What don't I know that I don't know?

[scarecrow7]

Hello, all. First post here, although I've been following for awhile.

I want to move towards upgrading my current setup, and want to know what I should beware of and questions I should ask.

Right now I have a six year-old 6.84 kW roof-mounted solar (24 x 285w) system with a SolarEdge SE7600A 7600w inverter and SolarEdge P300 power optimizer

It's a multifamily dwelling, so the above supplies around 40% of annual electricity use

I have the room, and would like to add at least as many panels again as to reach 100% of future use, which will ultimately I hope through attrition include the replacement of 2-3 ICE vehicles (and some farm equipment) with electric. If I end up overbuilding a bit and routinely put some extra into the system, I'm not going to worry too much.

So, I'm thinking: inverter upgrade, add thirty or so rack-mounted panels @ 300+w each, two Powerwalls.

The current panels are on top of the garage, with the inverter inside. The Powerwalls could also go inside the (climate-controlled) garage, and the rack-mounted panels around fifty feet away.

What about this is impractical, what technical hurdles will I be looking at, and what questions should I be asking the contractor(s)?

Thanks

-scarecrow

 

[BGbreeder]

Depending on how far you drive, the replacement of the two ICE vehicles and tractor could add substantially to what you might need. E.g. A Tesla model 3 could pull 11.5kW for eight hours or so to replace an empty battery system (353miles). So, how many miles do you drive, and how many gallons of diesel/gasoline does the tractor burn per year? FWIW: 1gal of diesel is about 40kWh of electricity, but diesel tractor to electric tractor isn't a well studied conversion and a great deal depends on whether your tractor spends most of its time mowing a lawn, or plowing, or chipping wood.

The other item I would ask about is what do you mean by 100%? Averaged over the whole year? 100% off grid? Very different amounts of solar needed. At your current rate of usage I am guessing 28kW of solar, but then you would need to add in the EV demands from there.

If you have the space, a ground mount is likely to be the lowest cost for a large installation.

In general, my $0.02 is if you can afford it, overbuild, but many utilities wont approve overbuilds.

Does that help?

All the best,

BG

 

[miimura]

It probably doesn't make sense to touch the existing solar. Just get a quote to add the new stuff. If you're adding about 10kW of new solar, you would need a third Powerwall to ensure that the system is stable when the grid is down with all that solar on the backup side. If you stayed with a new SE7600, it could work with only two Powerwalls.

 

[jjrandorin]

What is the driver for wanting the powerwalls?

Sometimes people think they want powerwalls "to fill up my EV" but thats not really very efficient use of energy. Using powerwalls to fill up a tesla ( for example ) is like saying you want to fill up a D battery by using an AA battery. Doable, but not something you particularly plan to do.

You are probably not looking at an "inverter upgrade", but an entirely new PV system, complete with its own inverter. Most installers wont touch the previous installation, unless they specifically were the ones that did it. Better to plan for an entirely new PV system (with its own inverter). It all can still offset your usage.

EDIT: I see @miimura already said that last part. Sorry for repeating it.

 

[scarecrow7]

Good, informed, thoughtful responses. Thank you!

The driver for Powerwalls isn't load/time offsetting (or whatever the correct term is), as that's not currently a factor here. In fact, I'd previously viewed having no Powerwalls in the initial solar install as a no-brainer. Local power outages are rare, and should a prolonged outage occur for whatever reason, I have a smallish propane backup generator and a transfer switch -- so outlets in living areas AND non-stove kitchen appliances AND sump pumps AND the basement freezer AND the (propane) furnace can be run for a considerable time. With a wood stove, an older farmhouse with windows, French doors and a few ceiling fans allowing good ventilation on warm days, then our only Achilles heel would be the ten days or so in the summer with 95+ temperatures. This is our thirtieth summer here, so we thought we had a pretty good sense for the climate

What's changed my mind is the events of the last couple of years:

• thirty-year high temperature was exceeded...by over seven degrees. In June.
• eighteen 95+ degree days already this year (early July), when our typical annual high temperatures are in the month starting in mid-July
• a wildfire last September way too close for comfort, while much of the county's fire equipment was nearly overwhelmed elsewhere
• murmurs of rolling blackouts at some point, maybe

So I'm readdressing my Powerwall conclusion.

