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

[BrettS]

So I’m starting this thread as more of a work in progress to help people size their solar/powerwall to be able to run off grid for extended times. In my case I have an on grid system, but I live in Florida and there is a real possibility of multi day power outages after hurricanes.

To keep things (somewhat) less complicated I’m not looking to discuss sizing an on grid solar only system or a solar/powerwall system that’s only intended to cover short outages. I’d like to keep this thread focused on systems that are completely off grid or can be run off grid for extended times (several days or more)

So, all that said, I think these are the things that you need to consider as you try to size your off grid capable system. Some of these points will probably be more obvious than others.

To start with you need to figure out the bare minimum size for your solar/powerwall system based on your daily usage.

-At a minimum your powerwalls need to have enough capacity to support your house from sundown to sunup. You’ll need at least enough capacity to get through the night when your solar system isn’t producing power. The good news here is that typically your house will use less power at night... everyone is asleep so lights and devices are off.

-At a minimum your solar system needs to have enough capacity to cover your entirely daily usage (both daytime and night time usage). The solar system needs to be able to provide enough power to your house to cover your power demand during the day while the sun is shining in addition to being able to fully charge your powerwalls so they will be ready to go when the sun goes back down.

Now, in a perfect world, that would be enough, but this isn’t a perfect world, and this is where things get more tricky. You’ll need to oversize your systems for the following reasons:

-There will be days where it’s somewhat cloudy or rainy and you want your solar system to be able to meet your entire daily power need even in less than perfect conditions.

-The solar system will produce less power during the winter than it will in the summer. Luckily (unless you have electric heat) your house will probably have a lower power demand in the winter, so that will offset the lower production to some extent. However, you need to make sure that the solar system will still meet your daily power need even during the lowest production times.

-There will be times when your powerwalls will be used to cover more than just your nightly power usage, so you need extra capacity in your solar system to be able to recharge your powerwalls even when they have been drained more than normal.

-You will need extra capacity in your powerwalls because there will be days where it’s very cloudy or rainy and even with an oversized solar system you won’t be able to generate enough power to cover your usage for that day. The powerwalls will need to take up the slack on those days.

So all that said, I would consider the following to try to figure out what size of a system to get.

Look at your usage history to try to figure out your average usage in each season. Look at the daily usage as well as nightly (sundown to sunup usage). You can use these numbers to establish a baseline.

Then consider the worst case scenario. Do you live in a climate where you might get heavy rain or snow for 4 or 5 days in a row? Or do you typically have good weather, then you might have a bad storm one day, then good weather again the next day?

If you could potentially have very low or no solar generation for several days then you need to make sure that you have a lot of capacity in your powerwalls to carry you through those times. You’ll need to size your powerwalls to be able to cover several days worth of your daily power draw and you’ll need to add a lot of extra capacity to your solar system to be able to recharge this huge powerwall system after it has been used for several days. (You don’t need a monster solar system that can recharge it all in one day... it might take a few days to be able to fully charge it, but you don’t want your solar system to be so small in that it takes a month to get your powerwalls charged again after a few days with little solar power).

On the other hand if you are only likely to have low or no solar generation for a day or part of a day, then you can size your powerwalls such that they are able to cover your daily power draw for a full day or two. Again, you’ll want your solar system to be sized such that it can recharge the powerwalls within a couple of days even when they have been drained way down.

Hopefully this information has been helpful to someone and I’d be happy to hear any suggestions or comments or other factors that are worth considering.

 

[wjgjr]

I think the other side of this to consider is the demand side. To me this involves figuring out what are the essential loads and what are optional, or at least deferable. For example, maintaining the refrigerator is essential, but things like A/C may be only partly essential - that is, you could possibly accept a slightly warmer house to reduce the amount of time the A/C is running. Other things like washer/dryer could be seen as mostly optional even during a longer outage (including options like doing less laundry and air drying) or could be delayed to a sunny day where there is excess generation. And maybe things like the stove/oven could be used less in favor of a grill, if that is available. The point being that you can potentially establish not only your typical usage but a minimum acceptable usage number. You could consider sizing with a goal towards supporting typical usage but which hopefully at least ensures minimum usage, even if generation is sub-optimal due to weather and/or seasonal factors.

