a NG here. be gentle.

[Fulthrotl]

i'm looking at putting an array up on the roof, 5KW range, with battery to give 24 hours reserve....
i'm doing a new roof on the house, and as they say, now is the time. i'll be upgrading to 200 amps
for the service before the roof goes on.

i'm looking for suggestions on a system. i'm browsing thru enphase, and looking for a microinverter system.
i'm put off by tesla. it's sexy, and looks way too overpriced for what you get. i already buy apple products.
it doesn't seem there is a path to get to tesla wholesale. i seems i can't just sign up with a license number,
and get there.

and, as i'm a C-10, i'm not looking for installation. that being said, if y'all were putting a good system on your
roof, what would you be using?

thanks for any help.

 

[wwhitney]

The battery part is the less mature market, with more limited options, and greater costs. So I suggest figuring that out first (not that I'm up the merits of Tesla vs Enphase vs Sonnen vs LG vs Generac vs whatever else I've forgotten). A couple basic questions:

1) 24 hour reserve--how many kWh is that? You have to look at your usage and make a design decision. A related question is battery inverter capacity, which will determine how large a load you can run from the batteries.

2) Is the battery strictly for backup/generation during an extended power outage? Or are you also interested in using it on a daily basis for either load shifting or generation shifting, in conjunction with a time of use rate plan?

Cheers, Wayne

 

[Fulthrotl]

The battery part is the less mature market, with more limited options, and greater costs. So I suggest figuring that out first (not that I'm up the merits of Tesla vs Enphase vs Sonnen vs LG vs Generac vs whatever else I've forgotten). A couple basic questions:

1) 24 hour reserve--how many kWh is that? You have to look at your usage and make a design decision. A related question is battery inverter capacity, which will determine how large a load you can run from the batteries.

2) Is the battery strictly for backup/generation during an extended power outage? Or are you also interested in using it on a daily basis for either load shifting or generation shifting, in conjunction with a time of use rate plan?

Cheers, Wayne

the enphase estimator says 28 KWH/day. i've not done a load survey to see what it actually runs,
but there is gonna be a 5 ton AC added shortly, there is an intelliflow pool pump, and two refers,
and a miele induction cooktop and electric range. and 60 8watt can lights.

i'd like it big enough to roll whole house, with any make up charging done off peak, but this
may get 'spensive, as ricky said to lucy.

 

[wwhitney]

the enphase estimator says 28 KWH/day

You can should be able to get historical daily usage data from your power company (I can at pge.com); if you not you could use your average daily usage from your monthly power bill and multiply by a factor like 1.2 or 1.3 to account for day to day variation (that's a guess on the proper factor).

but there is gonna be a 5 ton AC added shortly, there is an intelliflow pool pump, and two refers,
and a miele induction cooktop and electric range. and 60 8watt can lights.

A generator type load calc will tell you the battery inverter power you need. Starting current for a large motor (AC compressor) can require a lot of otherwise unused inverter capacity, so it may (likely will) be worth it to get an inverter type AC unit.

Cheers, Wayne

 

[Fulthrotl]

You can should be able to get historical daily usage data from your power company (I can at pge.com); if you not you could use your average daily usage from your monthly power bill and multiply by a factor like 1.2 or 1.3 to account for day to day variation (that's a guess on the proper factor).


A generator type load calc will tell you the battery inverter power you need. Starting current for a large motor (AC compressor) can require a lot of otherwise unused inverter capacity, so it may (likely will) be worth it to get an inverter type AC unit.

Cheers, Wayne

i've not selected that yet, so thanks. i'll see where that goes.

 

[jaggedben]

Tesla is not overpriced. It may even still be underpriced. And no, you cannot get it without being certified so you're probably better off finding someone else to do it for you if you go thatway.

Enphase also requires certification, but it's probably easier to get by simply signing up and taking the online training. I like the Enphase product over Tesla for several reasons (fewer weak points for total system failure, more modular, safer chemistry) but I can't argue that price is one of them.

Regardless, it is fairly quixotic to try to back up a 5-ton AC with today's batteries, unless money is no object. It's the locked rotor amps you need to worry about, Wayne is absolutely correct about getting an inverter type. I think you may still want a generator for that, much as I hate to say it as a solar guy. It is possible, or will be soon, to integrate a generator into either Tesla or Enphase's setup. Should be easier with Enphase but not quite released yet.