What I don't want/need is seamless peak power forever. Solar-wise, what I'm after is around 100% of my typical annual use, year-over-year, but dipping into the grid on dark days, midwinter days, high-AC-use days and the like

But what I would like a couple of Powerwalls to supplement the capabilities of my existing backup (as described above) is:

• ability of the air conditioner to run at least a couple of zones (LR, master BR) with AC if the outage is on a miserably hot week
• charging a couple of sub-panels so the greenhouse fans run, and power to the detached apartment (currently uncovered by backup)
• and, electric irrigation pumps. My current fire suppression system (besides calling 911) is a gasoline-powered pump and 300 feet of 1.5" fire hose -- drop the dipper in the creek or either of the irrigation canals and I can wet down a single 300-foot radius area in a hurry. But, this summer I'm adding for a total of four 1.5" impulse sprinklers that run off my electric irrigation pumps. which can comprehensively and simultaneously wet down approaches in three directions -- including my 1895 barn, my propane tank...as long as the power stays on. That means energizing the barn sub-panel and the detached apartment sub-panel, neither of which are on the current limited transfer switch as described above

Finally, as regards ICE transportation and farm equipment: not much. We drive fairly little, and there are no long daily commutes. When my 1995 4Runner finally dies, I might replace it with a 2025 Ford F150 Lightning, but even then it will be a few hundred miles a month, if that. The tractor is a 1980s John Deere that I've maintained and it just keeps going; likewise, the Gator (from the years before they branded them as Gators). It's the elderly Craftsman riding mower that will bail on me sometime this decade, I think, as well as in a decade or so replacing my wife's ride with a smallish electric SUV

So, two fundamental issues:

1. more solar panels, gorund-mounted, to bring peak output from 6.84kw to more like 14-16 kw
2. probably 2 Powerwalls to allow the augmented backup as outlined above. Or three?
3. assuming I use the same contractor as for the initial system -- what would the new system look like?

Thank you!

--scarecrow

 

[Southpasfan]

If you add (1) how many kwh your 6.83 system produces in a year, and (2) how much you pay per kwh its easy to estimate one of two things:

(a) is the proposed upgrade, with or without PWs, cost effective?

or

(b) is it a question, not a saving money, but getting the intagibles.

 

[astrorob]

two PWs might be OK but at least with the current PW (not sure about PW+) each can only supply 5KW of load. if you think the AC(s) will pull more than ~8kw together then you might consider having 3 PWs.

 

[jjrandorin]

The OP mentions both AC and irrigation pumps, so one of the "what might I not know that I dont know" things is, pumps / motors can have high start up loads that are different from their running loads, so OP would really need to ensure that whoever they go with on this has a good handle on what the startup loads for that equipment is if it is to be backed up.

Its possible that OP could only need 2 powerwalls for regular usage, but due to wanting to start up the AC and those pumps, could need 3 or even 4. I dont know, but its definitely something to investigate.

 

[BGbreeder]

This is from the perspective of someone who has had wildfires burn to the property line in four of the last six years, and a major fire burned to within a mile last year. So, yes, five out of the last six years, I have had the fire pump out wetting down the surroundings. We had basically no solar generation for several days last year when the fire was the closest, which is also when you want the fire protection the most.

I think it is fantastic that you have access to enormous water supplies for fire suppression.

Irrigation pumps move lots of water, so they are a high kWh item. Add in that @scarecrow7 wants this to work during fires, would lead me down the path of a either a standby generator with a buried tank (against fire) for fire suppression, or an engine driven irrigation pump. (Probably also with a flame resistant pump and generator house.) It is a lot of power that would be needed for potentially days depending on the fire conditions. I don't really see that solar plus batteries is a reasonable solution for that part of the need.

The rest of the house loads could be addressed by a suitably sized solar system and powerwalls, with possibly adding in a minisplit AC system that could provide off grid cooling to a small part of @scarecrow7's home during an outage. @scarecrow7 wrote that 6.84kW of solar covered 40% of the needs, which implies 17kW+ of solar needed before adding any more loads, with four Powerwalls to keep up with the solar charging. (17kW/5kW/Powerwall rounds up to four.)

I think the big decision is whether to cover the fire suppression system with solar plus batteries. I think it gets really expensive.

All the best,

BG

 

[scarecrow7]

Thanks, all. Really appreciate it.

So I'm leaning towards:

• see what original contractor has to say about supplemental vs independent system
• adding enough ground-mounted panels to achieve rough parity on a year-over-year basis consumption vs production
• two Powerwalls which are wired to back up whole-house supply, including subpanels

In case of outage in summer/AC months:

• existing manual transfer switch to backup propane generator for kitchen, living area plugs, router, furnace blower, sump pumps
• trip off main box breakers to unused areas
• which leaves Powerwalls (and linked solar) running only AC to a zone or two, and whatever else I choose to leave on

In case of needing irrigation pumps during summer outage for irrigation or fire suppression:

• it's only two pumps. IIRC, they're no more that 2 HP apiece, which means steady consumption of no more than around 3 kW. Even with the surge in turning them on (is that hysteresis? I don't remember. Have mercy on a biochemist), they won't be cycling on/off, and I won't be turning them on simultaneously as they're maybe 250 feet apart. In a fire situation I'll be supplementing these two pumps with my gasoline-powered fire hose, and whatever I can get out of my city water line. Then we leave, not wanting to do an Aldo Leopold.