It ends up to an extent being an individual cost-benefit analysis as to how much managing demand is acceptable vs. paying for more generation and storage, so it depends on your personal preferences. It also does potentially depend on whether your goal really is to manage to be completely off-grid or just to handle an extended outage of a month or so.

 

[BrettS]

Yeah, that’s definitely true and it’s kind of where totally off grid branches away from off grid for an extended outage. Obviously if you were totally off grid then you would want to be able to operate the house as your normally do, but if you are just trying to manage an extended outage then you can definitely make concessions and limit your power usage in order to get by with a smaller and cheaper system.

In my case, car charging is a big one. I believe that my system is sized such that I should be able to operate pretty much indefinitely off grid, but with little to no car charging, as well as trying to be reasonably conservative with my power usage.

 

[bmah]

Yeah, that’s definitely true and it’s kind of where totally off grid branches away from off grid for an extended outage. Obviously if you were totally off grid then you would want to be able to operate the house as your normally do, but if you are just trying to manage an extended outage then you can definitely make concessions and limit your power usage in order to get by with a smaller and cheaper system.

In my case, car charging is a big one. I believe that my system is sized such that I should be able to operate pretty much indefinitely off grid, but with little to no car charging, as well as trying to be reasonably conservative with my power usage.

You'd be surprised what you can do when you're faced with a multi-day outage of unknown duration. A few lessons from last year's PG&E PSPS events:

Our solar is definitely undersized for the house (it was installed ten years ago for peak shaving), but by starting with full Powerwalls (courtesy of Stormwatch) and being really careful about not using heavy appliances, we got through a 60-hour downtime pretty well. As @wjgjr said it's good to prioritize and figure out what's truly essential.

If the timing of the outage is known you can use that information to top-off your EVs, do the laundry, finish baking those cookies, etc. before the outage starts.

If you know what time of the year outages are likely to occur and what the weather patterns are going to be like, you could factor that in too. For example, PSPS events in NorCal seem to be more likely in the late summer and fall, when the weather is fairly clear (not maximum daylight but fairly clear skies) and temperatures mild (some but not a lot of HVAC usage). It'd be a very different calculus if your outages are normally caused by blizzard conditions taking down lines, etc.

Bruce.

 

[swedge]

Brett,

I think you have covered the main issues. It is not a simple question, and probably best not left to a salesperson. We did analysis as you describe, so I thought I'd share our case.

I suspect that for many situations, it will not be economical to have enough solar capacity to sustain power during an extended winter outage, so compromise may be wise. Fortunately, most winter outages are from tree-falls and hence local and short term, so this may be a rare situation. Still, we are keeping our little generator as a back-up to our back-up.

We recently added a single PowerWall to our (2016) 3.8kW solar system. Testing during the two months since the PW was installed proves we could go off grid during these summer months, except I want those summertime NEM credits. An extended outage during the winter will still require our little 2kW propane inverter generator.

Annually our solar provides ~85% of our consumption. This we know because we included consumption monitoring on our Enphase monitor. I analyzed a full year of hour by hour solar production, along with PG&E hour by hour grid data. From this we knew we very, very seldom draw over 5kW. Our average daily usage of ~15kWh means that, if fully charged, the 13.5kWh of PW capacity less a 20% backup minimum, handles the overnight portion. This data helped a lot, rather than relying on estimates and appliance power ratings.

