Things to consider when sizing your solar/powerwall system for off grid or extended outages

[gpez]

Per Tesla: "To ensure reliable operation during power outages, at least one Powerwall is required for each 7.6 kW AC of solar included in the backup circuit." So for the 12 kW system, you'll need a min of two Powerwalls and three Powerwalls for the 16 kW system.

As we talked about on the other thread Tesla's guidance is way too simplistic, IMO. It does not take in to account inverter ratings, panel orientation, local irradiance levels, static and dynamic house draw, usage patterns, etc etc.

To be relevant to this thread I'd add a bullet to the OP saying "don't follow Tesla's 7.6kW AC per Powerwall requirement blindly".

 

[Apoztel]

Is anyone completely off grid using PWs? Can you use electric heat, hvac, electric stove/oven, electric water heater and still be completely off grid or would you have to be grid tied?

 

[bob_p]

Our system was sized to provide 50% of our annual power usage. We've had several periods when we've operated without any grid power for multiple days in a row.

If we anticipate an extended power outage after a hurricane, we plan to manually turn off non-essential devices to allow us to operate off-grid longer - plus with Storm Watch mode, we'll be at 100% charge before the power goes out. At least in our experience in Houston, when a hurricane hits, power outages usually happen after the hurricane has hit the area and once that happens, the storm passes within a few hours and we've usually had clear skies for days afterward - when we would get good solar power.

But... We don't have all electric devices. We use gas for our water heaters, cooktop, clothes dryer and HVAC heater. The only major electric appliance is our double oven - which we would avoid using during a power outage. We have 3 HVAC systems - and the air conditioners draw a lot of power, so we'd probable turn 2 of the systems off during an extended outage. Our pool pumps are also a major power user - and would turn those off or limit the main pump to running only for a short period each day.

Using all electric devices will consume more power.

Each PowerWall provides 5KW of continuous power (13.5KW of usable energy). Check the power consumption for the electric devices - and if the devices are all running - how many PowerWalls would be required - and how long they would last. And then scale the solar power to be able to charge the PowerWalls for overnight use.

 

[gpez]

Each PowerWall provides 5KW of continuous power (13.5KW of usable energy).

Just a quick note that each Powerwall can provide 5kW of continuous power (7kW peak for 10s) and has 13.5kWh storage. Important to show that the storage is measured in kilowatt hours meaning it can run at the maximum 5kW for 2.7 hours before draining.

 

[mstgkillr]

Any guess as to the ideal Powerwall mode for an extended outage? Backup-only or self-powered? And, why the reserve setting for backup-only mode?

 

[miimura]

Any guess as to the ideal Powerwall mode for an extended outage? Backup-only or self-powered? And, why the reserve setting for backup-only mode?

Not sure I understand the question. If you're trying to prepare for an outage, use Backup Mode. Once the power goes out, there is nothing to set. The Powerwalls will just do their best to power the house given the available generation and demand.

 

[mstgkillr]

Not sure I understand the question. If you're trying to prepare for an outage, use Backup Mode. Once the power goes out, there is nothing to set. The Powerwalls will just do their best to power the house given the available generation and demand.

It is my understanding the with grid power available, backup-only mode charges the Powerwalls first, then goes to the house, with any excess back to the grid. With self-powered, the power goes to the house first, then the Powerwall, followed by the grid.

With a grid power outage, the house becomes priority, I just wasn't sure if it the mode changed how the power was delivered to the house or Powerwalls. After thinking about it, it probably doesn't change anything during a power outage.

 

[BrettS]

Once you are in an outage it doesn’t matter what mode the powerwalls are configured in. Power will always go to the house first, drawing from the powerwalls if there isn’t enough solar to power the house. Any excess solar will go to charging the powerwalls. It’s also worth noting that the reserve percentage doesn’t matter during an outage. If necessary, all available power in the powerwalls will be sent to the house during an outage.