In case of winter outage:

• rely as we can on the Powerwall/solar, switching to backup propane generator if PW drops below minimums. Wood heat & propane furnace.

Any critical mistakes here?

Thanks again for the informed replies. Appreciate your time

--scarecrow

 

[Will792]

two PWs might be OK but at least with the current PW (not sure about PW+) each can only supply 5KW of load. if you think the AC(s) will pull more than ~8kw together then you might consider having 3 PWs.

PW limit for charging is 5KW, so for 2 it would be 10. The part that is unknown is PWs ability to throttle SE inverters, if PV current gets close to the charging limit of PW. We do know that PW increases frequency if micro grid once battery is almost fully charged. Inverters usually have installable profiles with 2 key AC frequency points. One is for fast disconnect and one is for slow. Many inverters reduce output power once frequency reaches slow disconnect level.

You can figure out demands of the well pumps by checking size of circuit breaker. A pump is inductive load so it does need plenty of current, when it starts. It is substantially less if you have constant water pressure controller. Dividing breaker size by 1.25 will give rough startup current. Most pump list startup power requirements in the manual since sometimes pumps are powered by backup generators.

Needs of AC startup current also depend on compressor type. Variable level cormpressors have substantially lower startup current.

Do not overstate your ability to modulate power consumption, in case of blackout. If some extra consumption kicks in, based on human mistake or device automatic activation, PowerWalls will shut down. After you restart the system you will to orchestrate turning devices one at a time. It is always better to have healthy margins in power backup system.

in my case I calculated size of solar system by extrapolating previous house electricity consumption, converting prior heating oil consumption into heating pump equivalent and electric cars consumption using mileage estimate and 0.27KWh per mile efficiency. I was aiming to have 95% coverage. My electric company pays $0.03 for 1KWh of unused excess of electric production. I installed system myself and had access to wholesale pricing for panels and microinverters. Even with valuing my time at zero the amortized cost of electricity from my system is around $0.032 per KWh. My point is that economic considerations are against oversizing PV system.

 

[BGbreeder]

Thanks, all. Really appreciate it.

So I'm leaning towards:

• see what original contractor has to say about supplemental vs independent system
• adding enough ground-mounted panels to achieve rough parity on a year-over-year basis consumption vs production
• two Powerwalls which are wired to back up whole-house supply, including subpanels

In case of outage in summer/AC months:

• existing manual transfer switch to backup propane generator for kitchen, living area plugs, router, furnace blower, sump pumps
• trip off main box breakers to unused areas
• which leaves Powerwalls (and linked solar) running only AC to a zone or two, and whatever else I choose to leave on

In case of needing irrigation pumps during summer outage for irrigation or fire suppression:

• it's only two pumps. IIRC, they're no more that 2 HP apiece, which means steady consumption of no more than around 3 kW. Even with the surge in turning them on (is that hysteresis? I don't remember. Have mercy on a biochemist), they won't be cycling on/off, and I won't be turning them on simultaneously as they're maybe 250 feet apart. In a fire situation I'll be supplementing these two pumps with my gasoline-powered fire hose, and whatever I can get out of my city water line. Then we leave, not wanting to do an Aldo Leopold.

In case of winter outage:

• rely as we can on the Powerwall/solar, switching to backup propane generator if PW drops below minimums. Wood heat & propane furnace.

Any critical mistakes here?

Thanks again for the informed replies. Appreciate your time

--scarecrow

That seems like a good power plan overall. I would be tempted to have a separate generator(s) for your irrigation pumps. In case of fire without power, you could start the irrigation generator and leave the irrigation pumps to do their thing while you do additional preventative wetting with your gasoline powered fire pump, or even install hardline for your fire pump.

I am of the belief that fire prep, e.g. irrigation prior to fire onset, should be as automatic as possible, I.e. flip a switch, start a pump, and have all of your sprinklers plumbed in so you can take care of packing/fire hardening the house by cleaning gutters one last time and moving anything flammable more than 30' away from your house, or whatever it is that you need to do. I think it is human nature to have deck chairs near the house, a dog kennel, a propane BBQ near the house, and all of it needs to get moved away, plus you need to pack important documents, keepsakes, etc. It is a lot to do in a short amount of time, and I would be thinking now about how to do as much of that today so it doesn't have to be done in the hours before you evacuate, like putting in pipe for fire sprinklers.

All the best,

BG