We do not have EV (yet), which would roughly double our needs. When we get one, I figure charging during a grid outage will mean driving to a charger outside of the outage area. Sort of like driving to a gas station, shouldn't be too big a deal. ;-)

PSPS outages were in October, clear, hot, windy days. Solar production is still decent then, so with a bit of conservation we should be able to produce enough solar to sustain power as needed. During winter or stormy weather, an extended outage would require we fire up our 2kW generator on both allow the solar to charge the PW as much as possible and to run critical circuits when that is not enough.

Tesla says the PowerWall can not, but it sure would be great if itl could be charged off of the generator. However, that is a topic for a different thread. I only mention this to remind folks of this sad limitation.

I hope my case helps others through this process.

 

[gigawatt1010]

Great thread @BrettS

I'll be following this closely as I wait for mine to be installed. Mine is a 14.02kW PV system that is supposed to generate about 15,000kWh. My utility company says I used roughly 13,000kWh in the past 12 months so I'm thinking I should have enough. I'm in NY and my house is on a hill above any tree lines so the only shade will be my chimney flume. Oh and the crappy winter weather we get.

Your installation moved pretty fast. Do you have a date for your PTO?

 

[BrettS]

Do you have a date for your PTO?

I do not. I need to wait for the city inspection first, although the installers said that my city is usually pretty quick about the inspections and said it probably won’t be much more than a week for that. However they are a little worried because my irrigation pump is right in front of my main electrical panel and apparently code requires at least three feet of clearance in front of the pump. So we’ll see how that goes.

Once it passes the city inspection then I’m waiting for the utility and they said that can be as long as 5 or 6 weeks. Although they did say that sometimes calling the utility to let them know that the inspection is complete can help get the ball rolling there.

 

[Doom Man]

I've got a 6.7kw system and 2 PW's for hurricane backup in south Louisiana. The 6.7kW is based on about 90% of my annual usage, but I might add a few more panels if I get an EV. The 2 PW's are a bare minimum to be able to have some a/c during an outage. I'd like to have another pair of PW's to keep the a/c's running, but that's cost-prohibitive at this point. So in a long outage, I'll have to be careful about how much I run the a/c's, and if my PW's get under about 40% I'll probably just rely on ceiling fans.

Good factors are that hurricanes usually mean only about 24 hours of low light, generally followed by long, sunny days. When it's really overcast, there's less need for a/c, so the PW's charge will last longer. I'd think a northern wintertime backup system would be tougher, when gloom descends and days are short and maybe there's snow on the array just when you need the power.

 

[bob_p]

We sized our system to produce about 50% of our electricity usage (including EV charging). One of the goals was to use the system to power the house for days if we lost power after a hurricane, and figured we'd be able to achieve that by turning off non-essential devices, if necessary (running on one HVAC system, turning off pool pumps, ...).

After we ordered the system, we did an energy analysis, identified devices that were wasting energy and reduced their usage or replaced them (optimizing pool pump usage, replacing frequently used incandescents with LED, ...). This helped to reduce the power consumption, and increase the % of power we'd be able to supply from the new system.

On sunny days in the winter and spring, we were able to operate the house for 4-5 days off grid (excluding EV charging).

Rather than trying to power our house 100% of the time (which is difficult when there are cloudy days), we shifted to a Free Nights plans, getting free electricity from 9PM to 9AM, and using the system to reduce as much as possible any grid use between 9AM to 9PM.

On most days, we end up using less than a KWh of grid power during the day. Over the past week, with multiple days of heavy cloud cover and rain, we've pulled power from the grid during the days and the PowerWalls have stayed close to the reserve %.

A basic mistake in sizing a solar panel/PowerWall system is to rely on average power consumption. Power usage varies based on exterior temperature. Solar power production varies based on number of daylight hours, angle of the sun, cloud cover, ...

When planning our system, we used 2 year smart meter data (at 15 minute increments) to help in assessing our power consumption, and even though we sized the system for only 50% of our power, we were pleasantly surprised how many days we've been able to operate off grid. And now that we're on a 12 hour Free Nights plan, as long as we don't get a lot of no-sun days, we expect our electric bills to drop considerably.

