Powerwall Reserve - How Low Do You Go?

[GaitherBill]

I'm trying to decide on a reserve power level that will give us adequate power to get through the night if we lose grid power.

The big loads on the Powerwalls are our well pump, fridge, 2 NG furnaces, Fios ONT, Synology 10 drive NAS, and a small Ubiquiti WiFi setup.

Our AC units are not on the critical loads panel.

Since receiving PTO on June 2nd I've had it set to 50%. Depending on how much the AC is running The PW's will last however long they do.

Last night I set it down to 30% and we were self powered all night till the sun came up since the weather was cool and the AC stayed off for the most part.

I'm curious on what you all have yours set for.

 

[wjgjr]

Since we don't have TOU pricing and have full net metering, I'm planning on leaving ours essentially full. Not only does it give the most protection if there is an outage, but it avoids the efficiency loss and potential reduced lifespan that can come with using the battery.

If and when (and I do think it will happen in the next several years) our utility offers us TOU pricing, I could see changing how we use the battery.

 

[mmanner]

Since we don't have TOU pricing and have full net metering, I'm planning on leaving ours essentially full. Not only does it give the most protection if there is an outage, but it avoids the efficiency loss and potential reduced lifespan that can come with using the battery.

If and when (and I do think it will happen in the next several years) our utility offers us TOU pricing, I could see changing how we use the battery.

Hi, where I live we don't have TOU or net metering (at all). I don't have a system yet, but one is ordered and I'm waiting for install in a few months. For me, does this mean that I should pick the minimum reserve level that provides the backup security I want, so that I can allow the batteries to cycle up to 100% and down to the reserve % in order to save some energy cost?

 

[wjgjr]

Hi, where I live we don't have TOU or net metering (at all). I don't have a system yet, but one is ordered and I'm waiting for install in a few months. For me, does this mean that I should pick the minimum reserve level that provides the backup security I want, so that I can allow the batteries to cycle up to 100% and down to the reserve % in order to save some energy cost?

So you are saying that your power company pays you nothing for any power you export to the grid? If so, yes, from a cost perspective, you want your batteries essentially drained overnight so they can be filled during the day with solar and then used overnight. (You also don't really want a system that generates more energy than your batteries can store since your utility gets that for free - though that is probably a topic for another thread.)

 

[morbidz]

I ran backup only mode, essentially treating the grid as a battery that doesn't have a 10% loss. I prefer full battery should a power outage occur

 

[mmanner]

So you are saying that your power company pays you nothing for any power you export to the grid? If so, yes, from a cost perspective, you want your batteries essentially drained overnight so they can be filled during the day with solar and then used overnight. (You also don't really want a system that generates more energy than your batteries can store since your utility gets that for free - though that is probably a topic for another thread.)

Yes, at this point if we have excess solar generated that can't go into battery and isn't used to power the house at the time of generation, it is lost. So my system saves me money only if I am using the battery to power the house (if no sun) and the sun to power the house if the sun is out. Also acts as a back up if the utility power goes out. The backup part was the main reason that I decided to get a system--but saving some money is ok too.

 

[BrettS]

I'm trying to decide on a reserve power level that will give us adequate power to get through the night if we lose grid power.

I'm curious on what you all have yours set for.

I mean this is going to be something that’s unique to each situation... the number of powerwalls, the power draw of each house. It may be interesting to see what other people have their systems set to, but I don’t think you can really use that data to figure out where to set your own system. The best thing to do is to test the system and see how much power you need and set your reserve accordingly. But do keep in mind that this may change seasonally as heating and cooling needs change.

In my case, they just finished installing my system on Tuesday of this week and I’ve been operating entirely off gird since Wednesday morning as I don’t have PTO yet, so I can’t be feeding power back into the grid. Over the two days that I’ve been operating this way my solar system has had enough power to run my house and fully charge my powerwalls during the day (with even a bit of left over power. Once the powerwalls hit 100% in the afternoon it starts cycling the inverters on and off). From sundown to sunup the powerwalls go from about 100% to about 40%.

I still need to figure out how I’m going to operate once I get my PTO and wind up back on the grid, but since my utility doesn’t offer a TOU rate and they do have net metering I may just stay in backup mode and let the powerwalls stay at 100%. However, doing this off grid test is definitely helping to give me peace of mind that my system is sized to be able to run off fully off grid if necessary.