 

[jaggedben]

BTW I have Enphase on my roof and highly recommend it over anything else for the size you're considering.

 

[wwhitney]

Regardless, it is fairly quixotic to try to back up a 5-ton AC with today's batteries, unless money is no object.

Why is that? If it's an inverter type compressor, the required battery inverter power should be feasible. For example, from a 2013 spec sheet on a Lennox XP25-060 (5 tons), the LRA is listed as 29A, and the MCA is 35A. So I think it only requires 7 kVA of inverter power, and (2) Powerwalls should be to run it no problem (depending on other loads).

but there is gonna be a 5 ton AC added shortly

How'd you come up with 5 tons? Did you run a Manual J and have you taken the appropriate envelope tightening steps to reduce the heat gain? 5 tons seems very large. Also, I'd suggest considering a heat pump.

Cheers, Wayne

 

[Fulthrotl]

BTW I have Enphase on my roof and highly recommend it over anything else for the size you're considering.

that... was the information i was looking for..... thanks.

truth be told, running the AC off grid isn't mandatory, and i know that.
enphase was saying 30.2 kwh to do that, and reality is, most likely 10kwh is all i'd probably spend for initially.
i could add a second one later if need be.
the biggest draw i've got is the induction cooktop.

they have a 26 panel 7.8kw package for $6,700 with IQ7. dunno what that would go for wholesale,
but i'd need a controller, and a 10kw storage, and racking, etc. so i was trying to get a ballpark figure on
this to see if it was even feasable.

i'm at the bar napkin stage of the project, that is why i asked y'all to be gentle. ;-)
i dunno what i don't know, yet, but i'm a fast learner.

as for the A/C sizing, a buddy said he could slide me into 5 tons with a new FAU and decent
energy ratings for about $7k ish, with labor. that was just based on single story, 1,700 sq ft.

bar napkin calculation said 4 tons would be good, but 5 wouldn't be much more, and would
allow it to cycle more quickly.

 

[Todd0x1]

bar napkin calculation said 4 tons would be good, but 5 wouldn't be much more, and would
allow it to cycle more quickly.

I'm just starting to learn A/C but I thought the goal was to not have it cycle quickly as not enough humidity is removed when it doesn't run long enough.

On the solar / battery backup front, I too have a home project coming up and am looking at the Sol-Ark inverters.

Home

Sol-Ark® provides solar energy storage systems with our battery-agnostic hybrid inverters for commercial and home applications. Learn more.

www.sol-ark.com

 

[ptonsparky]

I know nothing about the Solar, but I do know oversizing a typical A/C is not suggested, as Todd stated.

I would hazard a guess that newer inverter style units changes things.

 

[GoldDigger]

I know nothing about the Solar, but I do know oversizing a typical A/C is not suggested, as Todd stated.

I would hazard a guess that newer inverter style units changes things.

I think you are correct to the extent that the inverter driven compressors can be run at a lower speed when less cooling is needed, allowing the equivalent of multi-stage operation.

Sent from my Pixel 4a using Tapatalk

 

[gadfly56]

Is there a reason you don't want to do a standard backup genny for this purpose? Or are you just geeking out? If you have city natural gas you could hook up a 20-22kW genny for the whole house and the money you'd save over the solar installation would let you run it for about a year. I did the calculation about a year or so ago either here or at another blog. Now, subsidies, tax credits and what all may change the economics, but on straight up numbers, that's my story and I'm sticking to it.

 

[jaggedben]

Why is that? If it's an inverter type compressor, the required battery inverter power should be feasible. For example, from a 2013 spec sheet on a Lennox XP25-060 (5 tons), the LRA is listed as 29A, and the MCA is 35A. So I think it only requires 7 kVA of inverter power, and (2) Powerwalls should be to run it no problem (depending on other loads).

Cheers, Wayne

Yeah, perhaps my phrasing was a bit strong, but the point was to urge anyone reading this thread to run through the numbers and check off the boxes at least twice before moving ahead with such a thing. It's critical that none of the ducks you've mentioned are out of their row. And the other duck you haven't mentioned is how quickly such an A/C will draw down your battery charge, leaving you without power for other, perhaps more critical, items. Running A/C the way many people normally do could mean having just 2-3 hours on 2 Powerwalls when the rest of the loads might last days. This is probably less of an educational problem with electricians than with the average homeowner, but the point still stands.