 

[fresnoboy]

I think you folks are being highly optimistic in trying to use PW's for multiday outages given that many times outages are caused by bad weather that also impacts solar production. Using a generator (gas/propane) is a much more reliable and can handle multiday outages much better than PW's. I just wish Tesla made it easier to integrate generators.

 

[wjgjr]

I think you folks are being highly optimistic in trying to use PW's for multiday outages given that many times outages are caused by bad weather that also impacts solar production. Using a generator (gas/propane) is a much more reliable and can handle multiday outages much better than PW's. I just wish Tesla made it easier to integrate generators.

Using solar+PW for backup is about probabilities since weather is a factor, but I don't think it is particularly optimistic to use this strategy for long-term outages as long as one is realistic about those probabilities. And while weather does cause many outages (and for now I am setting aside folks in CA or elsewhere who deal with fire shutoffs where they may well have full sun during the entire outage) it is also the case (and certainly in my area) that the weather that causes outages typically does not hang around for days. So, even if a major storm means it takes many days to restore power, it is likely there will be some better weather during that outage to at least help replenish the batteries. Having a full PW to start a backup is, of course, ideal, and so the storm watch mode is a nice feature to try and help with that.

With all of that said, I do agree that a generator still has the edge on reliability in an outage, but I don't think it is as significant as it once might have been. And, the nice thing about solar+PW - depending on the individual circumstances - is it can be more cost effective than a generator because solar+PW provides a financial benefit in regular use, not just in the case of an outage. I also don't blame Tesla for not necessarily making an effort to support generators that compete with powerwalls and also tend to run counter to their general business of promoting greener technologies.

In the end it has to be an individual decision, but I don't feel like the comments so far seem to show unrealistic expectations of what is possible. It does potentially require one to be proactive about managing load during an outage, but that can also be said of a generator depending on its size and fueling requirements.

 

[gpez]

I think you folks are being highly optimistic in trying to use PW's for multiday outages given that many times outages are caused by bad weather that also impacts solar production. Using a generator (gas/propane) is a much more reliable and can handle multiday outages much better than PW's. I just wish Tesla made it easier to integrate generators.

This is very fair and I think there's also a consideration of damage due to bad weather. The Powerwall is pretty hearty (the Ingress Protection ratings are really good) but they're not invincible and same with rooftop solar panels. I know there are design considerations that help mitigate against the driving rains and winds from a hurricane but it doesn't eliminate the risk. Of course outdoor standby generators have similar risk.

One other thing is to remember that not everyone has these weather related outage concerns. One of the big factors for me to go solar + Powerwall is earthquakes. Even a medium sized one could very easily disrupt natural gas and gasoline supplies for a period of time. Earthquakes don't really affect the sun, though!

All of that said, I do have a backup generator: in a pinch I can use my EV to power the most critical house loads.

 

[Doom Man]

I'd have probably gone with a generator if it was an option for the reasons stated above, but my tiny lot couldn't meet the setback requirements without putting the generator in the middle of our courtyard, which would look (and sound) terrible. I will say that I had a GE whole-house generator at my old house, and it was great when it worked. It did break down surprisingly often, required regular service, and it used a ton of natural gas when we had outages. PW's should be maintenance-free.

 

[miimura]

I set my Reserve between 25% (Summer) and 75% (Winter). The logic is like this - in the Summer it's easy to replenish the batteries, so I just let them cycle according to my TOU rate plan. The generation before 3pm is sufficient to power the house until midnight and still leave the batteries with more than 50% remaining. As the days get shorter and the sun gets lower on the horizon, solar production goes down. Eventually, it's less than the Peak period consumption. At that point, I start raising the Reserve so that the headroom above the Reserve is approximately the best case generation for a day. That way, the batteries cycle from the Reserve to about 95% daily during the late Fall through early Spring.
I have two Powerwalls and a small 4.32kW solar system.