 

[GaitherBill]

We can all do a bunch of math and get really precise about this, I'm sure.

Just off hand, I'd say our house has a constant 1Kwh draw when the AC units aren't on, with small spikes up to 1.3Kwh.

So some quick math would say leaving my 30% reserve would give us maybe 8 hours max? So realistically maybe 6 hours with zero PV input.

I think that would be just fine during the sunnier half the year. Once we get to winter, with shorter daylight hours and possible bad weather that could impede PV input, I'll want to increase the reserve.

I forgot to mention, we have NetMetering here.

 

[jjrandorin]

I'm trying to decide on a reserve power level that will give us adequate power to get through the night if we lose grid power.

The big loads on the Powerwalls are our well pump, fridge, 2 NG furnaces, Fios ONT, Synology 10 drive NAS, and a small Ubiquiti WiFi setup.

Our AC units are not on the critical loads panel.

Since receiving PTO on June 2nd I've had it set to 50%. Depending on how much the AC is running The PW's will last however long they do.

Last night I set it down to 30% and we were self powered all night till the sun came up since the weather was cool and the AC stayed off for the most part.

I'm curious on what you all have yours set for.

I change mine depending on what I think it will actually require to run the house overnight. Right now, I keep it set to 20% because we have plenty of sun, etc. When I got it installed in Jan of this year, I had the reserve set to 35 or 40%. If I know we are going to have rain the next day, or its going to be very overcast, I move the reserve up.

Yeah its micro managing it, but I dont mind, looking at the electrons flow into the powerwall gives me an incredible sense of self sufficiency and satisfaction, for some reason, as I have mentioned before.

 

[wjgjr]

We can all do a bunch of math and get really precise about this, I'm sure.

Just off hand, I'd say our house has a constant 1Kwh draw when the AC units aren't on, with small spikes up to 1.3Kwh.

So some quick math would say leaving my 30% reserve would give us maybe 8 hours max? So realistically maybe 6 hours with zero PV input.

I think that would be just fine during the sunnier half the year. Once we get to winter, with shorter daylight hours and possible bad weather that could impede PV input, I'll want to increase the reserve.

I forgot to mention, we have NetMetering here.

Based on your location, I'm assuming you are also in PEPCO territory. That is why, for me, leaving it maxed out is the way to go. I'm not as concerned about Tesla's "Self-powered" score since my goal is not to be off-grid, so keeping the powerwalls full is the best choice financially, and it conveniently means I'm always as prepared as can be in the case of an extended outage.

The other side of this that hasn't been discussed might be what is best in terms of environmental impact, which perhaps gets even more complicated. On that front, the ideal would presumably be to operate your home on battery during peak load times and export solar to the grid (and/or not import grid power.) But, since PEPCO doesn't provide an incentive in the form of TOU pricing, it would be something you would have to choose to do on your own (and, as a note, PEPCO's pilot TOU program sets peak in the afternoon in the summer but the morning in winter.) And the impact does depend a lot on what mix of fuels the utility uses during those times, taking into account also the ~10% energy lost by storing it in the powerwall.

 

[GaitherBill]

Based on your location, I'm assuming you are also in PEPCO territory. That is why, for me, leaving it maxed out is the way to go. I'm not as concerned about Tesla's "Self-powered" score since my goal is not to be off-grid, so keeping the powerwalls full is the best choice financially, and it conveniently means I'm always as prepared as can be in the case of an extended outage.

The other side of this that hasn't been discussed might be what is best in terms of environmental impact, which perhaps gets even more complicated. On that front, the ideal would presumably be to operate your home on battery during peak load times and export solar to the grid (and/or not import grid power.) But, since PEPCO doesn't provide an incentive in the form of TOU pricing, it would be something you would have to choose to do on your own (and, as a note, PEPCO's pilot TOU program sets peak in the afternoon in the summer but the morning in winter.) And the impact does depend a lot on what mix of fuels the utility uses during those times, taking into account also the ~10% energy lost by storing it in the powerwall.

We’ve got Potomac Edison/First Energy. We’re right on the edge. Couple streets over is Pepco.

There’s no TOU pricing as far as I know.

 

[GaitherBill]

I change mine depending on what I think it will actually require to run the house overnight. Right now, I keep it set to 20% because we have plenty of sun, etc. When I got it installed in Jan of this year, I had the reserve set to 35 or 40%. If I know we are going to have rain the next day, or its going to be very overcast, I move the reserve up.

