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Powerwall sizing - Essential / non-essential loads

mspohr

Well-Known Member
Jul 27, 2014
10,059
12,639
California
I wanted to share my thinking on solar/battery power.

Use Case:
Our power is very reliable with only occasional outages usually lasting only a few hours. This, in itself would not require any backup power. However, we are in an area subject to "Public Safety" power shut offs which can last for days and our area occasionally has multi day outages.
So, I want a battery backup system that can last multiple days and power essential services. I don't want or need to power everything but I want to remain comfortable.
We have solar and net metering with a flat rate so no opportunity for arbitrage (currently... that may change in the future).
The "payback" for this system is "security", not financial. I'm willing to invest what is necessary to provide energy security but don't see any return in oversizing the system.

What size system?
I want to be able to power essential loads (heat, lighting, cooking, house monitoring, communication) for multiple days.
We need to consider both the power (kW) and energy (kWh/day) requirements of our loads.

Lights, house monitoring, Internet, computers and other conveniences.
Fortunately, these do not take much power or energy. From my home energy use monitoring, it appears that all of these loads together use less than 1 kW of power and only about 15 kWh per day.

Heating and hot water. (No A/C required where we live)
We have a heat pump which heats half the house (hydronic) and provides hot water. This draws about 2 kW when running and ranges from 2 kWh in summer (hot water only) to a maximum of 20 kWh in winter.
The rest of the house is heated with a forced air gas furnace. 0.5 kW when running and up to 3 kWh in the winter.

Cooking.
Gas stove.

Hot tub.
Non-essential. It can go for days even in winter without energy use.

Tesla.
This doesn't need to be charged during a power outage. It just doesn't make sense to charge a large car battery from a small home backup battery. If I have not planned ahead, I can drive to someplace there is power and charge. If the Tesla battery is depleted, I can charge at a very low rate (2 kW) for a few hours to get the car drivable.

Solar.
My 9kW solar system generates about 70 kWh/day in summer peak and as low as 30 kWh/day kWh winter. (Cloudy days will decrease both these numbers.)

If I add up the power, I get a maximum load of 3.5 kW for essential loads.
For energy, I can get by with 17 kWh in Summer (probably less) and will need up to 37 kWh/day in winter.
So, my solar can provide all the power I need for extended outages during most of the year; falling short only it the darkest days of winter when we will need to turn the thermostat down (or, just turn off the hydronic heating and let the gas furnace take over).

So, what size Powerwall?
It seems that a single powerwall will handle both the maximum power (3.5 kW) and energy (running on solar during the day, charging the battery for use overnight... a 10-15 kWh battery) should be sufficient.

Footnote on Tesla Powerwall:
I placed a deposit a year ago with Tesla for Powerwalls. They have been silent since. I have called them several times and they give only vague statements "someday... maybe". I've given up on them.
I'm looking at the Outback Skybox which seems to be roughly equivalent to the Powerwall and seems to have much nicer (more flexible) software. Each puts out 5 kW and can be configured to use solar and also supplement grid power from the battery (useful for TOU). Up to 10 units can be chained together if my needs change. I'll need to provide batteries and fortunately have a brother in law who works for a battery company and can get me a good price on LiFePO4 batteries (safer and more durable). I can also install this system myself (with help from my electrician friend). Total cost should be much less than the Tesla Powerwall.

Any comments?
 

miimura

Well-Known Member
Aug 21, 2013
6,554
6,364
Los Altos, CA
With 9kW of solar, you really want at least two Powerwalls so that they can absorb your solar during an outage without any chance of overload. By the same token, if you are going with another kind of hybrid inverter like Skybox or Radian or Magnum that will be AC coupled, you need to size the battery inverters greater than the solar inverters. Maybe you can get away with a 8kW Radian with 9kW solar depending on the AC rating of the solar inverter.

