Critical loads backup for home - 12V inverter vs gas generator

[josephyancey]

I'm looking for some advice here.

I live in a suburban area but we do tend to lose power every few months for 4-12 hours from storm damage. So far we just tough it out but my home does have a critical loads transfer switch. I've tested it and the default load with all of this on is around 300W but could peak up to around 1000W if both refrigerators start at the same time and the blower for the furnace is on.

Since I've got a giant battery sitting in my garage, I thought that I might just get a 12V 2000W inverter and attach it to the car. It looks like the MME's 12V converter can do 160A and all I would have to do is leave the car "on" to keep that running. 5-10 days worth of critical loads backup power sounds pretty good. I've tested this out on my MME with a friend's inverter and a pair of jumper cables. Other than the cheap jumper cables getting very hot it worked fine for a 30 minute test.

Pricing out doing this right with permanently mounted 2awg wires to anderson plugs with a Renogy 2kw inverter comes out to a bit under $400. Now I'm thinking that if I'm spending $400 why not just get a decent 2000W gas powered inverter generator. Yamaha clones can be had for around that price.

MME As Inverter Pros:

1. No noise
2. much smaller / more portable than a generator (for camping or whatever)
3. no refilling it with expensive gas

MME As Inverter Cons:

1. Can't drive the car when the house needs power
2. possible risk in damaging the 12V converter

Gas Generator Pros:

1. No risk to car
2. Can drive the car
3. much more "common" use case
4. can more easily sell it if I wanted
5. would likely feel more comfortable going to bed with it running (outside)

Gas Generator Cons:

1. Noisy
2. have to buy gas(or propane)
3. quality of the electricity (although I'm looking at units with very low THD)
4. I've got to maintain a rarely used small gas engine

Is there anything I'm missing? What would you do?

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[DevSecOps]

I have a whole home generac and a honda generator plus solar that will work off grid in another building on the property.

I wouldn't use the car. I have a Rivian that has actual outlets that I might use if absolutely necessary, but using the MME LVB to run a couple fridges and a furnace (or the whole home) would prob be really bad for all the reasons you listed. I would go the generator route. We loose power for normally 3-5 days straight every year and being on well means that power is everything including showers and toilets. Those are things that the car just couldn't run.

You're going to have the anti-gas crowd responding emotionally so I would be prepared for that. Ultimately it should be about your safety and that of your family. Do what's best for you!

Lastly, there have been reported bugs with the idle time being disabled on the MME in the past. I'm not sure if that's been resolved or not. If it's still bugged then the max time the vehicle will stay on would be 30 minutes even if it's disabled. I have not tested it in a long time.

 

[kltye]

I'm looking for some advice here.

I live in a suburban area but we do tend to lose power every few months for 4-12 hours from storm damage. So far we just tough it out but my home does have a critical loads transfer switch. I've tested it and the default load with all of this on is around 300W but could peak up to around 1000W if both refrigerators start at the same time and the blower for the furnace is on.

Since I've got a giant battery sitting in my garage, I though that I might just get a 12V 2000W inverter and attach it to the car. It looks like the MME's 12V converter can do 160A and all I would have to do is leave the car "on" to keep that running. 5-10 days worth of critical loads backup power sounds pretty good. I've tested this out on my MME with a friend's inverter and a pair of jumper cables. Other than the cheap jumper cables getting very hot it worked fine for a 30 minute test.

Pricing out doing this right with permanently mounted 2awg wires to anderson plugs with a Renogy 2kw inverter comes out to a bit under $400. Now I'm thinking that if I'm spending $400 why not just get a decent 2000W gas powered inverter generator. Yamaha clones can be had for around that price.

MME As Inverter Pros:

1. No noise
2. much smaller / more portable than a generator (for camping or whatever)
3. no refilling it with expensive gas

MME As Inverter Cons:

1. Can't drive the car when the house needs power
2. possible risk in damaging the 12V converter

Gas Generator Pros:

1. No risk to car
2. Can drive the car
3. much more "common" use case
4. can more easily sell it if I wanted
5. would likely feel more comfortable going to bed with it running (outside)

Gas Generator Cons:

1. Noisy
2. have to buy gas(or propane)
3. quality of the electricity (although I'm looking at units with very low THD)
4. I've got to maintain a rarely used small gas engine

Is there anything I'm missing? What would you do?

