Making Power
Home Up


Making Power… and other things I’ve learned about Generators

So, you want a generator?   Over the years, I’ve used quite a few, taken care of them, even specified new generators.   All of this is a quick summary of what I’ve learned.  Of course your experiences might be totally different, but hopefully, this will give you some things to either think about, or ask about.

Why do you need a generator?   Most likely, to make power when the power company fails.   While we haven’t had any day-long state wide power failures like the ones in the 1960’s and 1970’s, I think there is still a potential for such an event.    And in these many years since the last major one, we’ve become far more power dependent.   You didn’t have a computer at home in 1965.  

This website might give you some historical perspectives.

A generator, by itself, isn’t enough if you have anything important.   You’ll also need an Uninterruptible Power Supply (UPS) to supply power while you or the automatic switch starts the generator.  



Lots of options, gasoline, while easy to get after the lights have been off a while, it’s very dangerous to store and transport.

City gas

Often a good solution since there’s no local storage.   But what if gas lines are interrupted?   What if the disaster that killed the power starts to effect the gas company?

Tank gases

Easy to store, very few environmental problems.    No way to transport more gas yourself.   You’ll have to wait for delivery.   Same gas supply could be used for other purposes, like cooking and heating.  


Popular especially for larger generators.  Safer to store, but new environmental demands make it expensive to store.   Usually large above ground double lined tanks are required.   Also, diesel fuel needs to be cleaned regularly if it isn’t used.   Here’s what happens:  The fuel and tanks sort of breathe.    As they expand and contract, humid air comes into the tank.  When that hits the colder tank walls, the water in the air condenses.   Now we have air, water and diesel.   Believe it or not, this is the perfect environment for a type of bacteria.   Yes, there is a living thing that thrives on diesel fuel.  When that grows, it can clog the filter and engine.   Unless you go through the fuel every year, you’ll need to treat and filter the stored fuel.  

UPS versus the Generator

Why does my new UPS reject power from my generator?

Actually, this confuses a lot of people.    A REAL UPS cares more about power than 'will it make light bulbs light up?'    There are many factors considered when a real UPS goes to UPS mode and gives up on the AC source.   Is the voltage within a narrow range and stable within that range?   Is the frequency 60 Hz plus or minus a certain amount, and stable?

They assume that you purchased a major UPS to protect your load, not just act like a dumb transfer switch.   It will rightfully reject power from a generator that doesn't keep the frequency stable when the load changes.    When I hear of this problem, I ask how the UPS reacts to the power line.   The answer is usually 'fine… it's just the generator.'   The UPS doesn't really listen for the exhaust... it's looking at the power coming in and decides that your gen set is putting out power that could do damage to a critical load, oh, let's say broadcast or computer equipment.   If the UPS has no problem with the power company, but rejects the generator, it's trying to tell you something.

What can you do about it?   All those 'bells and whistles' in that expensive UPS include a log that tells you exactly why it rejected the AC source.  Hook up a computer to the UPS serial port and look at what it wants to tell you.    Was it frequency or voltage?   

Frequency?   Add an electronic governor to the gen set that will get the frequency stable with changes in load.   That will keep the engine RPM steady even if the load changes quickly.   Sort of like a cruise control on your car.   Some will keep you more or less at 55 MPH, others are rock solid, no matter how steep the hill.  

Was it a voltage sag?   This means that a large load was added to your generator and it took some time to get back to the right voltage.  When you’re on the power company, they’re more or less an infinite source of power (at least your small load compared to all their generators.   But when 60 amps or air conditioning compressor comes on your 200 amp gen set, that’s quite a load.   You need to either get heavy cyclic loads off the gen set or get a bigger one.   Examples of large loads that go on and off and can cause a voltage dip include air conditioning systems and tower lights. There are some tricks if you’re on the border… does the A/C need to cycle while the gen set is on?   Constantly on might make the room cold, but it could help your situation.   Maybe you could live with the tower lights on constantly while you’re on generator.   Do you have a non-emergency panel?   There are some things that just do NOT need to be on the generator.   Sprinkler pump?   Water heater?   Or my favorite, the jacket heater on the generator!  

Look into your gen set specs.   It might just be the wrong model for the application.   Gen sets are not created the same.   I had one installation that used a Dayton genset that was designed for emergency building lighting.   Just fine to light hallways and exit signs, but it wasn't designed for computer loads.   It was replaced by a Katight and that ended all the problems.   I’ve been told it’s a function of the amount of copper in the generator.  

If you don't care so much about your load, you can tell the UPS to not be so picky.   Say that the +/- 0.5 Hz window is too tight for you.   You can tell the UPS to allow +/- 1 Hz.   It's up to you.  You might be able to tell the UPS that 90 volts is just fine.  But don't blame your UPS when your equipment starts dying. 

Would you take the battery out of a smoke detector that keeps going off when there's some smoke?   Or would you put out the fire?   Fix the generator and let the UPS work as it was designed to.


I learned a lot real fast about the batteries used to start generators.   Here’s a what happened with mine:

Remember, if the battery is bad, you have a hunk of metal during a power failure.   Make sure the battery is in good shape so it can start up the generator, even if it takes a while to turn over.

Air Supply

Air is a good thing for a generator.  Make sure your air intake is clean, and will be clean during a problem.   I don’t mean just the carburetor intake.  Also the fan.   I’ve heard of generators failing during a hurricane.   A few things have happened:   The storm surge water comes up and starts to get sucked into the fan intake.  Or the barometric pressure has gone so low it has effected the air/fuel mix.   Or the wind from the storm cancels out the needed air movement and the generator over heats.   Any of these causes a shut down.    If it’s an outdoor generator, you can’t get outside to hit the reset.  


Remember that a generator needs more than fuel to run for days.   Don’t get caught without engine oil if you have days of fuel on-site.   Some larger generators, even in top condition, can burn gallons of fuel oil in a day. 

Too cold? 

Yes, a generator, especially a diesel can be too cold.   You should have a crankcase heater, even in warm climates.   The bottom of the engine should be hot to the touch, all the time.   This makes sure the gen set will start and run smoothly within seconds of starting.   This electric heater does NOT need to be on the generator circuit.   No need to heat the generator if it’s already running.

Transfer switches

This device does a few things

1.  Is there a power failure?   What IS a power failure?   No power?  Or poor power?   One phase low?   How long does it need to be off before you start the generator?  

2. If there is a power failure, order the generator to start.

3. Decide that power from the gen set is stable and ready

4. Throw a big double or triple pole, double throw switch to disconnect from utility and connect to the generator.

5. Monitor the utility power.   When it has been stable for a certain amount of time, throw the big switch back to the normal position

6. Let the engine cool down

7. Shut down the engine

8. Every so often, usually weekly, go through all this as a test to make sure the generator is ready for an emergency.

How well it does all these things makes the difference between the good ones and the great ones.   Throw in some telemetry, gauges, alarms and remote controls, and you have a nice bit of equipment. 

Again, all of this is based on what I have learned over the years.   Your mileage may vary.  But if this gets you to ask your sales person, technician, architect, or neighbor some questions, then I’ve accomplished my goal.  

01/14/2008 13:11:33 -0500

Except for portions owned by others, Copyright: Ray Vaughan, 2008