The easiest way to separate what to back up is to eliminate any 240V loads from the backup side unless really needed. You probably should back up the hydronic heat pump so you have hot water. As you said, if it gets really cold during an outage you could retreat to the part of the house that is heated by the gas furnace. Since you have a relatively large solar system, there's a chance that you will have surplus generation on a cold sunny day and you could run the hydronic heat when the batteries are near full.
 

gpez

Member
Apr 25, 2019
735
602
USA
With 9kW of solar, you really want at least two Powerwalls so that they can absorb your solar during an outage without any chance of overload. By the same token, if you are going with another kind of hybrid inverter like Skybox or Radian or Magnum that will be AC coupled, you need to size the battery inverters greater than the solar inverters. Maybe you can get away with a 8kW Radian with 9kW solar depending on the AC rating of the solar inverter.

The easiest way to separate what to back up is to eliminate any 240V loads from the backup side unless really needed. You probably should back up the hydronic heat pump so you have hot water. As you said, if it gets really cold during an outage you could retreat to the part of the house that is heated by the gas furnace. Since you have a relatively large solar system, there's a chance that you will have surplus generation on a cold sunny day and you could run the hydronic heat when the batteries are near full.

Certainly having more Powerwalls is more efficient in this scenario however most modern inverters, specifically the Enlighten IQ series, support scaling back production without shutting down. This avoids the problem you mention here of being overloaded. I have an 8.55kW system and only 1 Powerwall with a grid profile applied to my IQ6 inverters that will reduce the production during a grid outage if the battery is full. See page 6 "AC Frequency / Watt" https://enphase.com/sites/default/f...Considerations-AC-Coupling-Micros-Battery.pdf

Removing the 240v loads is how I approached our backup scenario - the A/C, dryer, level 2 EV charger, and oven are all not backed up during an outage.
 
Last edited:

miimura

Well-Known Member
Aug 21, 2013
6,554
6,364
Los Altos, CA
Most modern inverters, specifically the Enlighten IQ series, support scaling back production without shutting down. This avoids the problem you mention here of being overloaded. I have an 8.55kW system and only 1 Powerwall with a grid profile applied to my IQ6 inverters that will reduce the production during a grid outage if the battery is full. See page 6 "AC Frequency / Watt" https://enphase.com/sites/default/f...Considerations-AC-Coupling-Micros-Battery.pdf

Removing the 240v loads is how I approached our backup scenario - the A/C, dryer, level 2 EV charger, and oven are all not backed up during an outage.
Of course, in theory, this is correct and can work with some combinations of equipment. However, you're inherently reducing your energy capture from your solar into your batteries. If the goal is to be able to survive extended outages, having only one Powerwall worth of energy storage is quite limiting. A two Powerwall system is much more robust because it provides more energy to ride through times of low generation and it provides the ability to support higher intermittent loads.
 

nwdiver

Well-Known Member
Feb 17, 2013
8,338
11,377
United States
I've really warmed to old-fashioned PbSO4 batteries mostly because they're so darn cheap. IMO lithium is a little overkill for a battery that's only going to be used a few days a year. Trojan has a line of industrial lead-acid batteries with an expected lifespan on par with lithium for a fraction of the cost. Are they heavy? Yes. Do they have a crap round-trip efficiency? Yes. Do they require water ~4x a year? Yes. But does that really matter for a battery that's going to sit in your garage until it's needed once a year?

My grid failure backup is a 4.4kW Magnum Energy Inverter that you can get for ~$2k + (16) 6v GC2 golf cart batteries you can get for ~$2k. The golf cart batteries provide ~400Ah total at ~48v for ~19kWh of storage. So I spent ~$4k for a 4.4kW / 19kWh backup system that can be AC coupled to a grid-tie inverter.

I've realized that old golf cart batteries still work ok for backup because even if they can kick out the power needed for a golf cart they can still put out a smaller amount to an inverter overnight so I'm hoping to pickup another 8 cheap... we'll see. If that fails I'm going to buy 8 'L16' 377Ah batteries for ~$2k to add another ~18kWh. That would give me >30kWh of storage for a total of ~$6k spent.
 