If you go with the car-as-giant-battery route, I would get a smaller capacity inverter (1kW-ish max, with a short "burst" capability not exceeding 1800kW or so) so your loads would "trip" the inverter first, rather than potentially exceeding the 160A of the DC-DC converter - because who knows what would happen if that limit is exceeded. You'll also have to consider the heat dissipation of the DC-DC converter at high loads, since it might not be designed to be continuously run at that amperage. (Someone else here probably knows that better than me)

Apart from buying gas for the generator, I'm not sure about the ins-and-outs of maintaining fuel that has been sitting around for some time. I assume you can use fuel stabilizers and whatnot, but that's just another thing to worry about.

Another option, of course, is just to get a bunch of LFP batteries that are periodically hooked up to a wall charger to ensure it's at 100% (since they're totally fine being at 100% SoC for extended times), and run your inverter off that. Since it sounds like your critical loads transfer switch is manual, that seems like an acceptable way to get your loads running. Downside, of course, is the initial cost. Upside is with some tweaking, you could potentially use the 12v outlet on the MME to trickle charge the LFP batteries at 120 watts ish if needed .

I'd personally go LFP because I dislike gas generators for lots of reasons, but there's nothing inherently wrong with that route.

 

[Fremont Kid]

We lived in the country for 27 years in rural Illinois farm country. Well water w/treatment system. Maintained a 1/4 mile long lane. Needed to provide our own electrical when the power was interrupted. I understand that in your position, you need to be your own utility municipalities.
Oh, and we heated 13 years totally with wood burned in an outdoor furnace.

We originally used a 10kW gasoline generator for @ 10 years. Then bought and used for 8-9 years a smaller 17kW generac then a whole-house 22kW which both consumed propane.

I recommend not using a gasoline generator because they require more maintenance, i.e. oil, spark plugs, etc. and if not careful the gas remaining in the carburetor can gel or harden. To shut down always turn off the gas supply valve, let the engine consume all gas in the carburetor, then turn off the on/off switch.

I recommend not using vehicle battery for reasons cited by DevSecOps and the analysis by Motor Trend: https://www.motortrend.com/reviews/...yearlong-review-update-1-sunrun-backup-power/
$18K to power home when needed using F150. Now, you can argue against some parts of their analysis but a Generac propane generator is far less expensive.

 

[josephyancey]

@kltye I would love to build a huge LFP powerwall system but that pretty quickly starts not actually making financial sense. 10ish hours worth of power at critical loads levels will cost around $1300 using prismatic cells with a good charger and BMS PLUS the cost of the inverter. At that point a generator makes much more sense. For the inverter with the MME approach I was planning on using a 100A inline breaker. That should trip if I tried to pull more than around 1200W.

@Fremont Kid I hadn't really thought too much about gas fouling in the carb. My mower generally doesn't have any trouble with that but it really only sits unused for about 2 months out of the year. Otherwise it's used once every 2 weeks. A generator could (hopefully) sit for months or even years without being used. Now I'm going to have to price out propane generators since I've got 3 people telling me that they wouldn't go with my car inverter approach.

 

[Fremont Kid]

@kltye I would love to build a huge LFP powerwall system but that pretty quickly starts not actually making financial sense. 10ish hours worth of power at critical loads levels will cost around $1300 using prismatic cells with a good charger and BMS PLUS the cost of the inverter. At that point a generator makes much more sense. For the inverter with the MME approach I was planning on using a 100A inline breaker. That should trip if I tried to pull more than around 1200W.

@Fremont Kid I hadn't really thought too much about gas fouling in the carb. My mower generally doesn't have any trouble with that but it really only sits unused for about 2 months out of the year. Otherwise it's used once every 2 weeks. A generator could (hopefully) sit for months or even years without being used. Now I'm going to have to price out propane generators since I've got 3 people telling me that they wouldn't go with my car inverter approach.

I learned the hard way. The second time I could not start the gasoline generator and second time my small-engine mechanic cleaned the carb he informed me how to avoid the problem.

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