Generator FAQ

In answer to multiple requests, here is a generator FAQ thread. Those with knowledge, please add to this.

We have become so dependent on electrical power that many of us have or will be soon purchasing generators for standby power during emergencies. Selecting a generator is not complex, requiring the answer to two questions: What do I want to power with the generator, and what fuel do I want to use?

1. It is VERY easy to overload a generator. Motor loads in particular, such as water pumps, refrigerators, and air conditioning, consume large amounts of power and will either damage an undersized generator or be damaged themselves if the generator is of insufficient capacity. Fortunately this is easy to avoid. Generators are rated in watts or kilowatts (kW) and it is a fairly simple matter to determine the wattage of appliances to be powered by the generator. Appliances have a serial number plate attached to the rear or bottom with the voltage and amperage necessary to run them. Multiply voltage times amperage to get the wattage necessary, then add 20% for the surge current necessary to get the motors up to speed. A few examples are: Refrigerators, from 100 to 12oo watts, oil or gas furnace 750 to 1000 watts, water pumps from 500 to 2000 watts, and TV/radio 300 watts. As you can see, this adds up quick and anything less than 5kW can run only one appliance at a time. As far as the generator itself goes, bigger is better.

2. The second consideration is fuel. Most portable generators use gasoline engines. This is fine for a unit, which must be portable, however, for a permanent installation; gasoline is not the best choice. Gasoline deteriorates rapidly in storage and is highly volatile, and the vapors are explosive. Diesel or natural gas/LP generator would better serve a permanent or semi-permanent installation. If you heat your home with oil or LP gas, then you have a ready fuel source for your generator. Home heating oil IS diesel fuel, and diesel engines are ideal for powering generators. Diesels have no throttle plate like a gas engine, and therefore are just about as efficient at 35% load as they are at full load, and will burn less fuel at part load. Diesels are heavier and more costly than gas engines, and it may take some looking to find a portable diesel generator.
If you heat with LP gas, a conversion kit is available to convert a gasoline generator to run on LP. This is a less costly installation than a diesel rig, even though a gas engine is less efficient than a comparable diesel. Natural gas is a judgment call: Natural gas is inexhaustible supply UNLESS the gas service is out, which, depending on the nature of the emergency, may indeed occur. I will add that most of the commercial buildings that I serviced used natural gas generators, although the purpose of the generator was only to evacuate the building in the event of an emergency. YMMV.
In any case, the opposite is true with engines: the smallest engine that will do the job is most efficient, and it is a balancing act between sufficient electrical power and engine efficiency.
The third alternative if you own a tractor is to use a PTO generator, driven by the tractor PTO. If you already own a tractor this is a valid choice, as you are buying the generator itself, not an engine/generator set. This is what I did; I live on a few acres and already had a diesel tractor. Photo below.

It is essential that your generator maintain the 60 cycles that grid current uses. Electric motors are highly sensitive to current cycles and will fail catastrophically (as in burn out) if the current is cycling slowly. Cycles are a product of generator speed and should be monitored by oscilloscope or other meter.

Installations vary from extension cords for a portable set to automatic startup and transfer switching. Most of us will be well served by a manual transfer switch. If you don't know what this is, then contact an electrician to install one. If you feed your electrical panel directly from the generator, you will back feed current thru the meter and energize the power company lines, possibly shocking and/or killing the utility workers! This is a VERY dangerous condition, so install or have installed, a transfer switch.

Last, but perhaps most important, is that generator power may not be the entire answer to a utility outage. Alternate heat sources such as kerosene heaters will greatly reduce your dependence on electrical power, as will kerosene lamps and battery lanterns. The most efficient use of the generator is to load it to about 80% capacity, so if you can run the generator for an hour or so every 4 or 6 hours, you will greatly reduce fuel consumption and wear. Consider reducing generator load as much as practical prior to determining your need for a generator. Ops

Ops, I will add a little here, FWIW.

Fuel choice: Operating cost on natural or LP gas is horrendous. Consumption of those gaseous fuels is quite large. In a cold climate, getting adequate LP gas evaporation is nearly impossible unless the engine is liquid fed. 100LB tanks just don't cut it. Gasoline stability is primarily affected by moisture. A little dry gas and yearly, or better, twice yearly drain & refill prevents most problems.

Exercise: ALL generators need run periodically! Many units have an exercise timer that starts unit and lets it run for 30 min every 7 to 30 days. On non-automatic units, YOU are the exerciser!! Put it on your calendar!

An annual oil change is mandatory. Use top quality full synthetic oil. It is the bee’s knees for air-cooled engines!

