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Zombie Apocalypse

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Generator input, charging the battery? That’s the feature I’d like to see added to the Powerwall. Enphase says they’re going to have future generator compatibility in their new system (but will it include charging?).

That seems...pretty inefficient, no? 90% roundtrip efficiency on top of the efficiency of the generator. I guess if your goal is to charge through the day so you don't have to run the generator at night then that makes sense but at the point you're running the generator might as well run it when you need it and not more, right?
 
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That seems...pretty inefficient, no? 90% roundtrip efficiency on top of the efficiency of the generator. I guess if your goal is to charge through the day so you don't have to run the generator at night then that makes sense but at the point you're running the generator might as well run it when you need it and not more, right?

Yeah that doesn't make much sense to charge the battery with the generator then use the charged battery to run the home.

The Tesla Gateway already has an option to power the home with a backup generator (thereby letting the PV charge the batteries). And then the homeowner can switch off the generator and use the charged battery to power the home.
 
One thing to note on using your existing solar with a different inverter - you have optimizers on each panel, and without a solar edge inverter to talk to, I don't think they'll let power flow (open strings just show 1V/panel).

Ah, good point. I made the assumption the Skybox would be able to talk to the optimizers. I sent an email to Outback sales to see if their system can talk to SolarEdge optimizers. If not, I would have to also add in $900-$1,400 for a 3kW - 5kW SolarEdge Inverter.
 
That seems...pretty inefficient, no? 90% roundtrip efficiency on top of the efficiency of the generator. I guess if your goal is to charge through the day so you don't have to run the generator at night then that makes sense but at the point you're running the generator might as well run it when you need it and not more, right?

I certainly haven’t looked into this or run the numbers or anything, but at first blush I think there could be some efficiency gains here. It’s my understanding that generators tend to be pretty inefficient at low loads. Normal (non inverter generators) have to run at a specific RPM in order to maintain the proper frequency, so even if you have a very low load (such as you would at night) the generator will still run at a high RPM and use a lot of gas.

If you could run the generator, say, for 4 hours at a high load to charge the powerwall, then use the powerwall overnight I suspect that even with the 10% round trip loss in the powerwall you would wind up saving a considerable amount of fuel versus running the generator for 12 hours to support a very low overnight load.

This is the same principal that (non plugin) hybrid cars work on. Other than getting a minimal amount of charging from regenerative brakes, the only way they can charge is by using the gas engine to drive a generator and there are losses in the generator and in the batteries. But by optimizing exactly when and how much the car charges the batteries it can make sure that gas engine is operating as efficiently as possible and is in that way able to save more energy than is lost in the conversion to electric power.
 
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Yeah that doesn't make much sense to charge the battery with the generator then use the charged battery to run the home.

The Tesla Gateway already has an option to power the home with a backup generator (thereby letting the PV charge the batteries). And then the homeowner can switch off the generator and use the charged battery to power the home.
This is an emergency power backup system. Of course you wouldn't run the generator routinely.
 
This is an emergency power backup system. Of course you wouldn't run the generator routinely.

Right - got that part. During an emergency though it seems like you're not gaining a ton from not simply running everything off of the generator rather than charging the Powerwall too.

I certainly haven’t looked into this or run the numbers or anything, but at first blush I think there could be some efficiency gains here. It’s my understanding that generators tend to be pretty inefficient at low loads. Normal (non inverter generators) have to run at a specific RPM in order to maintain the proper frequency, so even if you have a very low load (such as you would at night) the generator will still run at a high RPM and use a lot of gas.

If you could run the generator, say, for 4 hours at a high load to charge the powerwall, then use the powerwall overnight I suspect that even with the 10% round trip loss in the powerwall you would wind up saving a considerable amount of fuel versus running the generator for 12 hours to support a very low overnight load.

This is the same principal that (non plugin) hybrid cars work on. Other than getting a minimal amount of charging from regenerative brakes, the only way they can charge is by using the gas engine to drive a generator and there are losses in the generator and in the batteries. But by optimizing exactly when and how much the car charges the batteries it can make sure that gas engine is operating as efficiently as possible and is in that way able to save more energy than is lost in the conversion to electric power.

This is actually a good point. For some reason I thought that standby generators in particular were happiest/most efficient at half load which is why they suggest oversizing them for house loads. After looking at some of the actual specs (https://www.generac.com/generaccorp...6kw-7036/guardian-series-spec-sheet-16-22.pdf) I now wonder if that was just the generator salesman looking for a bigger comission. :)
 
I certainly haven’t looked into this or run the numbers or anything, but at first blush I think there could be some efficiency gains here. It’s my understanding that generators tend to be pretty inefficient at low loads. Normal (non inverter generators) have to run at a specific RPM in order to maintain the proper frequency, so even if you have a very low load (such as you would at night) the generator will still run at a high RPM and use a lot of gas.


Damn that basically kills my idea of putting a zombie on a big hamster wheel; it wouldn't be able to run at the RPM fast enough to get the proper AC frequency.

Plants-vs-Zombies-Comes-to-World-of-Warcraft-Through-Cataclysm-2.jpg
 
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your costs keep going up. At this point, how close are you to the cost of a Powerwall?

I am not going to let reality get in the way of my dream!!! ;)

Kidding aside, a small generator still looks to be the most financially practical for an emergency scenario. Something large enough to keep the freezer frozen.

Even with an inverter, the Skybox approach is still significantly less expensive than Powerwall(s). Though, a big cost savings is I can do the install myself.

I need to research what battery solution I'd actually use. Below would be almost 2 Powerwalls worth of battery backup. They are flooded lead acid batteries which require maintenance, where I believe Powerwalls are some type of no maintenance lithium. Also, the batteries will require more frequent replacement than Powerwalls. It also has the potential negative that it isn't designed to be a whole-house backup.

OutBack Skybox - $3,328
SolarEdge 5kW Inverter - $1,420
20kWh Lead Acid Battery Bank - $2,859
Critical Load Panel - $47.28
PV/Battery Feed Panel (for single Rapid Shutdown Disconnect) - $47.28
Rapid Shutdown Disconnect - $109.95
Wiring gutters - $300
Misc conduit, wire, connectors, etc. - $500
Total - $8,611.51

1-3 Powerwalls range from $12K - $24K. I'm still not sure Tesla would sell me a single Powerwall due to the size of my solar system.
 
Generator input, charging the battery? That’s the feature I’d like to see added to the Powerwall. Enphase says they’re going to have future generator compatibility in their new system (but will it include charging?).

generators can charge the battery in an off-grid config, see attachment. the same capability exists in grid-attached configs but is disabled via software. if tesla allowed you to charge their battery with a generator (non-green energy source) then they would lose their fed tax credits. after the tax credit program expires in another couple of years i would expect the functionality to be added to grid-attached configs.

generators are most efficient when they're running at full load. so if you had a generator it would be nice to use any excess not needed for powering the home to recharge the batteries.
 

Attachments

  • Tesla_Powerwall 2_AC_ApplicationNote_Off-GridApplication_NA.pdf
    3.6 MB · Views: 83