(Continued from Part 1. This concludes the article.)
Starting
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
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. Continue reading“Generators for Family Readiness – Part 2, by Greg X.”