Battery vs Generator for backup power

Do you need an 18 kWatt genset? I am guessing that you are using natural gas as fuel?

Gasoline/propane/natural gas generators (non-inverter generator types) tend to be most fuel efficient between 50% and 100% of rated load; When operating at 50% or less of rated load (for a first approximation), the generator will consume around the same fuel flow as 50% rated power... I.e., if loading genst at 20% rated output -- I.e., ~3,600 Watts), it will still draw around 50% rated fuel flow.

https://eepowersolutions.com/products/battery-monitoring-systems/battery-monitoring-solutions-vla-vrla-nicad-systems/bds-pro-battery-cell-voltage-monitoring-system/

18 kW propane fuel flow
1/2 load 169 cuft per hour
full load 247 cuft per hour

Roughly, 100 cuft = approximately 1 Therm of natural gas.
PG&E is charging me around $2.00 per therm right now.

So, my guess (unless you are running A/C, electric water heater, electric stove, electric drier, etc.) would be running much cells than 9-18 kWatts--probably closer to 3-4 kWatts typical worst case (no AC, natural gas appliances).

Estimated natural gas costs around 1.69 CCF (~1 therm) * $2 per therm = $3.38 per hour at 50% or less loading

Of course, if your runtimes are only a few hours at random times during the year... Not a huge fuel cost. 18 hours * $3.38 per hour = $60.84 for "yesterday's" outage.... Not insignificant--But probably worth it for your home/needs.

However if you were running for many hours per day (say during a 2 week outage)--That could be a real "surprise":

14 days * 24 hours per day * $3.38 per hour = $1,135.68

That is not easy to overlook.

If you were running off of stored fuel (gasoline, diesel, propane), reducing genset size and supported loads to something smaller (like a 3-7 kWatt inverter-generator)--You would save a good amount of change (example: very important for off grid-winter system usage).

-Bill

I ran my in-laws house on a single Honda eu2000i (1,600 Watt inverter-generator) for about 4 days during our last wildfire shutdown.

Used a extended runtime fuel cap (basically a barbed fitting in an aftermarket fuel cap and fuel line to 5 gallon tank--Honda has a small fuel pump and can draw fuel from an external tank).

Ran 2x full size energy star refrigerators, a full size upright freezer, and a koi pond with 1/3rd hp water pump, aeration, and UV lights.

In their case, they had natural gas (stove for cooking) and city water (no well pump). Probably about the limit I would put on a 1,600 Watt genset--Ran fine on something like 2.5 gallons of gasoline per day. (fridges and freezer probably averaged 120 Watts with 50% duty cycle plus another few hundred watts for koi pond).

You need to figure out the cost/benefits for your home+family. If you have a natural gas home (stove, hot water, drier, heating), your electric loads are going to be quite low. The big loads (stove, hot water, heating) are carried by natural gas.

If you have central A/C--That can be a big "killer" here. A typical A/C system can have high starting current (for the compressor, and sometimes even the fan(s)). And A/C system do consume a lot of energy.

More modern A/C system (mini-split, more modern window units) can be "inverter" based--Which reduces starting surge current to near zero over running current--And can run the A/C compressor at lower speeds/RPMs during less than 100% demand for cooling.

So, now you are looking at the questions--Do you have/need A/C? Do you have natural gas stove/hot water/heating. Do you want to use the microwave/toaster oven, etc.? Can you load manage (turn off A/C to run microwave OR toaster oven, etc.)....

Are you looking for a "near normal" home operation, or are you OK with just enough power to keep the fridge and freezer cold, run some LED lightning, maybe a wash and line/drier a couple times a week, etc.?

If you have an electric stove--Use a propane camp stove during outage? Go without electric hot water? Camping tankless water heater for hot showers?

What is the level of "pain" you accept vs the cost of the hardware/installation/fuel costs? Power outages for hours/days/weeks?

To give you a better idea of your energy usage--You might want to look at a "whole-ouse" energy monitor. There are quite a few out there these days. One company that has been out there for years (I have never used one--So just a link for you to study/start your searches):

https://www.theenergydetective.com/

You can get one sensor for your whole home, or multiple sensors (water heater, stove, drier, A/C, Heating).

Type typical PG&E utility meter these days usually has an LCD panel that shows you your energy usage (in kiloWatts?)... I.e. 0.340 is 340 Watts.... You can do a few quick checks (start up the A/C, lights, washing machine, TV, etc as you desire, as an example) and see how much power your home draws. Of course, if the refrigerator and freezer run on 50% duty cycle--Those may be on or off at any random time.

You can also get a Kill-a-Watt type meter and measure the 120 VAC plug in loads (microwave, TV, washer, window A/C, etc.)....

https://www.amazon.com/kill-a-watt/s?k=kill-a-watt

Appliances with induction motors will have a (relatively) high starting surge (a 120 Watt refrigerator motor may take 500-600 Watts to start, and run a 500 Watt defrost heater for a limited time once or twice a day).

So if your desire is a 3,000 Watt average home load, then a 5,000-6,000 Watt genset may be needed. If you want to run central A/C, then a larger genset may be needed. (and other questions--Do you need 120/240 VAC power, or just 120 VAC from a smaller genset).

There are other options out there too...

http://www.generlink.com/

They have transfer switches that just plug into your meter socket on the side of your home, and then plug the utility meter into the Generlink. Much cheaper to install.

Again, more questions. What do you feel comfortable with.. Rolling out a generator and plugging into the Generlink, then fueling/starting the genset? What about your spouse and kids when you are not home (can they wait until you are home from work)?

The generac with auto-transfer switch will auto start/stop. And auto start/run once a month(?) to ensure all is OK....

I really don't know how reliable (or unreliable) PG&E will be in the future... I could never justify backup power (solar, battery, genset). In my 65+ years, had one outage that lasted a week (coast south of San Francisco) ~60 years ago, and two years ago when we had wild fire shutdowns for 1-4 days for wild fire prevention... Personally, I went with a small Honda genset (I use gasoline+fuel preservative) as my backup. Basically save the food in the fridge+freezer... And figured "camping" for lights/radio (I have natural gas, so cooking and hot water was normal--Climate can do OK with no heat/no A/C during outages)... And figured my outages would be earthquakes or "stupid" California PUC+PGE green and other policies. Did not foresee "wildfires" on my bingo card.

-Bill

And for anyone interested--A link for the extended run fuel cap/hose/etc. kits for some Honda euXX00i models:

https://www.amazon.com/s?k=honda+eu2200i+extended+run

-Bill