Go Back  DoItYourself.com Community Forums > Interior Improvement Center > Energy Usage, Conservation and Weather Stripping
Reload this Page >

Calculating minimum energy requirements for selecting generator

Calculating minimum energy requirements for selecting generator


  #1  
Old 11-04-12, 09:18 PM
T
Member
Thread Starter
Join Date: Oct 2010
Location: USA
Posts: 563
Received 0 Votes on 0 Posts
Calculating minimum energy requirements for selecting generator

Hey all,

In light of the various natural disasters in the last few years, I'm trying to determine minimum requirements for a generator, should I ever decide to buy one. So far I have the following that must always be expected to operate:
  • Heatpump - 5 ton, R22 refrigerant, circa 2000 (live in the desert, being able to cool part of the house is a requirement)
  • Refrigerator - Subzero built-in circa 1979 (worst case 2400V @ 120V)
  • Freezer - Whirlpool Gladiator GAFZ21XXRK (worst case 1800W @ 120V, runs out of 15A receptacle)
  • Exterior safety/security lighting (about 2A, 240W @ 120V)
  • Interior lighting (about 4A, 480W @ 120V)
  • One 20A electric circuit's worth of small devices scattered about (2400W @ 120V)
Things that could be needed but don't necessarily have to run at the same time as each other:
  • Convection toaster oven (Breville BOV800XL, 1800W max @ 120V)
  • Microwave (1800W @ 120V)
  • Basement Sewage Pump (measured the load once, about 13A, 1560W @ 120V)
  • 40 gallon Hot Water Heater (240V, would be on timer, would be set to mornings and off by 6:30AM)
I have a propane barbecue grill and I try to keep one 5 gallon tank full in reserve at any given time, so I could cook for quite some time on 5 gallons. I have a side burner too, so I wouldn't need to even use the electric cooktop in the house, let alone the electric oven, whose duties are covered by both the grill and the toaster oven. Obviously I wouldn't run the microwave and the toaster oven at the same time so that 1800W only needs to be counted one time.

We have plenty of books and don't even have cable, so no need to run the television, and I doubt that Internet service would remain reliable for terribly long without power, even though the cablemodem also provides the landline phone, and uses a small battery backup to keep the phone working during short power outages. In any case, if Internet service does work we could limit its use to the most efficient laptop in the house and could charge that laptop when not using other devices.

I want to be able to power the sewage pump at any given time, but it won't necessarily even be needed. That would depend on how much time we spend in the basement, which would be more in the summer months due to temperatures.

I'm willing to forego basically everything else. The clothes washer falls into that misc 20A load and we'd just line-dry the clothes, we wash 'em cold anyway so no need for hot water there.

Rough power draw estimates for the heatpump, the hot water heater, and any knowledge for refining fridge and freezer estimates would be appreciated.
 
  #2  
Old 11-04-12, 09:35 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 62,170
Received 1,528 Votes on 1,411 Posts
You've got quite a list there. That's the way you need to do it though. You've got some power hungry loads there. What fossil fuel will you be using. You mention propane for cooking so I'm going to assume you don't have natural gas which leaves gasoline, diesel or propane. Based on your wish list you're going to probably need to use diesel. You will need to carefully add up the wattage for the items you must power. No way you can run it all. You'll need select which items to run at a given time. I'm working with a customer right now in same situation as you. Propane hot water though. He's looking at a 40Kw generator. That unit at full load and running on diesel will yield 24 hours of service with it's own self contained fuel tank which I believe is 21 gallons. He's not happy with that consumption and is looking towards propane. Propane yields less Kw output than diesel. I'm going tomorrow to measure his propane tank and investigate an additional one. We're talking bucks here.
 
  #3  
Old 11-04-12, 09:48 PM
F
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,495
Received 36 Votes on 28 Posts
The water heater is easy, simply look at the nameplate and read the wattage. Most common are 4800 watt units. It will either be taking the full 4800 or nothing.

The motor loads, heat pump, sewage pump, refrigerator, freezer and washing machine are much more difficult in that you need to allow for the starting surge which in a worst case scenario could be as much as six times the running wattage.

Your heat pump at five tons is roughly five horsepower, not including the outdoor unit and indoor unit fans. Five horsepower is roughly 5,000 watts and the starting surge is probably close to 15,000 watts minimum. Like PJ states, you are looking at some serious machinery to run everything in the home.

I think the first thing you need to do is to access how often and how prolonged the power outages have been in the past and (hard to do, but try) what is the likelihood of prolonged power outages in the future. Outages of only a couple of hours are mostly just nuisances. Outages of more than a week often mean moving temporarily. For only a week people can often make do with far less than they originally think. I woulds say to forget about powering the laundry equipment. A larger water heater (80 gallon) can hold enough hot water for three days worth of navy showers along with hand and dishwashing. A couple of electric space heaters and closing off the major part of the house can keep you warm without the central heat. Same with a smaller window unit (or portable unit) for cooling.

Even with all that you will still probably want a slow-speed watercooled diesel unit and that, my friend, won't come cheaply.
 