If you are in backup only mode before the outage happens, then you might be in better shape during the outage because your powerwalls will start fully charged. If you were in self powered mode with a low reserve then it’s possible that when the outage happens you could only have a limited amount of power in your powerwalls to cover you during the outage.

 

[h2ofun]

Once you are in an outage it doesn’t matter what mode the powerwalls are configured in. Power will always go to the house first, drawing from the powerwalls if there isn’t enough solar to power the house. Any excess solar will go to charging the powerwalls. It’s also worth noting that the reserve percentage doesn’t matter during an outage. If necessary, all available power in the powerwalls will be sent to the house during an outage.

If you are in backup only mode before the outage happens, then you might be in better shape during the outage because your powerwalls will start fully charged. If you were in self powered mode with a low reserve then it’s possible that when the outage happens you could only have a limited amount of power in your powerwalls to cover you during the outage.

Interesting. I was talking to a person last night about how his tesla batter was doing. He said during our last power outage it was great. They just behaved as normal. But the next morning, while taking a shower, the battery was all used up and turned off. Your post implies this is normal? Good for the battery?

But worse, when the sun came back up, the battery system would not turn back on, so his solar would not work recharging battery since GW I guess still was not going to power the inverter, since battery was dead.. When he call Tesla support, he was told it was a fw bug and many in Calif had the same issue since this was the first power outage for many with their new battery.

So, how does one avoid this issue?

 

[BrettS]

Interesting. I was talking to a person last night about how his tesla batter was doing. He said during our last power outage it was great. They just behaved as normal. But the next morning, while taking a shower, the battery was all used up and turned off. Your post implies this is normal? Good for the battery?

But worse, when the sun came back up, the battery system would not turn back on, so his solar would not work recharging battery since GW I guess still was not going to power the inverter, since battery was dead.. When he call Tesla support, he was told it was a fw bug and many in Calif had the same issue since this was the first power outage for many with their new battery.

So, how does one avoid this issue?

If the house uses more power than the you have battery capacity for then it’s normal for the battery to get totally drained and shut off. I’m not sure what else you would expect to happen. At some point it will simply run out of power. The way to avoid it is to get more battery capacity or set your reserve higher so more battery power would be available during an extended outage.

If the battery does drain completely during the night the system is supposed to start up for a few minutes periodically during daylight hours. If it discovers that there is enough solar power to run the house at that time, then it will stay on and run the house and charge with any excess power. If there is not enough solar power then it will shut off and try again later. (Kind of a like a ground hog, I guess). So it is normal that the system might not start up immediately when the sun comes out, but it should start up on it’s own at some point in the morning. If it didn’t do that, then perhaps there was a firmware bug like tesla said.

 

[h2ofun]

If the house uses more power than the you have battery capacity for then it’s normal for the battery to get totally drained and shut off. I’m not sure what else you would expect to happen. At some point it will simply run out of power. The way to avoid it is to get more battery capacity or set your reserve higher so more battery power would be available during an extended outage.

If the battery does drain completely during the night the system is supposed to start up for a few minutes periodically during daylight hours. If it discovers that there is enough solar power to run the house at that time, then it will stay on and run the house and charge with any excess power. If there is not enough solar power then it will shut off and try again later. (Kind of a like a ground hog, I guess). So it is normal that the system might not start up immediately when the sun comes out, but it should start up on it’s own at some point in the morning. If it didn’t do that, then perhaps there was a firmware bug like tesla said.

There was plenty of sun. He told me Tesla said many had the same issue. I do not ask exactly how the system came back up. His battery was at 100% before outage. I told him my generator worked great.

 

[BrettS]

There was plenty of sun. He told me Tesla said many had the same issue. I do not ask exactly how the system came back up. His battery was at 100% before outage. I told him my generator worked great.

It sounds like tesla acknowledged that there was a firmware issue, so I imagine that’s what the problem was. It definitely doesn’t sound like it was working the way it was supposed to.

