How Big a Generator do You Need

How much Electricity do Common Household Appliances Use?

The following information is only an approximation, since different brands and models of tools and appliances power usages vary.  However the following list is useful in helping you determine the size generator or alternate power system you need.

Remember that every appliance that is plugged in may consume energy even if it is not turned on.  For example, TVs, computers, VCRs, and other electronic devices use energy to stay in standby mode. Even your mobile charger for your phone or iPod consumes about 4 watts of electricity per hour.

Electricity is measured in kilowatts. One kilowatt equals 1000 watts, and one thousand watt-hours (one thousand watts used for an hour) mean one kilowatt-hour (1kwh) or one kilowatt used over one hour. Your utility bills show kilowatt-hour usage.  The following chart shows the approximate wattage use for the following appliances for one hour

If you look on the nameplates located on many tools and appliances, they will tell you the exact voltage that they draw.  Another method is to measure the amperage draw with an ammeter. You can usually find these at your local hardware store.  Remember that the starting voltage on an electric motor is higher than the voltage to keep it running.

If you are thinking about running a computer or other sensitive electronics with your generator be aware that running home computers from a standard generator is not advisable.  It requires a generator that utilizes AVR (automatic voltage regulation such as the Briggs Pro Max 6000A or an Inverter such as the Honda EU & EM.  The other alternative is to use an Uninterpretable Power Supply (UPS).  These are typically sold for use with computers to prevent  data loss in the event of a power cut.

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2 thoughts on “How Big a Generator do You Need”

  1. While the portable inverter-generators provide clean sine wave AC which is better for running computers and sensitive electronics, I am not aware of any which are full service panel rated, which are adequate to power a whole-house system with well pump, HVAC equipment, water heater, washer-drier, refrigerator, freezer, etc.

    My home backup genset is not a “forever off-the-grid system,” but intended as storm backup sufficient for 10 days to two weeks until its 500-gallon fuel in the tank runs out, I get an LPG delivery to enable me to keep going, or the AC mains come back up.

    A smaller backup genset must work harder at a higher duty cycle. Most of the smaller portable Honda and similar systems in the 5kw range are marginally sized, not continuous-duty or full service panel rated and not suitable for to provide seamless failover in whole-house set-up. This means that you must either isolate or stagger high capacity loads from the system.

    An electric 50-gal water heater uses about 4500w, a well pump about the same in starting loads, ditto a heat pump. So with a smaller genset you can’t run multiple high loads pieces of equipment at once, you must choose which ones and then alternate their use. Auxiliary power systems must also be sized for the starting loads and not the running loads of compressors, pumps, etc.

    If you are on municipal water so there is no well pump to worry about, that simplifies things. If you don’t have forced air central HVAC, that also reduces normal household loads. Having lived through a 2-week power outage during a record heat wave in which daytime temperatures exceeded 100 degrees F, and having lost over $500 in food which spoiled, I wanted a full-panel system able to run two 10,000BTU A/C units, well pump, water heater, washer, drier, refrigerator and freezer, etc. without having to make any adjustments to “normality.” After age 62 you don’t need to practice how to be uncomfortable anymore.

    In winter food storage is less an issue because we can keep perishables outside on the screened porch. But we need to maintain a limited amount of electric baseboard heating, just enough to keep water pipes from freezing when the house is vacant. Otherwise in winter we minimize electric heat use by using a woodstove and a gas fireplace, keeping wall thermostats set to 60 degrees and wearing sweaters. Just like living in Scotland!

    My intent was to “flip the whole house,” maintaining a comfortable retirement lifestyle with as few inconveniences or adjustments as possible. I wanted a full panel rated system providing seamless failover without requiring any conscious action or adjustment. I sized the system larger than I needed to provide surplus capacity to run machinery in the workshop and a trash pump in case the basement should flood. For normal needs for the average house I would not plan for less than 10kw, and for a larger house 20kw is better so that the system does not have to run at full capacity all the time. Sizing larger than you need gives both surge capacity and better reliability because the unit does not have to run at maximum capacity all the time.

    Installing a smaller 10kw vs. 20kw genset saves only about $1500-1800 on cost of the generator, but unless you do your own site work, can do your electric connections to code, pull any needed local permits to get it inspected, the cost of contracting a full turnkey system for 20kw vs. 10kw only about 20% more. Swap out your smaller LPG tank used for the stove and water heater for a bigger one, at least 500 gallons. Pay the electrician and the gas company for the connection changes, either unit costs about the same.

    The approximate cost to contract the whole thing is about $8000 for a 10kw system vs. $10,000 for 20kw, this for a full turnkey installation powered by LPG, with double-pole, double throw automatic transfer switch, including an annual service contract from the gas company, who installed it.

    While fuel consumption is less with a smaller genset, a 10kw using LPG uses 1.9 gallons/hr. at full load. This is almost the same as a 20kw unit uses at 50% load. A typical 10kw LPG unit consumes 1.25 gallons/hr. at 50% of capacity. That means a 300 gallon LPG tank would last about ten days at 50% duty cycle if you didn’t use your gas stove, water heater or fireplace.

    A 500 gallon LPG tank powers a 20kw unit for ten days at 50% load. To provide the same capacity the 10kw unit must run at 100% of its capacity consuming almost the same amount of fuel, give or take about 5%.

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