keep up.

The room is only 224 sq ft. (14’ x 16’) but it has 22 windows (most of three walls), 4 skylights, and an exhaust fan at the peak that has to run sometimes to keep the humidity off the windows. There goes the heat…

Does anyone know what it costs to run a mini split? Looking to save on installing something else. Was wondering if we could add the room on to the existing furnace and central A/C. Maybe it was not possible and that is why the contractor installed this unit. Any ideas?


Pat Barry
02-01-2014, 10:48 AM
Wow - lots of windows Ray. Here is what I suggest. 1) make a very careful inspection and find and seal any drafty things that you can find, ie: window seals, doorseals, skylight seals, electrical outlets, etc. Any place at all where you are wasting heat. 2) do something with the windows. For me, with no where as many windows as you have, we put the 3M window insulator kit up. Remarkable to me how much difference that makes. If I were you I'd start at the top and try to do something with the skylights 3) add curtains so you buffer the room from the window glass - maybe rollups if you want the nice views during the day. I have no experience with reflector films - maybe someone else does. 4) find ways to stop using the vent - the window film will hep immensely with this. Good luck - remember green houses are usually very humid places - its part of the package

PS - can you get LED grow lights? They would save a ton on lighting costs

Dan Hintz
02-01-2014, 12:46 PM
PS - can you get LED grow lights? They would save a ton on lighting costs

Yeah, but in the winter you'd just have to use a bigger heater to keep the space warm... net gain of zero (at best).

Mike Lassiter
02-03-2014, 3:42 PM
our shop the laser is in is 12'x32'. When we set it up and wired it I put a window unit from Home Depot that was combination heat and cooling. Insulated good and sheetrock on inside walls. Exhaust for laser is recirculated so no heat loss due to trying to heat the world when running the laser. Our laser has been climate controlled it's whole life, and we found during the summer our electric bill ran $30-$40 month and that depended on how much laser work was done. We have a 3-1/2 HP blower and Craftsman 240 volt compressor that are in a closet for lack of better description that is insulated for noise control. During the winter the electric bill ran $90-$100 month during normal weather here, again depending on laser use.

The heat & cooling unit had a remote control and digital temperature monitor. On cooling once the unit shut off when set temperature was reached the blower fan also shut off. It would start every 5 minutes or so to do a air temp sample. If temperature was still good, blower turns back off. If getting warm, compressor would start and unit would run unit set temp again reached then again shut off. When running heat however, the fan never turned off. the unit was purely electric strip heat and only the heating element shut off, never the fan. It cost me as much to heat our shop as it did our house. We have 2x6 exterior walls with R19 insulation and R30 something in ceiling and gas furnace, also running a auto setback thermostat in home so it was a bit aggravating the small shop cost what it did to keep laser in controlled temperature environment.

About 3 years ago now, we removed the heat and air window unit that we installed when the shop was build and setup, filled the opening in the wall and replaced that with a Fujitsu mini split system that has a SEER rating of 25 and EER 13.8 the highest SEER rating available at the time. One thing I truly like is the temperature can be set and regardless of outside weather the unit will maintain the inside temp automatically switching from cooling to heating automatically as needed to maintain the inside shop temp without me having to change the unit from heating to cooling. You know how there are those times of year it gets cold at night but warm during the day were the building could cool down too cold or get too hot for the recommended temperature for the laser to be in, unit will automatically do what it needs to, to maintain the room temp so it is set and forget. Oh, yea - the heating bill now is $30-$50 a month instead of $90-$100; cooling didn't change that much, but did drop some. This unit is a scroll compressor and inverter design that doesn't exactly ever stop running. It will "throttle down" to almost nothing when not needed, but the fan stays running always.

Recommend you check these units out. I think the cost was around $2100 in winter of 2011, where the other unit cost around $500 that we replaced. Highly recommend the one we got.

Kev Williams
02-03-2014, 4:45 PM
Any of you guys trying to heat a garage-sized shop, and have natural gas (or cheap propane) available should consider a direct-vent gas heater...

7-1/2 years ago we sold our home to buy a houseboat. That winter we lived in a 36' fifth wheel. Since most of the heat from the RV furnace went outside, our propane bill in 10 weather was about $40 every 3 days! So for a couple hundred bucks on ebay I bought a direct-vent thermostatically controlled gas fireplace. Propane bill went down to $20 every 5 days. Here's a pic--

HOWEVER, we were soon overcome by condensation. RV's just don't have much insulation, which means the paneled walls were cold, especially around the ceiling. Burning a non-vented indoor fire means outside air, and all the water in that air, must replenish the air the fire used. And those cold walls just loaded up with water, big time!