Water-cooled engines are quieter, use far less fuel, last longer and cost MORE. 4 pole generators run half as fast (1800 RPM vs. 3600) resulting in less noise and wear.

Unless your generator is very large - 20 or 30 KW - it is senseless to install a transfer switch that takes your 200-amp panel off city power and connects it to a 30-amp generator! 200 amp xfer switches are expensive, too! Better to install a 2-pole breaker in your existing panel slightly larger than your generator's capacity. Feed from that breaker to a transfer switch then to a sub panel. Move all your critical loads from your original panel such as well pump, furnace, alarm system, a TV or radio, minimal lighting etc. DO NOT put more load in the sub panel than the generator can operate!! This scheme prevents overloads and requires little on the part of wife or kids when the power fails. Start generator, throw transfer switch, enjoy. Clearly this is a more complex method but it is legal in all jurisdictions, very safe and all around great. Most commercial installations use this "emergency panel" scheme - for a reason.

Domestic hot water: If your water heater is electric, consider that it requires 5 kw - then forget about connecting it, the cook stove or clothes dryer to the generator unless your pockets are deep!

FREQUENCY: I can't stress this enough!! Motors are PICKY about frequency!! The power company maintains frequency to .01 cycle (Hertz for you kids). This MOST critical factor can be checked and monitored several ways. A "frequency meter" is really a COUNTER that compares your generator's frequency to a stable reference in the instrument. A counter will work as will certain VOM (volt ohm meters) such as a Fluke or Field Piece. Between $100 & $200 for a top quality VOM. Radio Shack also offers counters. A surplus reed type freq. meter is great if you can locate one. Foolproof to use, extremely reliable and inherently accurate. Military used lots of them. If you are really tight on $$, here is an expedient way to test frequency; Obtain two motor operated clocks with a sweep second hand. Test the clocks by setting to exact same time and running them on city power for a few days. If they are identical, fine. If not, toss the SLOW one and try again. Now to test generator. Connect one to generator and load it up. Connect second clock to city power. Exactly synchronize the second hands. Any difference after a period of time is the generator's frequency error! An error of 1/2 cycle is acceptable which is an error of 1 in 120 or 1 second in 2 min. or 10 seconds in 20 min of the units exercise time - all for $5.00 worth of flea market alarm clocks! To correct, adjust the engine governor. Engine faster, generator clock runs faster. Once properly set it will require little further attention. Easy isn't it?

Voltage: Far less critical than many folks believe. A good VOM can test it (and many other things). Great SHTF tool if you go to the trouble to learn to use it. Generator voltage is controlled in many different ways. If it is not right, and you are SURE your meter is right, dig into the unit's service manual.

Generator selection:

Here is a link to a PDF page in the WW Grainger Catalog:
www.grainger.com/Grainger/wwg/catalogPDF.shtml

Start on page 305.

Look through the next 10 pages or so for great info on generators, transfer switches and accessories.

Once you get into Grainger's catalog, there is a wealth of available information!!!! Fuel consumption rates, surge power ratings, weight, sound ratings etc. Far more information than I could post here. They have branches throughout the country and will sell to anyone. I do not work for them but find the information invaluable and they can provide what they have in the catalog. Read, study, learn!

Surplus generators:

Uncle Sam uses a lot of generators - and sometimes a few become available through surplus sales. The are generally of EXCELLENT quality and can be a real bargain IF you know what ones are suitable to use - and how.

400 HZ or cycles:
These were built for use with aircraft. Limited use for anything else. You can run heating elements with them or incandescent lights. Will NOT work with motors, refrigerators, freezers florescent lights, radios, clocks etc. Avoid.

3 phase:
Three-phase power differs from single-phase power. These generators CAN be used to power your home - you just need to know how.

If you have some mechanical ability, surplus generators can be an unbelievable value!
120/240 4-wire units work as - is. Use the center-tapped phase (two lines and neutral), ignore the 3rd phase leg. Output will be less than unit is rated. As a guide, divide the rating by 1.73. (15kw unit could supply 8.7 kw.) Some will do better, some worse. With a little ingenuity, it is possible to make use of the 3rd phase, automatically, for a 240-volt load like a water heater, well pump or domestic space heating and take advantage of that wasted capacity. Beware of that 3rd leg though - it is 208 to neutral and can ONLY be used for 240-volt load. ONLY.

120/208 units (and Uncle used a LOT of these!!) These are nearly always 4 wire units. One is neutral, three are hot lines like above. Any ONE phase is 120 volts to neutral, 208 between any of the three. Almost all of your 240-volt stuff will hum along on 208 just fine.

