Generator Sizing

[casingpoint]

Does any one know the correct method for determining electrical generator capacity based on anticipated watt usage?

[Purplehood]

This is what I pulled from an article:

Total the watt ratings of all the AC-powered components on your boat. A boat's total current draw is expressed in kilowatts (kW), a term that means one thousand watts of electricity. As a rule of thumb, select a genset rated from 10 to 25 percent greater than the boat's average draw. In other words, if your totals add up to a load of 1,500 watts, then 2kW worth of genset is just about right. Extra capacity allows you room for future appliances, yet doesn't leave the engine loafing

[anygunanywhere]

Does any one know the correct method for determining electrical generator capacity based on anticipated watt usage?

To hitch hike on Purple's post, you have to add up the watt rating of everything you plan on powering with the generator.

Your house hotel load is going to be pretty high.

Equipment like air conditioners and refrigerators/freezers need to account for surge capacity as well.

Anygunanywhere

[Kerbouchard]

If its for your home, the easiest way would be to look at your electric bill and take the Kw/hrs and divide it by 744(hours in a month with 31 days, adjust depending on month). This will give you your average usage over time. Next throw in a conservative factor such as 120% and you should be in the ball park.

If its for a boat or RV or other appliances, you can take the nameplate data and it should give you amp draw. Multiply your amp draw by your voltage and you'll get the wattage that the item consumes. Add all of the wattages together and again, multiply by 1.2 to get your 120% wiggle room.

[flintknapper]

Here ya go: http://www.findgenerators.net/Generator ... -Site.html" onclick="window.open(this.href);return false;

If the generator will be for your home... then you'll need to determine what items you want to power up (and when).

Generators run the gamut from emergency portable units (provide power for lights, run fans, refrigerator, etc..) to large "standby" units that can power the entire house.

Most folks opt for medium sized portable units: 5,000-8,000 watts typically. One consideration is the "surge rating" of the unit. All generators have a "continuous use" and a "surge" rating. The surge occurs when items such as refrigerators start and stop.

Items such as light bulbs, burners on a stove, heater, are "resistive" and have a constant wattage. Be sure to purchase a unit with a generous surge rating if plan to run a lot items with motors.... or anything that has a start up draw, (tv, computer, etc).

DO NOT buy any of the cheap Chinese "knock off" generators, you will hate yourself later.

Consider your fuel source and how often you anticipate needing to use your generator. Gasoline gens. are the most common, but if you store one with gas in the tank (and carb. bowl), don't be surprised if it won't start a year or so later because the fuel has turned to varnish. Fuel stabilizers such as "Sta-Bil" will help, but its best to run the carb dry and empty the tank if you are going to store it for a long time.

You can convert most gas generators to a "tri-fuel" configuration by adding a kit (or they can be bought that way). Then you have the choice of running it on Gas, Propane, or Natural gas. Just turn off one fuel source and turn on the one you want to use. Propane has no real storage drawbacks (it remains stable for years). You can always find propane, but not always fresh gas (hurricane evacuations and the days afterward).

You'll also need to consider how you want to transfer the generated power to your home/appliances/devices. Most folks use appropriately sized extension cords, but you can also have an electrician install a transfer switch that allows you to just plug into the gen. set and then the receptacle. Of course, there are at least a half dozen ways to rig up your own power inlet box...but I can't list them here for liability reasons.

You would do well to study up on the subject before making a purchase. There is actually more to know than the average homeowner realizes. Good luck...and let us know if you have any questions.

Flint.

[mr.72]

If its for your home, the easiest way would be to look at your electric bill and take the Kw/hrs and divide it by 744(hours in a month with 31 days, adjust depending on month). This will give you your average usage over time. Next throw in a conservative factor such as 120% and you should be in the ball park.

This is a good idea, but I think the 120% fudge factor may not be the best number to pick for Texans in the summer.

The problem is that the average current draw in your household may be X, but the instantaneous current draw may be far more than 1.2X. In fact it can be much higher. Your air conditioner operates only a small percentage of the time and it draws huge current. Same for your clothes dryer, refrigerator, etc. Even though the average may be, say 1000 watts for everything, in practice you may see 5000 or more watts for an hour at a time. I am just picking round numbers.

[flintknapper]

Here is kind of an idea of "surge" (although your appliances/devices may be much more).


Light bulb: 60 watts 60 watts surge
Fan: 75 watts 150 watts surge
Small TV: 100 watts 150 watts surge
Color television: 300 watts 400 watts surge
Home computer: 400 watts 600 watts surge
Electric blanket: 400 watts 400 watts surge
Microwave oven: 750 watts 1,000 watts surge
Furnace fan: 750 watts 1,500 watts surge
Refrigerator: 1,200 watts 2,400 watts surge
Well pump: 2,400 watts 3,600 watts surge
Electric W/H: 4,500 watts 4,500 watts surge

[mr.72]

well the "surge" current has to do with turn-on current, but there is also the issue of the dynamic load.

So my refrigerator may be average of 200 watts, when measured over a 24-hour period.

However it's 1200 watts if you measure it while it's running, and zero watts measured while it's not running.

So if I try to run it on a 240W generator, then I will trip the breaker as soon as the fridge comes on.

Now taking surge current into effect, using Flint's numbers, it takes 2400W to start the fridge. So even though it may only draw an average of 200 watts, it requires at least a 2400W generator to operate.