  #4  
Old 11-05-12, 06:04 AM
T
Member
Thread Starter
Join Date: Oct 2010
Location: USA
Posts: 563
Received 0 Votes on 0 Posts
Yeah, I know that this won't come cheap. I did see a generator on craigslist, manufactured in the early eighties, using an old Chrysler slant six engine for the prime mover, 30KW electrical capacity. It's currently on a trailer, he's asking around $3000. It's tempting, and he says that the usage is low, like 200 hours or so, but he states that the voltages are 120V/206V, and I don't understand the 206V number. That doesn't make any sense to me.

Kind of funny, I'm actually not trying to run everything at the house. Between the house and the detached workshop I have three heatpumps and two hot water heaters, and about twenty 15A and 20A circuits for the receptacles and lights.

One of my long-term plans is to rebuild an '82 Dodge D350 crew cab truck that I have sitting and to put the Cummins 5.9L drivetrain into it. I've been considering a transmission with a power takeoff, so I can attach a generator head to the PTO and use the truck itself as a generator when necessary. That's VERY long-term though, as my wife will kill me in my sleep if I start working on the truck before I finish restoring the car in the workshop...
 
  #5  
Old 11-05-12, 05:41 PM
T
Member
Thread Starter
Join Date: Oct 2010
Location: USA
Posts: 563
Received 0 Votes on 0 Posts
Checked the water heater, 240/208V, says "Upper 4500/3380, Lower 4500/3380, Total 4500/3380". I assume that either there are two elements and only one runs at a time, and that the 4500W number would be for the 240V mode, while the 3380W for 208V...

Would a memory clamp meter around one of the hot wires supplying power to the air conditioner possibly allow me to determine the amperage? I assume that both hots would draw about the same, so I could multiply the measured amperage times 120V and times two to get wattage. There's a junction box that I can get to in order to access the wires individually.
 
  #6  
Old 11-05-12, 06:06 PM
F
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,495
Received 36 Votes on 28 Posts
You are correct with the water heater. On the A/C you would need a peak holding clamp-on ammeter and then just long enough to measure the inrush current when the thing starts to learn the staring amperage. After it starts the steady-state amperage is the running amperage. Take either and multiply by 240 for the watts, starting or running respectively.
 
  #7  
Old 11-10-12, 07:43 PM
Nashkat1's Avatar
Member
Join Date: Mar 2012
Location: USA
Posts: 8,469
Received 2 Votes on 2 Posts
I assume that... the 4500W number would be for the 240V mode, while the 3380W for 208V...
It should be the other way around. It's Watts that stay steady.
 
  #8  
Old 11-10-12, 08:39 PM
F
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,495
Received 36 Votes on 28 Posts
Sorry, Nash, it is the resistance of the elements that stays the same. The lower wattage IS the result of using 208 volts rather than 240 volts.
 
  #9  
Old 11-11-12, 12:29 PM
T
Member
Thread Starter
Join Date: Oct 2010
Location: USA
Posts: 563
Received 0 Votes on 0 Posts
Okay, I took a look at the labels on the two HVAC units and on the air handlers.

The new, smaller Lennox compressor that takes care of the kitchen, the family room, the dining room, and a storage room on the ground floor shows an RLA of 16.67 and an LRA of 82.0. The Fan unit on it has an FLA of 1.7, horsepower of 1/4. The air handler for this unit has an FLA of 6.1 and Horsepower of 3/4.

The older, large Carrier unit that takes care of the upstairs and downstairs bedrooms and bathrooms, downstairs family room, and upstairs living room shows an RLA of 24.4, an LRA of 140.0, and the fan has an FLA of 1.4, horsepower 1/4. The air handler has an FLA of 4.3 and Horspower of 3/4.

To my knowledge neither air handler unit has auxiliary heat strips.

I assume that RLA is while the unit is running at any given moment, and LRA is when it kicks on and has to overcome initial pressure to start up. These units both say 230V instead of 240V, but I assume that for safety's sake I should do the wattage calcs assuming 240V.

That means, again assuming 240V, to operate the Carrier I need 35 kilowatts at startup and 7200 watts minimum to run, and to operate the Lennox I need about 22 kilowatts at startup and 5900 watts minimum to run.

If I cheat and calc for 230V I get about 33.5kW start and 6900W run for the Carrier and just over 21kW start and just over 5600W to run for the Lennox.

I can see why putting electric HVAC on a generator is a headache. Without something to store up power for the start modes for the devices it's difficult to supply enough current.

I've been mulling installing solar panels- I have a detached workshop and large flat roof above the daily driver garage, over the kitchen, and over the integrated patio roof. For the price of the generator I'd need I could go a long way toward making solar work, even if I have to buy expensive gear to let the system intentionally island the house during an outage or to add a battery system for handling momentary high demand loads like compressor starts.
 
  #10  
Old 11-11-12, 05:45 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 62,170
Received 1,528 Votes on 1,411 Posts
You would need a massive solar cell array, back up battery system and invertor to run any type of air conditioning. If you get a power outage.....it would be most cost effective to rough it and use fans.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: