Powerwalls for whole-house backup in NE US winter?

[twocoasts]

Is anyone using Powerwalls as their sole source of power backup, in the Northeastern US, in a place where the grid goes out often? Is It workable in the winter if you’re heating with heat pumps (assuming they have soft starts installed)?

My specifics: rural area with outages a few times a month, mostly a few hours at a time. Longest outage ever was four days. 15.9kw solar generating capacity. Panels pole mounted and seasonally adjusted to maximize output, and they shed snow quickly. I’m not in the lake effect snow belt, but it does snow, and we can have multiple successive days with enough cloud cover to seriously reduce solar generation.

Electrical usage hard to estimate because we”re now switching to electric appliances and geothermal heat pumps. Because of that, power demand will normally be high. In the winter I expect we we will normally consume more power than we produce and make up the balance from the grid. During grid outages we just need to cover necessary loads: one 5 ton heat pump to keep the pipes from freezing, well pump, a few lights and a power outlet, and one cooking appliance (microwave or stovetop).

The cost of installing and running a propane-guzzling generator big enough to back up the minimum necessary loads is enough that it could make economic sense to go with a Powerwall-only backup strategy, even if it took 3 or 4 Powerwalls (past that, the math gets tougher, but I might be able to swing it).

Is a Powerwall-only backup realistic in this scenario? Reliable? Other thoughts?

Thanks for any advice!

 

[Tam]

...Is a Powerwall-only backup realistic in this scenario..

It's all in the math.

How much do you use each day?

An average household uses about 28.9 kWh or 869 kWh per month.

If you use a lot or want to go off grid then let's thinking about PowerPack instead.

 

[ewoodrick]

For many days outage, a generator is probably going to be your best bet. That's a LOT of battery sitting there unused to pay for. Whole House Generators have gotten cheap lately and available at Home Depot. I think that they've got options, installed for $5k

 

[miimura]

I can imagine that your worst case low solar generation and electric heat pump heating would significantly limit your runtime to make it through longer outages. You would need to collect some data about how bad your solar generation gets and how much energy you typically use for heating.

Honestly, other battery systems have good generator integration while Powerwalls do not. The fundamental problem with the Powerwall generator integration is that once the batteries run out, your solar and batteries are offline until the grid comes back. Other inverter and battery systems can charge the batteries from the generator so your generator runtime is drastically reduced.

 

[twocoasts]

Any tips for how to calculate likely usage in kWh for heat pumps and appliances that are not yet installed?

A $5k generator would be nice, but the minimum generator needed for these specific loads costs more like $12k just for the hardware. Add the big propane tank needed, burying the tank, running gas and electric lines, etc, and you’re near $20k+ range—I’ve got actual quotes. And it’ll cost more if they hit rock while they’re digging or we have to build a structure to screen or protect the generator (unknown). Plus annual running costs of about $1100-$1300 (fuel at $4-$5/gal x 4 gal/hr x assume 48 hours outage per average year, required annual service @ $350).

So the question is, given that that’s what it’s going to cost, can it be done with Powerwalls in this climate?

 

[twocoasts]

Other inverter and battery systems can charge the batteries from the generator so your generator runtime is drastically reduced.

Any suggested brands/models?

 

[miimura]

Any tips for how to calculate likely usage in kWh for heat pumps and appliances that are not yet installed?

Do you have utility metering that shows hourly or daily usage or just gross monthly meter reads?

Any suggested brands/models?

Schneider, Outback Power, and SMA all make good battery inverters that have generator support. Since the runtime would be reduced, you could probably get away with a smaller, possibly gasoline powered generator. The question with those inverters is what kind of batteries to use. Therein lies the strength of Tesla's Powerwall - they have the best and cheapest batteries. However, you have to work with the inverter and system limitations that come with them.

 

[twocoasts]

Do you have utility metering that shows hourly or daily usage or just gross monthly meter reads?


Schneider, Outback Power, and SMA all make good battery inverters that have generator support. Since the runtime would be reduced, you could probably get away with a smaller, possibly gasoline powered generator. The question with those inverters is what kind of batteries to use. Therein lies the strength of Tesla's Powerwall - they have the best and cheapest batteries. However, you have to work with the inverter and system limitations that come with them.

Just gross monthly meter reads. And of course, since going forward the by far biggest and most critical loads will be the ones that are not yet installed—the heat pump especially—past meter reads don’t address what the usage will be that I have to back up.

 

[miimura]

Just gross monthly meter reads. And of course, since going forward the by far biggest and most critical loads will be the ones that are not yet installed—the heat pump especially—past meter reads don’t address what the usage will be that I have to back up.

That's going to be really tough to estimate. Even if we knew what the continuous draw was on the heat pump, we don't know how much it's going to run to keep your house at the set temperature because it depends on so many things like the insulation and air leakage in your house.

Do you at least have a handle on your daily solar generation? How many consecutive days do you get little or zero generation?

 

[shs1]

I have a similar setup with 11 kW of solar and currently a single Powerwall 2. I also have a geothermal heat pump and an 18 kW Kohler propane powered generator. There is no way the heat pump can run off of a single PW2. I am hoping to get a second one any day now. The heat pump works just fine under generator power and hopefully will with a 2nd PW2 and a soft start thingie.

My Powerwall/generator interface will automatically switch to and fire up the generator if the PW2 fails either because it can't handle the load, or if the battery SOC falls below a certain level - not sure what that is. I also have the ability to remotely manually switch to generator operation which I typically do during power failures as my single PW2 can't really handle the house load for very long if the heat pump tries to start up. While the house is powered by the generator the solar can charge the PW2, but with the current single PW2/current firmware situation that has sometimes not been reliable as the solar can generate more that the single PW2 can take.

I also get a fair amount of snow here. If the sun is up and most of the snow is removed from the ground mounted solar panels, I can run the whole house till the sun goes down, and then no way I can keep the heat pump going overnight when it gets really cold and it is most needed. And some mornings if we have had lots of snow and the power is off, it is frankly easier to let the generator do its thing rather than walk out to the solar panels with my snow shoes and try and get the snow off the panels so I can heat the house.

In the last few weeks I have had 5 different power failures andI have had power outages (big snow storm) of up to 10 days in the last few years. I love my Powerwall (although it has its issues right now), but again am glad I have both the PW2 and the generator to deal with power failures. Hopefully I'll soon have a second PW2 and can reevaluate things.

 

[twocoasts]

Do you at least have a handle on your daily solar generation? How many consecutive days do you get little or zero generation?

Agreed that the lack of consumption data makes it tough. Generation data at least I have. If we define “little generation” as < 20 kWh, the longest stretches during the low generation / high demand months of December/January/February have been four days. That happens once or twice a winter. The longest stretch below 30 kWh/day was 6 days, but that has only happened once. One- or two-day stretches happen more often. Typical day generation about 40 kWh. Peak generation days of 50-70 kWh happen between 12 and 20 days over those three months, Finally, February is usually the coldest month and by then peak days are more like 80 kWh.

The house’s air sealing, insulation, and window system efficiency are all pretty good: spray foam insulation in walls, roof, and under slab that all exceed code requirements significantly. A lot of glass, but about 90% is triple pane argon filled, and about 70% faces south or west so lots of solar heat gain. Renovation in progress now using a builder who prides himself on tight buildings and knows I’ll do a blower door test at the end. There’s an efficient wood stove for additional heat when I’m there. When I’m not there, just one of the two heat pumps can keep the house at a safe temperature.

Question for Powerwall owners: I understand from other posts that you can’t typically charge the Powerwall from the grid when the grid is up (except during severe weather alerts), but can you automatically direct all your solar generation to charging the Powerwall while the grid powers all your consumption?

 

[miimura]

Agreed that the lack of consumption data makes it tough. Generation data at least I have. If we define “little generation” as < 20 kWh, the longest stretches during the low generation / high demand months of December/January/February have been four days. That happens once or twice a winter. The longest stretch below 30 kWh/day was 6 days, but that has only happened once. One- or two-day stretches happen more often. Typical day generation about 40 kWh. Peak generation days of 50-70 kWh happen between 12 and 20 days over those three months, Finally, February is usually the coldest month and by then peak days are more like 80 kWh.

The house’s air sealing, insulation, and window system efficiency are all pretty good: spray foam insulation in walls, roof, and under slab that all exceed code requirements significantly. A lot of glass, but about 90% is triple pane argon filled, and about 70% faces south or west so lots of solar heat gain. Renovation in progress now using a builder who prides himself on tight buildings and knows I’ll do a blower door test at the end. There’s an efficient wood stove for additional heat when I’m there. When I’m not there, just one of the two heat pumps can keep the house at a safe temperature.

Question for Powerwall owners: I understand from other posts that you can’t typically charge the Powerwall from the grid when the grid is up (except during severe weather alerts), but can you automatically direct all your solar generation to charging the Powerwall while the grid powers all your consumption?

Your home efficiency sounds excellent. During an outage, you would clearly want to have the wood stove going to reduce the load on the heat pumps.

Yes, depending on your relative solar production and battery state of charge, the Powerwalls will charge from All Solar while the grid supplies the household loads. This was a low production day in November and the Powerwalls were running up against the reserve on consecutive days. You can see that they only powered the house during Peak for less than 2 hours.

 

[GottaGetRidOfICE]

Do you have utility metering that shows hourly or daily usage or just gross monthly meter reads?
Schneider, Outback Power, and SMA all make good battery inverters that have generator support. Since the runtime would be reduced, you could probably get away with a smaller, possibly gasoline powered generator. The question with those inverters is what kind of batteries to use. Therein lies the strength of Tesla's Powerwall - they have the best and cheapest batteries. However, you have to work with the inverter and system limitations that come with them.

We are installing 2 Sonnen batteries (2x16kw) that will work with large solar array and 8w generator. I expect to survive any outage without pipes freezing, and probably comfortably. Our Boston area geothermal (vertical closed loop) sees 50 degree underground. Sonnen is not the cheapest battery, but Tesla had hard limits on Backup Gateway that would have limited what we could back up, required a lot of trenching, etc., and in the end we would have gotten much less for same amount of expenditure. “Sometimes the cheap turns out expensive.”

 

[miimura]

We are installing 2 Sonnen batteries (2x16kw) that will work with large solar array and 8w generator. I expect to survive any outage without pipes freezing, and probably comfortably. Our Boston area geothermal (vertical closed loop) sees 50 degree underground. Sonnen is not the cheapest battery, but Tesla had hard limits on Backup Gateway that would have limited what we could back up, required a lot of trenching, etc., and in the end we would have gotten much less for same amount of expenditure. “Sometimes the cheap turns out expensive.”

From what I've found in a quick search, it appears that Sonnen uses Outback inverters in North America, integrated inside their cabinet. They are a very good choice because they can be arrayed and scaled into larger systems.

 

[rkarlberg]

Check with your utility - ours just offered a program where you can buy 2 PowerWalls for $25 per month each, free installation, and they will use off peak power to charge the batteries at night, which you can then use during the peak times. Unfortunately we didn't get in this time, sold out very fast.

 

[nyprepper]

twocoasts, I am in a rural area in the Hudson Valley too and had a 6.6kwh system and two Powerwalls installed in October. first off, let me say that the Tesla/Solar City covering our region is out of New Windsor and they are awesome. If you choose to go this route, you will be very happy with the install.

i made this investment solely for backup power in the event of grid outages. they are very frequent here..... typically 3-4 per month ranging from a few minutes to a few days. I have a 2800 square foot house and heat with oil and wood. I have replaced all lights inside and outside the house with LED's. I have 2 refrigerators. The only way we know that the grid goes down is because of the notification from Tesla. It is that awesome. Lights don't even blink.

Both fridges, a few lights, TV/modem/router, and the occasional use of the well pump will draw about 4-500 watts on average over a 24 hour period. I leave my two Powerwalls set to back up only so they are always at 96%+ capacity. So by doing the math, 27kWh capacity, divided by 500 watts, equals 54 hours...... so figure 2 days. When I am using an electric range, dryer, dishwasher, etc.... the house can draw as much as 8-9kw..... e.g. only 3 hours of run time at that rate. So when the grid goes down we go into bare essentials mode only.

The big variable in being able to handle a multi-day outage is solar production. As you know, rain, fog, heavy overcast, and snow on the panels can cut production down to zero. If I was generating 12kWh of solar I would be pretty much self sufficient in an extended outage (like Sandy, Irene...both 7 days plus).

I still have a Yamaha portable gas generator and I keep plenty of gasoline on hand. I have my setup wired so that if I did deplete the powerwalls in an extended outage I could run a subpanel off the Yamaha. Kind of a 3rd layer of contingency.... the first being solar, the second being the powerwall stored energy, and the third being the gas generator.

I hope this helps. I am very very happy with the system overall.

 

[twocoasts]

Thanks for coming through with some great info and alternatives. Sounds like a good solution may be either the Sonnen battery system with its Outback inverters, and possibly the third layer of a modest-sized generator. Meeting with my solar company Thursday and you guys have given me some ideas to talk through with them.