Rarely you will find a unit with no neutral available as it was used only for three phase loads. If the thing is 120/208, the N connection is in the junction box as it is the star of the three windings. For 120/240 units, it is more problematic. The N MAY be present in the junction box but not necessarily. A dry transformer rated 240 each side with a centertap on one side will solve the problem. The centertap becomes your neutral (Non-tapped side toward generator.) Such a transformer can be purchased surplus at a decent price. New would be pricey! There are other ways, too but the part is harder to find.

480 volt: Some are reconnectable internally to deliver 240. Many are not. A transformer, as above, can make these usable.

BE SURE THE UNIT IS GROUNDED!!!!!!!!!! ALWAYS!!!!!

28-volt DC units. Lot of these around too. For those of you into solar, it is possible to combine solar cells into a 28volt array. Two car batteries are the same as a military 28 (24) volt battery. Half the current vs. 12volt. Not to be overlooked in some applications.

Surplus engines:
Many surplus units use engines exactly like ones available on the civilian market. Quite often they use a 28-volt electrical system but are otherwise the same. Dealing with that 28-volt system is not too hard but can get a little expensive. Don't let it put you off from buying a fine unit though.

SOME surplus generators used engines that were contract built for the military. Parts can be a challenge.

Shielded ignition: Most gasoline-powered units will have shielded ignition. Plugs for some of these are a problem. If you can find them surplus, OK but they are expensive and hard to find otherwise. If they are standard plugs in a little shield box, don't worry about it.

Surplus can be of very high quality and CHEAP - if you have some mechanical ability the value can be outstanding.

Domestic hot water:

If you have a gas or oil fired hot water tank, skip this post. If yours is ELECTRIC, read on! An electric water heater draws about 4500 to 5000 watts (4.5 to 5.0 kw). It will run on any frequency such as 50, 60, 400 cycles (or even DC) and whatever voltage you can get - up to it's rated 240. A 50-gallon tank requires 12 hours to fully heat if you supply it with 240 volts and it begins with 50-degree water. NOT a good use of a generator!! There are exceptions - if you have a LARGE generator, if you happen to have a 3 phase one and use the ofdd phase jut for the water heater etc.

There IS a SHTF way to solve the problem, however. An engine recovers some 25% of the fuels heat for mechanical work (driving the generator). The balance, 75%, is rejected to the coolant and exhaust. It is normally wasted. A water cooled engine can be fitted with a heat exchanger and small circulator pump to heat domestic hot water. Energy withdrawn via this method is free and does not add to fuel consumption. Beware of freezing.
A couple turns of 3/8" or 1/2" copper (stainless even better!) wrapped around the exhaust pipe along with circulating pump (1/20 hp, <$80.) can provide LOTS of hot water. Be certain to apply proper controls (who wants boiling hot water in the shower??) and a RELEIF valve so the thing can't blow up!! Any who want to try this, contact me via e-mail for more details. For long term SHTF or those who would just die without a hot shower, it can be a great idea!

The hard core among us might even consider a horizontal plate clamped to the exhaust to cook on. They do get HOT and the heat is free........

Boilers:

Those of you lucky enough to have hot water (even steam) heat have another alternative for domestic hot water - an indirect heater. The boiler is easy to run off a small generator, as the motors required are small. The indirect heater is very effective and a good idea even when SHTF is not considered. Tankless heaters work too but I hate the things.

If you run HW heat, the heat exchanger can also be used to heat the boiler water, so that no heat is wasted. This is a mini version of the "co=generation" plants seen in industrial applications - the waste heat from the genset is used to heat the building, domestic hot water, etc. If you have a water-cooled stationary diesel plant with hot water heat, no energy is wasted and you will bet the maximum value from your fuel.
MM, Is your set up like this? Mine is not, I use a PTO set. Ops

WRONG!!

My gas water heater MUST have electric or she don't run. It's got an A/C fan for forced ventilation & electronic ignition with a cut-off if it's not working.

I'm thinking about a battery backup w/inverter, since I don't believe the draw could be all that much

Yep, I screwed the pooch above!! Of course, oil fired requires power as does SOME gas units and I did not address that.

The amp draw of an oil fired, induced draft or electronic ignition heater will be spelled out on the data plate. AMPS x VOLTS = watts (Well, VA really, but close enough in this case.) An oil burner motor is moderate starting torque but the induced draft/electronic ignition gas heater is low torque, therefore moderate starting demand.

Little known tidbit: Most generators have a 12v DC output that is in addition to the rated AC watts. That 12v DC can be used to charge batteries used to run lights, inverters etc. Can add to the total energy available from solar or other sources.

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My hot water gas heater worked for a week without power after Isabel. I think link belongs in the genny FAQ for multi fuel applications.

Tri Fuel conversions