 

[mstgkillr]

Researching a whole home standby generator is exactly how I ended up on this forum, with solar and Powerwalls being a direct replacement for a generator. Actually, the solar to offset my utility bill and the Powerwall as a generator alternative.

Quick background... I live in SW Florida where most power outages occur during hurricane season (Jun-Nov), with the most activity in Aug and Sep. I have a variable speed A/C and can run my entire home with the exception of the hot water heater, dryer, and range, on a 7 kW peak, 5.5 kW continuous inverter generator (Honda EU7000is). All other energy efficiency upgrades are complete (variable pool pump, LEDs, etc.) Unfortunately, I lost the generator in a house fire, so here I am. Ideally, I am looking for a solar/Powerwall system that can run my entire home anytime from Jun-Nov, minus the hot water heater, dryer, and range, almost indefinitely (at least 1-2 weeks).

For the solar array, I am considering either a 12.24 kW or 16.32 kW system. According to PVWatts, the 12.24 kW system would offset almost 95% of my average electric use over the last year. However, in December, I added a 210-gallon reef aquarium, which is what started the search for a generator in the first place. And, in March, I added a 21 cuft freezer. I assume next years electric use will be higher. The 16.32 kW system would cover my current electric use and about 20,000 miles/year of EV use. It would also be a buffer for panel degradation over the life of the system.

For Powerwalls, I am unsure how many I need. I've read anyway between 2 and 4. I understand the Powerwall can only charge at 5 kW each, and if utility power fails and I cannot backfeed into the grid, it is possible for larger arrays to produce too much power for the Powerwalls and completely shut everything down. I've read of people that have a 12 kW system with 2 Powerwalls, but couldn't the panels overwhelm the 2 Powerwalls if the house wasn't using much energy?

Right now, I am leaning towards the 16.32 kWH system with 4 Powerwalls. Any suggestions?

 

[BrettS]

Right now, I am leaning towards the 16.32 kWH system with 4 Powerwalls. Any suggestions?

I think the bigger the better. The system I just put in is 15.12kW with 4 powerwalls. I’ve been running off grid since Wednesday morning and so far it’s met my needs perfectly. Other than limiting car charging I haven’t really done anything differently with my power usage. The powerwalls will drain from 100% to about 40% from sundown to sun up and the solar system will power my house and recharge the powerwalls the each day. I’ve even had some excess solar power at the end of the day after the powerwalls hit 100%. My power usage is a bit higher than yours (I used about 30,000 kWh last year) but a lot of that usage is car charging.

I had actually looked into solar briefly a year ago and at that time I was looking at a 14kW system with 2 powerwalls, but at this point I’m very glad that I went with 4. 4 powerwalls is easily enough to carry me through the night and if I make some effort to conserve energy I think they would be enough to get me through two or three days of bad weather with very limited solar production. It looks like the weather is predicting some rain for me next week, so I’m interested to see how that goes with my continued off grid test.

 

[gpez]

For Powerwalls, I am unsure how many I need. I've read anyway between 2 and 4. I understand the Powerwall can only charge at 5 kW each, and if utility power fails and I cannot backfeed into the grid, it is possible for larger arrays to produce too much power for the Powerwalls and completely shut everything down. I've read of people that have a 12 kW system with 2 Powerwalls, but couldn't the panels overwhelm the 2 Powerwalls if the house wasn't using much energy?

Right now, I am leaning towards the 16.32 kWH system with 4 Powerwalls. Any suggestions?

View attachment 556619

Remember there is a difference between the panel ratings and the inverter ratings. Many times inverters are smaller than the aggregates of the panels, so it's likely that even a 16.32kW system is limited by a 13kW inverter. On top of that unless all of your panels are in the same spot in the same direction its unlikely that all of them are maxed out at the same time. And of course your home load subtracts from that so It's possible 3 Powerwalls is plenty for you.