OK, that's the 'bad' of direct vent heat. But note that condensation is only a problem on COLD surfaces. We now use this fireplace to keep our houseboat warm (maybe 4 weekends a year)

BTW, yes, it's safe. The top of the thing never gets above about 100, and even the grill never gets too hot to touch. There's louvers just above the firebox and below the top, THAT's where all the heat comes out. And we've never had any condensation problem in the boat. However, we're not out there in REAL cold weather, so it isn't fired up all that often. And 'tight' this boat is not. It ventilates all too well.

Back to garages, most garage walls won't get cold enough to cause a condensation problem. And since October I've bought new machines, and I've had to turn my garage into shop space. I brought my fireplace home from the boat and tried it out, and it worked great. So I went back to ebay and bought a "Kozy-World" direct-vent wall heater. $217 shipped, 30k BTU, thermostatically controlled, and it's setup to use either LP or natural gas....

The thing works awesome! My shop isn't all that 'tight' either, but the walls aren't cold, so I have absolutely no condensation problem, even on the one window. You CAN feel the extra humidity, however, it's bone dry in Utah, so the extra humidity is welcome! One neat thing about this heater is that it has a 2-stage burner, most of the time it operates at only 20k BTU. See the 2 pics below, second pic I cranked it up so it would do a full-flame, you can see the difference. I put a small fan there in the corner to help move the air. I have it about 65 during the day and about 58 after I shut down for the day, I also turn off the fan at night. In the mornings I don't notice anywhere in the garage is warmer or colder than anywhere else, so I probably don't even need the fan. When it's in the 20's out, it seems to kick on for a few minutes about 2 or 3 times an hour. Probably about the same time the house furnace runs. I know that's it's not costing me that much in NG to run it, and it's a WHOLE lot less than electric would be!


Dan Hintz
02-03-2014, 6:59 PM
Back to garages, most garage walls won't get cold enough to cause a condensation problem.


I'm really hoping the water staining on the wall surrounding the A/C unit is coincidental ;) :D

Steve Peterson
02-03-2014, 7:21 PM
I estimate the yearly cost at about one dollar per watt for a device that runs all the time. This works perfectly when electricity is 11.4 cents per kilowatt-hour. Adjust the cost depending on your actual electric cost. In my area we are around 12c for the baseline, but then the cost shoots up to 20c, 30c, and even 50c for going way over the baseline.

A 400W device running 24 hours per day will cost around $400 per year. 400W * 24 hours * 365 days * $0.114 / 1000W-hours = $400. Your lights are only on for half the day and your electric rate is about 0.75X, but each bulb will still cost about $150 per year or $12.50 per month.


Brian Elfert
02-03-2014, 8:34 PM
Our residential electricity rates are still a flat rate. We don't pay more when we use more, yet! At work we get charged based on peak usage. We replaced some old equipment with new stuff that uses about the same amount of power, but reduces our peak loads to save money.

Raymond Fries
02-03-2014, 10:02 PM
Mike - I like your option. Did you do the install or hire a contractor? I did some research on installs today and it appears that I could do this and save some money. Fujitsu is highly rated and I will go with one of those.

Mike Lassiter
02-03-2014, 10:36 PM
I did everything except open valves on compressor to allow charge into lines. I bought unit and made deal with dealer to do install completely but for warranty he had to pull vacuum and check my work. I have a/c gauges and vacuum pump but his pump pulls deeper vacuum than mine and he had digital gauge were mine are not. Also due to much higher pressure of the refrigerant this uses you have to use thicker walled copper tubing and special double flaring tool. I have double flaring tool used in running steel lines like brake lines but to my recollection the angles are different on these fittings.
You would have to have install certified by dealer for warranty. I think on mine it is 6 years parts 2 on labor.

Mike Lassiter
02-04-2014, 12:08 AM
Here's a few pictures to give you an idea of what I did. Much left out but after removing the old heat & air unit I had to block opening to put cedar boards on outside, then insulate wall cavity. This was done in December and had to keep building heated and opening covered when I got old unit out unit wall closed back in. Also had to run wiring outside thru wall into breaker box, and since I had the wiring stapled to stud powering old unit, I just cut wire off and sheetrocked over the receptacle box and cut wires in breaker box and pushed then into wall. Reused breakers in box for new system and installed disconnect box outside by unit.
I bought a wall bracket kit to mount compressor unit on outside wall up off of ground. Could possible want to move this building some day, and with unit on wall rather than concrete pad I don't have to worry about breaking system open to do that - if ever. Also with unit on wall weed eating around unit will never be a problem as I raised it up high enough to just weed eat under it. Also bought line cover kit which you can see running across wall over window and down to unit. This encloses both refrigerate lines, power from outside unit thru wall to inside unit and a condensation drain tube which you can see in close up showing everything. I am quite anal about details and probably went to a lot more trouble than the selling dealer would have. Some parts of the job are aggravating to do. Don't know how much I saved by doing everything myself, I never considered having it done otherwise and didn't ask about it. You can get these with multiple inside units running from a single outside unit. It has gotten down to low single digit temperatures this last month here, and a time or two previously. The unit I have operates down to a lower outside temperature than some of the other units. I supplemented heat with a small kerosene heater during the very cold spell we had last month, not so much because we had to, but trying to help the unit not run as hard.
As I said, I am extremely pleased with this system. We got some money back on it, but I think because we have home based business and it was installed on the shop building rather than our home we didn't get to get a $300 tax credit. Was explained tax credit was not for business but residences so we didn't qualify for it. Money saved every month will pay for it in a few years. I would guess we have saved $200 - $250 each winter because of the difference in cost to heat shop over the other unit we originally installed.