Yeah its micro managing it, but I dont mind, looking at the electrons flow into the powerwall gives me an incredible sense of self sufficiency and satisfaction, for some reason, as I have mentioned before.

I think I’ll also be nerding out on the reserve.

 

[getakey]

Keep ours at 30%. In PG&E land where we face PSPS (Public safety power shutoff) outages in dry high wind weather. Have advance warning to switch to 100%

 

[Doom Man]

Does anyone consider the effect on battery life of having a fully charged PW exposed to summer heat? My PW's are outside because I don't have a garage. I've read that it's not good to have a fully charged lithium battery exposed to heat, because it reduces battery life. I know "100%" on the app is not a true 100% thanks to battery management, but would it still be better to use the battery daily so it doesn't sit at 100# in the heat?

FWIW, I'm running 40% reserve right now, so it moves from 40% to nearly 100% most days. I was on backup-only during winter months. I'm still playing with it.

 

[BrettS]

Does anyone consider the effect on battery life of having a fully charged PW exposed to summer heat?

Yes, Tesla considered that for you. The powerwalls actively heat and cool themselves. So even if your powerwalls are outside in hot weather they will cool themselves to keep the batteries at an optimal temperature. It’s more than just a fan, they actually have a mini air conditioner type system in there with a mini compressor.

 

[zanary]

Yes, Tesla considered that for you. The powerwalls actively heat and cool themselves. So even if your powerwalls are outside in hot weather they will cool themselves to keep the batteries at an optimal temperature. It’s more than just a fan, they actually have a mini air conditioner type system in there with a mini compressor.

This feature is called preconditioning, and grid power is used for it (not it's own stored power).

 

[BrettS]

This feature is called preconditioning, and grid power is used for it (not it's own stored power).

View attachment 556677

Yes, preconditioning is something the powerwall does, but that is to keep the batteries warm when the weather is cold. It will also actively cool the batteries to keep them from getting too hot, which is what the previous poster was concerned about.

 

[aesculus]

I think you just have to run some periods and see how it reacts via different weather and usage scenarios.

In the winter, where there is not much solar potential, I was running mine at 70%. During spring and now summer I have it set to 50%. May go a bit lower but I need at least 10 kWh to run the house at base load (no cooling) until solar kicks in so I doubt I will go lower by much.

I am on Cost Based control to the real answer there is what SoC reserve do you need to have to keep from using grid power during peak. Assuming it charges to 100% before peak, the reserve would be whatever power you consume during peak subtracted from your total storage capacity.

We have been hitting around 100F all week long here and yesterday was the first day I had to use grid energy during peak because I hit my reserve 30 minutes prior to peak ending. I am going to try 45% today and see if that carries me through. It's rare for me to have a full week of 100F temps so I will probably move it back to 50% once this heatwave is over.

 

[getakey]

I think you just have to run some periods and see how it reacts via different weather and usage scenarios.

In the winter, where there is not much solar potential, I was running mine at 70%. During spring and now summer I have it set to 50%. May go a bit lower but I need at least 10 kWh to run the house at base load (no cooling) until solar kicks in so I doubt I will go lower by much.

I am on Cost Based control to the real answer there is what SoC reserve do you need to have to keep from using grid power during peak. Assuming it charges to 100% before peak, the reserve would be whatever power you consume during peak subtracted from your total storage capacity.

We have been hitting around 100F all week long here and yesterday was the first day I had to use grid energy during peak because I hit my reserve 30 minutes prior to peak ending. I am going to try 45% today and see if that carries me through. It's rare for me to have a full week of 100F temps so I will probably move it back to 50% once this heatwave is over.

We are in that 100F area for 3 days now. I have reserve set at 30%, but on 95+ days I cannot make it on PW to 9pm. Come close at 95. Run out at 7ish on 100+. Fairly big house though, but keep Therm at 78/79

 

[aesculus]

We are in that 100F area for 3 days now. I have reserve set at 30%, but on 95+ days I cannot make it on PW to 9pm. Come close at 95. Run out at 7ish on 100+. Fairly big house though, but keep Therm at 78/79

I move the thermostats from 76 to 82 at 2pm after pumping the house down.That seems to help the house ride through but you have to be able to stand the large delta and the higher heat. So they really don't go on again until around 6PM.