In addition there are other things to accommodate the higher PV output. Shamefully stolen from myself:

1) Take advantage of your inverter's frequency power scaling. Many new PV inverters (and some old ones) are configurable to scale down production based on the grid frequency. SolarEdge inverters all it "P(F) Power Frequency" (https://www.solaredge.com/sites/default/files/application_note_power_control_configuration.pdf, page 6) and Enphase calls it "curtailment" or "ramp down" (https://enphase.com/sites/default/files/downloads/support/Design-Considerations-AC-Coupling-Micros-Battery.pdf, page 6). Since the Powerwall will increase the power frequency during off grid operations to instruct the inverters how to behave you can match the Powerwall's frequency profile with your inverters to ensure that you never exceed the Powerwall limits. This is what I do for my 8.6kW PV system which just barely reaches the PV inverter rating of 6.6kW during the sunniest of days. If your system supports it this is probably the best way to go.

2) Shut off a PV inverter/string off during an outage. This is not a great option but one that some have floated on this board for large PV systems that aren't looking to add many Powerwalls. With this configuration you're essentially reducing the size of the PV system during an off grid operation keeping it under the Powerwalls' inverter limits. Open the breaker on one of your two inverters during the sunniest of days and close it back during cloudy days. More manual but keeps the Powerwalls from tripping and costs nothing.

3) Increase your Powerwalls. As you noted this is an expensive but the most doable option.

​

 

[BrettS]

Remember there is a difference between the panel ratings and the inverter ratings. Many times inverters are smaller than the aggregates of the panels, so it's likely that even a 16.32kW system is limited by a 12kW inverter. On top of that unless all of your panels are in the same spot in the same direction its unlikely that all of them are maxed out at the same time. And of course your home load subtracts from that so It's possible 3 Powerwalls is plenty for you.

I’m not sure what inverters Tesla is using for the 16.32kW systems, but for my 15.12kW system they gave me two Solar Edge 7600H inverters, rated at 7600W each, or 15.2kW together, so it’s definitely a good match for my panels. However, like you said, there are conversion losses and panel orientation losses and such. For my 15.12kW system I have been seeing AC generation in the low 12kW range at peak times. The highest I saw it go was 12.9kW.

 

[gpez]

I’m not sure what inverters Tesla is using for the 16.32kW systems, but for my 15.12kW system they gave me two Solar Edge 7600H inverters, rated at 7600W each, or 15.2kW together, so it’s definitely a good match for my panels. However, like you said, there are conversion losses and panel orientation losses and such. For my 15.12kW system I have been seeing AC generation in the low 12kW range at peak times. The highest I saw it go was 12.9kW.

My math was off - should have been more realistic at 13kw inverter on a 16kw PV for an ~80% ratio which I've seen here. I didn't go Tesla so it's very possible they line up inverters closer, also may be locale specific based on your solar irradiance profile. I edited my post

 

[GenSao]

For Powerwalls, I am unsure how many I need. I've read anyway between 2 and 4. I understand the Powerwall can only charge at 5 kW each, and if utility power fails and I cannot backfeed into the grid, it is possible for larger arrays to produce too much power for the Powerwalls and completely shut everything down. I've read of people that have a 12 kW system with 2 Powerwalls, but couldn't the panels overwhelm the 2 Powerwalls if the house wasn't using much energy?

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 others have stated, the Powerwall can control the solar panel inverter(s) by limiting output or tuning them off.

Right now, I am leaning towards the 16.32 kWH system with 4 Powerwalls. Any suggestions?

View attachment 556619

Compare your daily energy usage to see if there is enough capacity to cover your power use in a backup senario. For summer, look to cover overnight use as solar can assist during the day. For winter assume you need 100% of your usage as solar may be impacted by weather. As a factor of safety, you can discount the fact that non essential loads can be shed (such as running the cloths dryer) in a back up scenario.

Although your peak power load can be supported by two Powerwalls, Your winter daily power average looks like you may need three Powerwalls. The 4th Powerwall is only needed if you are worried about a multi-day power outage in the winter.