Raymond Fries
02-04-2014, 10:36 PM
Mike thanks for your information. Looks like you are set. Good idea to mount the unit on the building. Glad you are saving money.

I will have to have a contractor do part of the install but I can remove the existing unit, and plug the wall. I have some vinyl siding left over from the room addition and a piece of the interior wood paneling that was used so I can make it look like the unit was never there. Going to get some quotes and get this done in the spring.

Thanks again for your help.

Enjoy Life...

Ty Williams
02-05-2014, 12:18 AM
You're approaching this from the wrong side. You'll save more money by reducing heat loss rather than trying to reduce the cost of heat gain. If your windows are single pane (which is common for garden rooms/greenhouses), they're about R1. If they're double pane, argon purged, low-e^3, they're around R2. The best commercially available residential windows I've heard of are only about R5 and you can't even buy them in the US. You're losing a tremendous amount of energy out the glass (and that's before you even look at any air-sealing issues which can account for up to 40% of thermal loss). The best thing you can do is to reduce your heat loss. The cheapest and biggest improvement for the least work is to make "shutters" that you put into the windows at night from a rigid, foil-faced foam like Dow Thermax or Super Tuff-R. You can cover 32sqf to R13 for about $30. However, you would want to put them in each night and take them out each morning (if the sun is out). Another option is to use a multi-wall polycarbonate panel as your shutter material so that you can just leave in place all winter and the sun will come in if it happens to be out. These are often used in greenhouse construction as the "glass" because it lets the light in and has some insulation value for a lot less cost than top of the line glass. The triple wall 1.5"-thick panels are about R3-ish and the really spiffy ones are about R5. This may not sound like much, but if you have single pane R1 glass and you add cheap R3 polycarb, you've actually reduced your heat loss by 75%!

You should take a look at what the passive solar community is doing for insulating windows. Many of them are very ingenious people and they're doing everything they can to hoard all the BTUs they manage to get a hold of.

Dan Hintz
02-05-2014, 5:52 AM
if you have single pane R1 glass and you add cheap R3 polycarb, you've actually reduced your heat loss by 75%!(through the glass)


Raymond Fries
02-05-2014, 8:33 AM

Thanks for the tips but not sure if there is much opportunity to pursue your ideas. The room was built only two years ago and has Anderson double pane windows that crank open. As I recall, the cost for these was around $10K.

Here is our room:


Your idea for the polycarbonate panels might work if they would look nice and we cound get them sized to our windows and the windows could still crank open.

Kev Williams
02-05-2014, 11:05 AM

I'm really hoping the water staining on the wall surrounding the A/C unit is coincidental ;) :D
lol.. yup, it is. That was there when my parents bought the place in 1969 when it was 4 years old. I believe that happened while the home was being built...

And I just went thru my gas bill history, and and using their "weather adjusted" figures. My wall furnace has been running since the end of December.

This January I used 18.22267 decatherms of NG
Last January I used 14.11266 decatherms.

We pay an average of $8.36 per decatherm, and I used an extra 4.11141 decatherms this year, so that comes to $34.36 plus tax (of course) to heat my 375 sq. ft. garage for the month. There's still snow on the ground from mid December so it's not been particularly warm either. Since it's costing me nearly that much to keep 550 gallons of well-insulated water electrically heated every month, I shudder to think what electric heat in my garage would cost me...

Dan Hintz
02-05-2014, 2:56 PM
Since it's costing me nearly that much to keep 550 gallons of well-insulated water electrically heated every month

Holy moly! You have an indoor swimming pool to keep filled? I'm hoping that was a typo and you meant 50 gallons?

Ty Williams
02-05-2014, 10:21 PM
Holy moly! You have an indoor swimming pool to keep filled? I'm hoping that was a typo and you meant 50 gallons?

Probably a hot tub. 550gal is waaaaaay too small for a pool. That's large fishtank territory.

Dan Hintz
02-06-2014, 5:51 AM
Probably a hot tub. 550gal is waaaaaay too small for a pool. That's large fishtank territory.

Didn't think I needed my ;) icon on that one...