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Reactions: mspohr

mspohr

Well-Known Member
Jul 27, 2014
10,059
12,639
California
With 9kW of solar, you really want at least two Powerwalls so that they can absorb your solar during an outage without any chance of overload. By the same token, if you are going with another kind of hybrid inverter like Skybox or Radian or Magnum that will be AC coupled, you need to size the battery inverters greater than the solar inverters. Maybe you can get away with a 8kW Radian with 9kW solar depending on the AC rating of the solar inverter.

The easiest way to separate what to back up is to eliminate any 240V loads from the backup side unless really needed. You probably should back up the hydronic heat pump so you have hot water. As you said, if it gets really cold during an outage you could retreat to the part of the house that is heated by the gas furnace. Since you have a relatively large solar system, there's a chance that you will have surplus generation on a cold sunny day and you could run the hydronic heat when the batteries are near full.
Thanks for your insight.
The Skybox is DC coupled for solar with 5 kW max into the batteries. Even in winter I should be able to get 25 kWh/day out of the solar. (I also have a set of Enphase inverters to supply excess solar directly to my local loads.) I can fully charge 15 kWh of battery each day. I could use the rest of that during the day (heat pump has two storage tanks... hot water and hydronic buffer). (Even run the hot tub with excess.)
 

gpez

Member
Apr 25, 2019
735
602
USA
Of course, in theory, this is correct and can work with some combinations of equipment. However, you're inherently reducing your energy capture from your solar into your batteries. If the goal is to be able to survive extended outages, having only one Powerwall worth of energy storage is quite limiting. A two Powerwall system is much more robust because it provides more energy to ride through times of low generation and it provides the ability to support higher intermittent loads.

It's not a theory, it's how the components operate. I linked the spec and my system is set up this way.

Like everything else it's an ROI calculation with the cost. Your post I was responding to said nothing about the limiting factor of capacity. 10 Powerwalls is better than 2 for extended outages but of course the price is much different. My statement was simply that the 5kW inverter shutdown limit of 1 Powerwall can be avoided with the current inverter tech and that if @mspohr believes their calculations are correct that is a very viable way to go.
 

mspohr

Well-Known Member
Jul 27, 2014
10,059
12,639
California
It's not a theory, it's how the components operate. I linked the spec and my system is set up this way.

Like everything else it's an ROI calculation with the cost. Your post I was responding to said nothing about the limiting factor of capacity. 10 Powerwalls is better than 2 for extended outages but of course the price is much different. My statement was simply that the 5kW inverter shutdown limit of 1 Powerwall can be avoided with the current inverter tech and that if @mspohr believes their calculations are correct that is a very viable way to go.
Thanks. I want to survive an outage, I'm not necessarily needing total comfort. I'm willing to adjust.
 

mspohr

Well-Known Member
Jul 27, 2014
10,059
12,639
California
I've really warmed to old-fashioned PbSO4 batteries mostly because they're so darn cheap. IMO lithium is a little overkill for a battery that's only going to be used a few days a year. Trojan has a line of industrial lead-acid batteries with an expected lifespan on par with lithium for a fraction of the cost. Are they heavy? Yes. Do they have a crap round-trip efficiency? Yes. Do they require water ~4x a year? Yes. But does that really matter for a battery that's going to sit in your garage until it's needed once a year?

My grid failure backup is a 4.4kW Magnum Energy Inverter that you can get for ~$2k + (16) 6v GC2 golf cart batteries you can get for ~$2k. The golf cart batteries provide ~400Ah total at ~48v for ~19kWh of storage. So I spent ~$4k for a 4.4kW / 19kWh backup system that can be AC coupled to a grid-tie inverter.

