Battery vs Generator for backup power

rmk9785e

We have a 7KW solar system installed in a home northern California (east bay) and power outages are becoming more common in our part of town. We typically have outages lasting 3-5 hours every couple on months and yesterday it lasted 18 hours. Wondering if we should get a Tesla Powerwall or a Generac 18KW generator. We have been quoted ~$12K for the 10KW Powerwall and $25K for a generator. It's another surprise that Generac site says generator price starting at $4939 (Transfer Switch, Installation, Taxes and Shipping Not Included) but the installed quote is about 5 times the MSRP. How much should transfer switch and installation cost?
What factors should we consider in choosing one of these options?

Wheelman55

Get a second and third quote on installing the generator. 

BB.

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

Ralph Day

Any time you're running a genset during an outage it only needs to run for say, 1 hour, off for 3, on for 1, off for 3.  No need to use constantly!  I don't know why people think they need constant power even in an outage.

littleharbor2

Ralph Day said:

Any time you're running a genset during an outage it only needs to run for say, 1 hour, off for 3, on for 1, off for 3.  No need to use constantly!  I don't know why people think they need constant power even in an outage.

How does that work?

Ralph Day

not very conveniently for sure.  But I had to do it once when my inverter was away for service...1 day only.  It lets refrigeration run enough to keep things safe, water pressure system stays up, not good for uv light though.

littleharbor2

Ralph Day said:

not very conveniently for sure.  But I had to do it once when my inverter was away for service...1 day only.  It lets refrigeration run enough to keep things safe, water pressure system stays up, not good for uv light though.

I guess that would work if you're trying to conserve on fuel. You will then still be living with a power outage except you are stuck firing up and shutting down the generator. Personally if I had a backup generator I would run it full time till it wasn't needed any more. All the other electrical loads we take for granted are out as long as you have the genny shut down. I guess if you knew you were going to be out of grid power for many days and had a oversized generator and limited fuel then the 0ne on, three off plan would need to be implemented. Sounds like a bad design if you find yourself in that situation.

rmk9785e

BB. said:

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

Thank you all for sharing your valuable advice.

Honestly we don't know the size of generator we should buy. We would like to size it accurately before we are given a cost to acquire and install. Yes, NG is our fuel. The Costco authorized installer who came to provide an estimate said that we need 10KW generator for the base essentials and 18KW for the whole house. He did not provide us an inventory worksheet. The generatorist site provides good information for this calculation.

BB.

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

BB.

I should add there are other options too... Run the "big $$$" genset 1/2 day (keep firdge/freezer cool, and run battery lights/radio or those new fangel suitcase battery pack+AC inverter for light loads overnight (and recharge when genset is running). That can save you lots of fuel--If those costs are an issue.

-Bill

PS: For example, run big genset 4-6 hours in the morning (getting ready for work/school/cooking) and again 4-6 hours in the evening. Keep family happy, and fridge/freezer cold. Again if multi-day outage and fuel costs begin to look "scary".

BB.

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

Marc Kurth

rmk9785e said:

We have a 7KW solar system installed in a home northern California (east bay) and power outages are becoming more common in our part of town. We typically have outages lasting 3-5 hours every couple on months and yesterday it lasted 18 hours. Wondering if we should get a Tesla Powerwall or a Generac 18KW generator. We have been quoted ~$12K for the 10KW Powerwall and $25K for a generator. It's another surprise that Generac site says generator price starting at $4939 (Transfer Switch, Installation, Taxes and Shipping Not Included) but the installed quote is about 5 times the MSRP. How much should transfer switch and installation cost?
What factors should we consider in choosing one of these options?

There are always regional differences but my neighbor had a 9/10kW Generac whole house backup installed last year and it cost him $8,400. grand total. It was about $4,000 for the installation. It is set up for propane, which he already had.