 

[aesculus]

If it discovers that there is enough solar power to run the house at that time, then it will stay on and run the house and charge with any excess power.

Hmm. This implies that if you don't have enough solar to power your house (and maybe never will with pure solar) then you would be advised to start turning off anything you can to get below the solar threshold. For example if your house is drawing 1kw and your solar will not produce that much until a few hours, it might behoove you to turn off as much load as you can to get the solar back on. Then after the PWs have charged sufficiently or the solar has gotten above the threshold to start turning on critical breakers.

I can't tell from the SLD why the PWs cannot be charged from the solar even when the house needs are higher. Must be something in the GW logic

 

[power.saver]

Because the load (your house) would have to match the solar output exactly. That is very unlikely. But the GW can control the PW charging to do just that. Once charged, the PW can balance the load and solar by sourcing or sinking the difference.

 

[BrettS]

Hmm. This implies that if you don't have enough solar to power your house (and maybe never will with pure solar) then you would be advised to start turning off anything you can to get below the solar threshold. For example if your house is drawing 1kw and your solar will not produce that much until a few hours, it might behoove you to turn off as much load as you can to get the solar back on. Then after the PWs have charged sufficiently or the solar has gotten above the threshold to start turning on critical breakers.

Yes, that’s true, and the powerwall user’s guide even suggests turning off as much as possible if you get into a situation where your powerwalls drain completely before morning. However, hopefully that’s not a situation that happens frequently. Ideally your powerwalls would be sized to your power demand and you’ll be able to make it through the night without completely draining the powerwalls.

I can't tell from the SLD why the PWs cannot be charged from the solar even when the house needs are higher. Must be something in the GW logic

The situation I was referring to is when you are in an extended outage and you have no grid power. In that case the house always has priority for solar power and the powerwalls only get excess solar power. The powerwalls and solar are on the same circuit as the critical loads, so there is no way for the system to direct the power from the inverters to the powerwalls without also powering your critical loads.

When you are on grid, then it can use the grid to power the house and use all of the solar power, if necessary, to the powerwalls.

It’s not super clear in your SLD, but if you look closely you can see that panels 3 and 4 are connected with no switch between them in the gateway. The gateway cannot isolate the generation panel from the backup loads panel. In fact, in some installations there aren’t even two separate panels, but everything is on one panel.

 

[aesculus]

It’s not super clear in your SLD, but if you look closely you can see that panels 3 and 4 are connected with no switch between them in the gateway. The gateway cannot isolate the generation panel from the backup loads panel. In fact, in some installations there aren’t even two separate panels, but everything is on one panel.

I see that now. If Tesla had a bit more logic and an isolation switch for 3/4 to be independent, than this issue could be adverted. But as you said the real answer is to make sure you have enough solar to accommodate at least 24 hours of critical load.

When I built my system I was right on the edge. I decided last fall that this was not practical (smoke days) and then decided to almost double my solar to ensure I could make it through at least 24 hours of critical loads and maybe even have enough for good solar production days to live high on the hog.

 

[BrettS]

I see that now. If Tesla had a bit more logic and an isolation switch for 3/4 to be independent, than this issue could be adverted. But as you said the real answer is to make sure you have enough solar to accommodate at least 24 hours of critical load.

They could have, but is it really worth it? I feel like it’s a pretty fringe situation and I think that for the most part people would want their house to be powered, rather than having the powerwalls start charging while their house remains dark. Even if that means a loss of a tiny amount of solar power before the sun is strong enough to start powering the house we’re talking about what... probably less than 1kWh. I’m not sure it’s worth adding additional complexity and cost to the gateway just for that.

 

[SMAlset]

Something useful to check for the day/days ahead is a weather app with cloud cover projections. When we want to charge one of our cars, my husband likes to see what tomorrow might bring in the way of recharging sunshine before deciding to charge or how much to charge. He cut back charging today given tomorrow’s cloud forecast. He’d rather charge using solar than from grid if he can help it. This is Dark Sky app on an iPhone.