I've realized that old golf cart batteries still work ok for backup because even if they can kick out the power needed for a golf cart they can still put out a smaller amount to an inverter overnight so I'm hoping to pickup another 8 cheap... we'll see. If that fails I'm going to buy 8 'L16' 377Ah batteries for ~$2k to add another ~18kWh. That would give me >30kWh of storage for a total of ~$6k spent.
I like this idea of using old lead acid batteries. When I think about it, these batteries will only come into service maybe a few cycles a year so it's really not worth it to have new, gold plated, best quality batteries, especially when these lead batteries are so cheap.
If I do get to the point where I have TOU and want to daily cycle to arbitrage, I can upgrade to some better batteries. Until then, these batteries should be good.
 

bkp_duke

Well-Known Member
May 15, 2016
5,582
19,623
San Diego, CA
Not sure if it was mentioned, but the round trip efficiency on the PW2 is ~90% (you put 10 kWh in, you get about 9 back out). I would say with a 9 kW solar system, and not needing to power an AC compressor, 2 PW2s would be a perfect fit for you, even with a long-term outage (you will of course keep recharging them during the daytime from your solar, even if the grid is out).

We have 3 PWs at our house, but only because it can get hot hot hot here for 2-3 months during the late summer (which is also during fire season), so being able to run at least one AC unit was high up on our want list.

While Lead Acid batteries can work, they take up a lot of space compared to PW2s given that the energy density is lower, and the wiring for them . . . . can look ghetto (call me a snob, but I like my gear to have a clean and tidy look).
 
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abasile

TSLA shareholder
Supporting Member
Oct 21, 2012
1,658
3,569
San Bernardino Mountains, California
Keep in mind that those "Public Safety" shutoffs will generally occur only in dry, windy conditions, in which case you'll probably have enough daily solar generation to keep everything going. While two Powerwalls would be nice, I would think you'd be fine with the capacity of just one.

Here in the SoCal mountains, it's not unusual to experience power outages during inclement winter weather. Last February, our solar panels were buried by multiple feet of snow. In such conditions, it's helpful to have more battery capacity. Also, we have an electric induction range and a 50 amp subpanel, and two Powerwalls were required to back those up.
 

bob_p

Active Member
Apr 5, 2012
3,725
2,922
If you have access to smart meter reports, that can help verify your power usage assumptions.

When planning our recent order for panels/PowerWalls, we pulled down actual energy usage for the last 12 months, in 15 minute increments, and did an analysis on the small number of peak loads to understand what was running, and determine how many PowerWalls we would need.

Our two largest energy users were EV charging (22 KW when charging both our S 100D and X100D at the same time) and pool equipment (5-5.5 KW, when all of the pumps are running). After that, the remaining consumption, even in a worst case scenario with ovens/HVAC running, should fit within the PowerWalls we are ordering (which can sustain 5KW per PowerWall).

Based on our analysis, 4 PowerWalls should be sufficient to handle everything for short outages, except for EV charging. And for operating off-grid for days, we'd turn off the pool equipment and other non-essential items to operate for as long as needed until the grid is back online.
 

mspohr

Well-Known Member
Jul 27, 2014
10,059
12,639
California
The Myth of Whole-Home Battery Backup

There are two fundamental engineering limits that make it impractical to run a whole house on battery power alone. First, the energy capacity of typical lithium-ion battery systems is insufficient to power an entire house through a nighttime blackout. Second, battery backup inverters are not powerful enough to start and run many large appliances.


Of course, multiple batteries and inverters can address these energy and power limitations. But the cost of 20+ kilowatts of inverters and 40+ kilowatt-hours of batteries is prohibitive for the typical homeowner.

A more practical approach is to design a battery backup system to power critical loads only: no large appliances such as air conditioning, 240-volt EV chargers or electric stoves. Instead, just four to eight smaller circuits in the house for refrigeration, lighting, entertainment, communications and convenience outlets.
 

miimura

Well-Known Member
Aug 21, 2013
6,554
6,364
Los Altos, CA
The Myth of Whole-Home Battery Backup

There are two fundamental engineering limits that make it impractical to run a whole house on battery power alone. First, the energy capacity of typical lithium-ion battery systems is insufficient to power an entire house through a nighttime blackout. Second, battery backup inverters are not powerful enough to start and run many large appliances.


