While small 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, being adequate to power a whole-house system with well pump, HVAC equipment, water heater, washer-drier, refrigerator, freezer, etc.
My home genset is not a "forever off-the-grid system," but is intended as storm backup sufficient for several weeks until its 1000-gallon LPG tank runs out, I get an LPG delivery to enable me to keep going, or the AC mains come back up.
A smaller genset must work harder, at a higher duty cycle. Typical "portable" systems in the 3-5kw range are marginally sized for nhousehold loads, are not continuous-duty or full service panel rated and do not provide seamless failover when used in a whole-house set-up. This means that you must either isolate or stagger high capacity loads from the backup system.
An electric 50-gal water heater uses about 4500w, a well pump about the same in "starting" load, ditto a heat pump. With a smaller genset you can't run multiple high-load pieces of equipment at once, so you must choose which ones and then alternate their use. Auxiliary power systems must 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 rated 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 household "normality." Trust me that after age 70 you don't need to practice how to be uncomfortable anymore.
In winter food storage is less of an issue here because we keep perishables outside in the unheated garage pantry. But we need to maintain a limited amount of electric electric baseboard heat, just enough to keep water pipes from freezing when the house is vacant. Otherwise in winter we minimize electric 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 any conscious action or adjustment. I sized the system larger than I needed to provide surplus "reserve" capacity to run machinery in my workshop and a trash pump in case the basement should flood. For normal needs for the average suburban house I would not plan for less than 10kw. For a large house, small farm and home business 22kw is better so that the system does not need to run at full capacity all the time. Sizing larger than you need gives both surge capacity and better reliability.
Installing a smaller 10kw vs. 22kw genset saves you only about $1500-1800 on cost of the basic 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 22kw vs. 10kw is only about 20% more. Put the genset on a dedicated fuel supply. Do not "piggy-back" household appliances onto the same fuel supply. You do not want routine household appliance use to reduce the available reserve capacity for your backup power system. To power the genset get a BIG LPG tank, not less than 500 gallons. MUCH better to get 1000. The approximate cost to contract out the whole job to a licensed contractor, including pulling permits and getting the proper inspections so that your homeowner's insurance will cover it when the house burns down, is about $10,000 for a 10kw system vs. $12,000 for 22kw, for a full turnkey installation powered by LPG, with solid-state double-pole, double throw automatic transfer switch.
Pay the electrician and the gas company for the connection charges. Get the work done professionally and safely, and also get an annual service contract. Having an annual service contract ensures that your unit is covered under the manufacturer's warranty and it protects you under your home owner's insurance from accidents or damage resulting from equipment failure. It is a BIG mistake not to do this when ground bounce from a near lightning strike fries the circuit boards on your controls, or when you get sued because you didn't install an ATS so current backfeeds into utility lines and kills a line worker 1/2 mile away, who was trying to restore the power during an ice storm. Or maybe when you changed the oil you didn't get the drain plug tightened correctly so that it vibrated loose an blew out during the weekly test, spraying the entire inside of the generator enclosure with oil and leaving you with a dry oil sump, and causing a fire which burns down the house. I've seen ALL of these happen to neighbors who thought that they "knew it all," wanted to save a buck and did the work themselves.
Propane produces 92,000 BTU’s per gallon, non-ethanol blend gasoline is capable of producing 114,000 BTU’s per gallon, and diesel is capable of producing 129,500 BTU’s per gallon. This means that it takes more propane per hour that either gasoline or diesel to run a generator. But the advantage of propane is that it is stable for long term storage, so it doesn’t go “stale” like gas or diesel. So, how much propane will my generator burn per hour?
It requires 2 horsepower to produce 1000 watts of energy per hour under load
Under load each horsepower consumes 10000 BTU per hour
Propane contains 92,000 BTU per gallon
Propane weighs 4.2 pounds per gallon
While fuel consumption is less with a smaller genset, a 10kw using LPG uses 1.9 gallons/hr. at full load. This is about as much as a 22kw unit uses at 50% load. A typical 10kw LPG unit consumes 1.25 gallons/hr. at 50% of its rated capacity. That means a 500 gallon LPG tank would last only about a week at 50% duty cycle if you also use your gas stove, water heater and gas fireplace. This is because an LPG tank can be filled only to about 80 percent of capacity, because "headspace" is needed for the gas to volatilize and function properly. A 500 gallon unit filled to 80% powers a 22kw genset for about a week at 50% load. To provide the same reserve power capacity a 10kw unit must at nearly full capacity consuming very nearly the same amount of fuel, give or take about 5%.
Using these factors consider how long can a 5000-watt portable generator run on a 500 gallon propane tank at 50 capacity:
10 horse power at 50% would use 5 HP to generate 2500 watts of electricity
5HP X 10,000 BTU would consume 50,000 BTU per hour
500 gallons X 92,000 = 46,000,000 BTU of energy in a full 500 gallon tank
46,000,000 BTU divided by 50,000 BTU = 920
A 500-gallon tank that is "full" would run a 5000-watt generator at ½ its rated capacity for 920 hours.
After comparing the various fuels, I decided upon propane for a 22kw Generac Guardian in my fixed setting with 1000 gallon LPG tank. For small, portable generators I consider a
tri-fuel generator which can run on either propane, gasoline or natural gas.
