Four Letters Re: Sizing a Retreat AC Power Generator

Mr. Rawles:
I saw that you recently posted my question to the blog, so I thought I’d update you. I ran the tests again and got what I believe to be a more accurate assessments.

My second test showed the refrigerator consuming right at 2.7 KWH (2,700 watts) over a 24 hour period for an average of 112.5 watts-per-hour. Now mind you, that includes all the hours we were asleep and so no one was opening the door, using up ice, etc.. During hours of heavy usage it was using about 150 watts-per-hour.

Test #2 for the chest freezer yielded the following results: KWH usage for the full 24 hours came to 1.02 KWH or 1,020 watts. This is an average of 42.5 watts-per-hour. Mind you, this freezer basically only gets opened once per day when we take out whatever we’re defrosting for dinner. All in all, I’m pretty happy with those results.

The next step is to test our other refrigerator and our upright freezer and to calculate the Amp Hours required (how many deep cycle batteries I’ll need) to build my homemade UPS system.

FYI, I found a really good deal the other day on a 4 KW emergency gasoline genset, and went ahead and bought it. My next big purchase will be a tri-fuel conversion kit from US Carburetion, so I can run her on propane. I know you guys usually endorse diesel as a primary genset/retreat fuel, but I really like the stability and shelf-life of propane – in my area, I can rent a 300-gallon tank (I own two 100-gallon cylinder tanks) from the propane provider for around $50 per year and fill it a little at a time as opposed to making an expensive all-at-once fuel purchase. My logic there being that I can dump a little in each month, so that it’ll be full when I actually need it to be. – JSC in West Virginia – A “10 Cent Challenge” Subscriber


Dear JWR:
I was catching up on SurvivalBlog this weekend and noted the article on generator set sizing. The main issue here is that there is a significant difference in the average electrical energy consumption of an appliance and its peak usage. This issue is compounded by electrical devices such as motors which are not purely resistive (i.e. inductive load) and thus have up to 3 times the energy demand to start as opposed to running. This is commonly referred to as “starting current” verses “running current”. When sizing an electrical generator, one needs not only to calculate the total energy consumption of all electrical appliances one anticipates to be running simultaneously, but also to cover the starting current for the item with the heaviest draw. Most electrical motors are labeled with their electrical current needs, commonly listed as starting or peak current and continuous current. In regard to an appliance which doesn’t list this information (such as a refrigerator), the owner needs to use his Kill-A-Watt [meter] to determine the current used while running (typically 3-5 amps) and multiply this by 3 to get a good estimate of the starting current demands.

The process should be to add up the total draw for all the appliances, and then double the highest one and add that also to the total. This will give a rough estimate of the peak current draw, in Amps. To convert Amps to Watts, one simply needs to multiply by the operating voltage (typically 120 or 240 Volts). This assumes that no more than one heavy draw appliance starts at the same time, but to cover all the starting currents would require a much larger generator.

Several years back, during an ice storm, we were living off of an emergency generator rated at 5,000 Watts (6,200 peak Watts ). One should disregard the “peak” rating of typical portable emergency generators since they are uniformly overrated (I have noticed that recently, peak rating is what is listed, look for the “continuous rating”). Our water heater (a purely resistive load, hence no “starting current”) consumed 4,500 Watts. In order to take a hot shower, we needed to turn off all other circuits and allow the water to heat up. After an hour, the water heater was disconnected to allow the well pump to be operated to provide water through the water heater to the shower. This constant switching of loads was a real nightmare.

As a caveat, typical consumer portable electrical generators are not up the rigors of continuous use. Their fuel economy is atrocious; our 5 KW unit uses about 5 gallons of gas in an 8 hour period. They are also typically powered by the equivalent of an air-cooled lawnmower engine. Consider taking your lawnmower into heavy wet grass and mowing continuously for 200 hours. After a week of trying to keep this loud and hungry beast fed, thankfully the power came back on-line. We went with a diesel powered 15KW unit which would even cover the arc welding unit and it uses about 1/4 gallon of fuel per hour during typical household test uses. The gas generator seemed to use virtually the same amount of fuel regardless of the load, but the diesel unit just sips fuel when it is just loafing along, with consumption roughly linear with the load.

When choosing a generator for long term use, I would make several recommendations:
First, if you pump water or want to run a welder or air conditioning unit, you will need at least 10 KW and 120/240VAC capability.
Second, get a unit with double windings so it can run at 1,800 rpm instead of 3,600 rpm (to make up 60 Hz AC power). This vastly improves fuel economy and noise level as well as longevity.
Third, the unit needs to be water cooled. While some air cooled units are built for longevity, they are the exception.
Fourth, think of fuel storage requiring long-term stability. This effectively rules out gasoline, and leaves us with NG/LPG or diesel.

While electrical generators are very useful and highly recommended, their Achilles’ Heel is fuel availability. We store adequate diesel fuel to run the generator full time for approximately two months use, which would extend to one year or more with limited part-time use, but it is still a finite resource. They can be useful as a bridge for short duration (till the power comes back on or we learn to live without). Except in the hottest climates, running a refrigerator or freezer a couple of hours twice a day is adequate with limited door opening. Once the foodstuffs in the freezer and refrigerator are used up, you will still need a manual pump for your water well in TEOTWAWKI. Hope this helps, – NC BlueDog


The Kill-A-Watt meter is a great tool but [KSC] really didn’t give it a chance to work. If you want to find out how much power your refrigerator uses over the course of the day leave it plugged into the meter for a few days at the minimum.

Most watt meters have the option to see how much power is currently being used by whatever is plugged into it. You’ll want to look at that while the appliance is cycled on. The refrigerators and freezers that I’ve dealt with generally don’t use more than about 150 – 200 watts while running, figure they use about three times that during startup.

In your situation, figure 600 watts startup power, times four appliances would be around 2,400 watts. I’m guessing that there will be other things that you will want to run also (lights, grain mill, battery charger etc.) so you may want to go with a 3,500 watt generator but as long as you aren’t looking to power your whole house from top to bottom with it you don’t really need a huge generator. – MercCom


Here’s a helpful site for figuring power requirements.

By the way, we all have useful generators sitting in our garages–in our car and/or truck. An inverter will let you tap that power. COSTCO has a 1,000 watt inverter for $65. If you use good sense in using power, and keep your vehicle tank(s) full, you can ride through a temporary power failure. Not bad for $65. But you also will have to buy or make up a pair of cables that will clip to your battery. The provided cables have useless terminals (closed end type) for the battery end of the cables. – Bob B.