Of course, multiple batteries and inverters can address these energy and power limitations. But the cost of 20+ kilowatts of inverters and 40+ kilowatt-hours of batteries is prohibitive for the typical homeowner.

A more practical approach is to design a battery backup system to power critical loads only: no large appliances such as air conditioning, 240-volt EV chargers or electric stoves. Instead, just four to eight smaller circuits in the house for refrigeration, lighting, entertainment, communications and convenience outlets.
I have a 125A 24 space sub-panel that has no high drain loads, but lots of circuits for outlets and lighting that can be easily backed up and last through the night from a single Powerwall. I agree that most or all 240V circuits should be excluded from backup, but paring all the way down to 8 circuits is crazy talk.
 

woferry

Member
Mar 4, 2019
406
478
San Jose, CA
Indeed, I didn't read the article but just from what was quoted it clearly doesn't apply to my house. I can run my whole house from a single PW2 all evening in the summer without any trouble (not surprising, since my typical daily consumption is 12-13kWh/day). So I didn't draw from the grid at all from May (PTO) through November except for one night I did a bunch of laundry (PW ran out of energy about 30 minutes before the solar was able to power the house loads), and one night where an RV was drawing power from the house (and it's fridge was busy cooling from 70F to normal fridge temperatures, so it ran non-stop almost all evening, PW ran out around 3am that evening). But the inverter loading was never an issue, and the battery capacity is plenty for me in most cases. Now the winter is a different story, partly because some days haven't had enough solar to fully-charge my PW, but also because the hours the solar can power the house are less, so the battery has to power the house for a few hours longer, and that is enough to make the difference. If the usable capacity was ~10% higher AND if it could start every sundown fully-charged (this is the real challenge) I'd probably be fine all year. So of course there's no one-size-fits-all solution, since houses can draw vastly different amounts of power depending on a lot of different factors.
 
Last edited:

Dan123

Member
Jun 19, 2018
451
322
Miami
The Myth of Whole-Home Battery Backup

There are two fundamental engineering limits that make it impractical to run a whole house on battery power alone. First, the energy capacity of typical lithium-ion battery systems is insufficient to power an entire house through a nighttime blackout. Second, battery backup inverters are not powerful enough to start and run many large appliances.


Of course, multiple batteries and inverters can address these energy and power limitations. But the cost of 20+ kilowatts of inverters and 40+ kilowatt-hours of batteries is prohibitive for the typical homeowner.

A more practical approach is to design a battery backup system to power critical loads only: no large appliances such as air conditioning, 240-volt EV chargers or electric stoves. Instead, just four to eight smaller circuits in the house for refrigeration, lighting, entertainment, communications and convenience outlets.

I have a whole-house backup with 2 Powerwalls. 2 Powerwalls are enough to power the house through the night, and enough to start my 4-ton AC unit. I think a whole-house backup is not a myth, but it does require an efficient house to begin with.
 

Lloyd

Well-Known Member
Supporting Member
Jan 12, 2011
6,334
2,200
San Luis Obispo, CA
I can run my entire house with the exception of car charging with two powerwalls. A/C will use the stored power quickly, but so far no problem running any of the major appliances individually.
 

jboy210

Supporting Member
Supporting Member
Dec 2, 2016
5,469
3,468
Northern California
If you trying to run you house on a one powerwall do you need to have a separate critical loads panel installed? As long as you do not turn on big power consumers like the A/C, oven, car charging, or dryer are you not be doing the same thing?
 

bkp_duke

Well-Known Member
May 15, 2016
5,582
19,623
San Diego, CA
If you trying to run you house on a one powerwall do you need to have a separate critical loads panel installed? As long as you do not turn on big power consumers like the A/C, oven, car charging, or dryer are you not be doing the same thing?

In theory, yes, but Tesla won't do the install like that. They will argue (rightfully) that you could overtax the powerwall inadvertently and they still have to service it under warranty.
 

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