Generating SizingOkay, you have made the decision to buy a whole-house generator for your home, or at least you are considering the possibility. You should have lots of questions running through your mind such as "What size generator do I really need?", or "What is the impact of getting one that is too small, or too large?
What is the "right" size generator? This depends on several factors, but really comes down to the question of What do you consider to be the "essential" things in your home that need electricity?
- Well pump
- Septic pump
- Your local climate. In other words, if you need to provide heating and/or cooling
- Number and size of refrigerators and freezers that contain perishable foods
- Other things that require power, such as fish-tanks, medical equipment, and alarm systems
- Comfort items such as televisions, stereos, computers, game systems, and garage-door openers
My advice to you is to buy a generator large enough such that your maximum typical load is no more than 80-85% of the derated capacity of the generator. Okay, we just introduced a new term here: derated capacity. That means, the rated capacity of the generator (e.g., 12KVa) decreased for the following:
- Type of fuel - Propane (LP Vapor) gives you more power than natural gas. As a practical example, the Kohler 18RES generator can be configured to use either Propane or natural gas. Using Propane, it is rated at 18KVa (75 Amps), and with natural gas, only 17KVa (70 Amps).
- Altitude - At higher elevations, the engine in your generator is not as efficient. As a practical example, the Kohler 18RES generator specifies derating (i.e., reducing) the capacity by 4% for every 1000 feet above 500. Every generator is a little different, so be sure to read the fine-print.
- Temperature - Again, the culprit is the engine. At higher temperatures, engines loose efficiency. For the example Kohler 18RES, it needs to be derated by 1.5% for every 10 degrees above 60 Fahrenheit.
Another approach is let a computer handle this for you. Several Automatic Transfer Switches (ATS) -- read more about these elsewhere on this site -- provide the ability to monitor the load on your generator and turn-off specific loads when the generator is nearing its capacity. For example, the Gen-Tran Ovation ATS has the ability to control up to six loads through a process called "Load Shedding". As a practical example, you could tell the ATS to temporarily turn on/off several high-load things such as the hot-water heater or air-conditioner.
What Happens if I Overload? - If you overload your generator, it will stop providing power to your home. Clearly, that is a bad thing and completly negates your purchase of a generator. Typically, a circuit-breaker inside the generator will "trip", and the generator will continue to run. To fix this problem, you will have to stumble around in the dark and turn a few things off before attempting to reset the breaker inside the generator. Believe me, you do not want this hassle.
Should I power the whole house, or just parts of it? - There are two ways to provide power to your house. You can power everything, or you can power certain parts. Easy, right? Well, not really. A typical house has a 200 Amp service, though smaller homes might have 100 Amps, and newer "McMansions" generally have 400 Amps. A typical generator provides only 50-75 Amps of power. There is now way your generator cannot power every light, computer, dishwasher, etc., at the same time -- you have to excercise some restraint. An Automaic Transfer Switch with Load-Shedding capabilities can help you avoid most overload conditions, and keep the generator's load down to approximately 85%. By the way, 85% is the maximum load you should target. This gives you a little buffer to handle transient loads such as opening a garage door.
The alternative is to power only certain areas of you home. This might include bathroom and hallway lights, refrigerators, freezers, and other critical systems (e.g., alarms), as well as a few comfort items such as a television, microwave oven, and gas/oil furnace. The benefit of this is that you can buy a much smaller and less expensive generator. the disadvantage of this is that your quality of life is impacted. The choice is yours, and the driving factor is cost. In my personal case, my wife and I could not agree on which parts of the house to power, so we bought a larger generator and powered the whole house. We incorporated intelligent load-shedding to avoid any overload situations. This works very well, but is a little more expensive.