(Continued from Part 1. This concludes the article.)
Electric starting is a nice feature. An electric starter motor spins the engine faster than pull starting increasing the probability that the engine starts. The starter keeps the engine spinning longer because a starter rope always runs out of length. My wife can push the start button and if your back is hurt “the button” still works. Gasoline engines are easier to pull-start than diesels. There are some small diesel engines with compression release that can be pull started, but it can be tough even for a 3-kW engine. We put starters on even the small diesels at work, for cold weather starting. Both my snowblowers have electric start too for the cold weather advantage. I have a 2.8 kW gasoline set that starts better electrically because the rope is so short. I can quickly pull it up to speed but then I run out of rope and it starts slowing down. With a bad back I can still start my 5.5 kW gasoline set but I have to electrically start my 7 kW even though it has a pull-start rope. My 7 kW should have around a 12 h.p. engine plus additional drag from the alternator inertia. Consider electric start an advantage for any genset and necessary for gas power over 5.5 kW and diesel power over 3 kW.
Sizing is important and tricky. The last thing a user wants is a generator that can’t carry the load or start the motor in your appliances. You also don’t want to purchase more generator capacity than you need. (As I’ll explain in a later paragraph). Lastly, you cannot trust generator name plate rating. Consumer Reports has tested consumer generators. Some produced rated capacity or more while others couldn’t do rated capacity. Some will have significantly greater motor starting (“peak load”) capability than others. We’ve tested commercial sets at work. One company’s 100 kW set might do 120 kW while another vendor’s set might only produce 80 kW using the same test method. EGSA publishes test methods but I don’t think many commercial companies follow them, unless forced to–say to meet military specifications. Test methods and test equipment affect test results too since companies don’t have the same test equipment. I’ve spreadsheet compared specifications for consumer gasoline powered sets. The fuel consumption numbers were too inconsistent to be accurate, and should be used for no more than rough comparison purposes.
There is plenty of advice online for calculating loads and sizing generators so below are some issues not typically covered. Remember induction motors can draw 3-4 times steady state current to start. One of my set’s surge capacity is about 150% of its continuous rated load capacity. When doing your load calculations consider that not all the loads will always be running at the same time. I’ve run two refrigerators and two freezers on a 2.8 kw generator and rarely did I see more than two appliances running at the same time, and never starting at exactly the same time. Remember you can also control what is running using circuit breakers and switches to allow a smaller set to power your house loads, without overloading yoru generator.
I’ve monitored my current draw powering the whole house, less AC, and it was surprisingly low, rarely above 3 kW. The refrigerators, internet, television sets, lights, parasitic loads and heat pump water heater didn’t use that much current at my house. If you don’t have one, buy an inexpensive clamp on amp meter at Harbor freight (or go buy a Fluke if you love to play with electricity). It takes a good meter designed for surges to measure surges. Testing is really the only way to be sure you have the right size generator. Buy the clamp on meter, and borrow from friends, rent or buy generator sets and see what loads they can power and how much extra capacity they have. I see quite a few used generators for sale online (on Facebook and Craigslist) for reasonable prices. Run your house load on a borrowed/rental/used generator while monitoring current draw with a clamp on ampmeter. My 1800W won’t turn over my universal motor chain saw but has no problem with my large weed eater. My 5500 W generator will start a 2 horsepower sewer pump. I wouldn’t know this if I had not run these real world tests. And just like your other preps: Test before you need the capability.
Generator fuel consumption is not proportional to the load. A typical 10 kw generator powering a 2 kW load will generally burn more fuel than a 3 kW set powering the same 2 kW load. In a grid-down situation fuel can be burned quickly so stretching fuel becomes important. My street was without power for five days a couple years ago. Gas stations had power so I ran my larger generator and burned through over 40 gallons of gasoline even though I shut the set off at night. This won’t work in a SHTF situation. At work we spend over 3+ times the cost of a generator on fuel, over the life of a generator.
I have generators ranging from 1.8 kW that can run some refrigerators and Internet up to a 7 kW that will run everything except for my central air conditioner. I can optimize which set I run based on house load needs, fuel supplies, cause of no power, and convenience. Two are inverter generators for quieter operation and less fuel consumption. Two run on propane to provide alternative fuel options. Every one of these will have an optimal load where it will produce the maximum Wattage at the lowest fuel consumption per Watt. It’s potentially difficult to find these specifications, it will vary based on the engine type, and testing isn’t easy either.
Diesel engine manufacturers will frequently publish a curve of fuel consumption versus power, so start with the engine manufacturers specifications and do the math to bring in the Watts. Work experience suggests the highest fuel efficiency load is fairly high, say 75, 80, maybe even 100% load. One of these days I may test one of my inverter generators. At work we use a scale for fuel consumption, load banks, and calibrated electrical measuring equipment. Theoretically, an inverter generator should be more efficient at partial loads than a fixed speed generator. The inverter allows the control system to run the engine no faster than necessary to develop the HP needed for the load.
One of the National Labs built prototypes and conducted testing proving this concept. I’m not sure how well commercial vendors are executing the concept. Variable speed also reduces engine wear and noise. In a true grid down situation noise tells the world that I have electricity and resources. So I want to be quiet.
One last word of caution, don’t run liquid-cooled diesel engines for weeks at low loads as it can lead to “wetstacking” (slobbering out exhaust), clogged mufflers, frozen rings and premature engine failure. Diesels run best hot and they get hot powering loads. We’ve replaced engines at work from this problem. If you see signs of wetstacking, then every couple hundred hours run your generator at 80-100 percent load and the engine should become hot enough to burn out the diesel residues.
Lastly, General Dwight D. Eisenhower said “You will not find it difficult to prove that battles, campaigns, and even wars have been won or lost primarily because of logistics.” I’m a big fan of logistics. Spare parts extend the life of your generator set. I have exploded view breakdowns of all my generators including part numbers. I’ve even called OEMs before buying to ensure I could purchase specific parts that I knew were likely to wear out or that could fail unexpectedly. According to my small engine shop there are low price generators that have no spare parts available besides filters and spark plugs. These gensets are literally designed to throw away when they break. Buying a set with spare parts available might cost you a bit more in the beginning, but it can pay off over the life of the generator. I think of this as an extension of the “two is one…” rule.