 

[wwhitney]

And the other duck you haven't mentioned is how quickly such an A/C will draw down your battery charge, leaving you without power for other, perhaps more critical, items.

As you say, cooling can require a lot of kWhs and really impact battery sizing.

But the primary variable is the amount of cooling you use (BTUs you want removed from your house), not the size of the A/C unit itself. Barring extreme cases like severe short cycling, the efficiency should be pretty comparable. So if you have the inverter capacity to start and run a 5 ton unit, and run it 30 minutes, that should use about the same energy as running a 1 ton unit for 2.5 hours, and provide the same BTU removal. (For comparable SEERs).

Cheers, Wayne

 

[jaggedben]

Is there a reason you don't want to do a standard backup genny for this purpose? Or are you just geeking out? If you have city natural gas you could hook up a 20-22kW genny for the whole house and the money you'd save over the solar installation would let you run it for about a year. I did the calculation about a year or so ago either here or at another blog. Now, subsidies, tax credits and what all may change the economics, but on straight up numbers, that's my story and I'm sticking to it.

Where you said solar it should say battery. The economics of the solar is in a different category.

 

[jaggedben]

As you say, cooling can require a lot of kWhs and really impact battery sizing.

But the primary variable is the amount of cooling you use (BTUs you want removed from your house), not the size of the A/C unit itself. Barring extreme cases like severe short cycling, the efficiency should be pretty comparable. So if you have the inverter capacity to start and run a 5 ton unit, and run it 30 minutes, that should use about the same energy as running a 1 ton unit for 2.5 hours, and provide the same BTU removal. (For comparable SEERs).

Cheers, Wayne

True, as long as we don't lose track of assuming an inverter type A/C drive, which is fine if you're choosing a new one but not if it means replacing an existing one. Also, people tend to have larger units because they want more cooling.

 

[Fulthrotl]

i was chatting with my wholesale house, and he mentioned the premise that a roof, racking, and conventional PV's
is about the same as a tesla roof, and the tesla roof is better looking, yada yada.... but a tesla roof won't work on
my house. i can pretty easily fit 20 300 watt panels on my roof, and they won't particularly be obnoxious. a 2X10 array
on the bottom of the photo. and i'm not in love with running the AC standalone..... it's a lot of oomph.

 

[BandGap1.1eV]

I installed a system at my house this past Spring. ~7.5 kWdc / 7.6 kWac SolarEdge system with DC coupled StorEdge inverter. The DC coupled nature of the StorEdge and Solark is a great feature as you only have one grid connected inverter. The Enphase and Tesla systems use dedicated AC coupled batteries if I recall correctly. This makes interconnecting to your house and the grid a bit trickier since both devices can independently dump power to the loads. In our area (National Grid) there is a 10 kW limit on net metered grid tied systems. You could easily surpass this limit with AC coupled solar+storage and the utility could hit you with a price tag of a new transformer at the street.

DC coupled systems also have the inherent benefit of DC clipping recapture, meaning you can safely oversize the PV and have any normally clipped power from the array get dumped into the batteries rather than being curtailed when the inverter reaches nameplate.

Some residential scale energy storage systems are limited to 5 kW of output power meaning you cannot reasonably back up your whole house (an electric dryer is a 5 kW load, for scale). SolarEdge recently rolled out a system called EnergyHub, which essentially is a "smart" transfer switch that gets installed between your utility meter and your main loads center. This 200A rated device allows you to safely island the whole house with a set of StorEdge inverters and up to two batteries (9.8 kWh each) per inverter.

Adding a significant amount of energy storage will raise an eyebrow from a savy local wiring inspector. There are sections in the International Fire Code that will require a 2 hour rated fire room, sprinklered, with a fire alarm and monitoring to a central dispatch. In my research, there seemed to be a threshold at 20 kWh under which these strict and expensive requirements were not not mandated.

Lastly, residential energy storage does not make much financial sense unless 1.) there is an active incentive plan in your area by the utility or the state, and 2.) you lose power once or more per month. The storage equipment for my system was ~$12k.

 

[wwhitney]

DC coupled systems also have the inherent benefit of DC clipping recapture, meaning you can safely oversize the PV and have any normally clipped power from the array get dumped into the batteries rather than being curtailed when the inverter reaches nameplate.

Interesting, although clipping even with a 1.5 DC/AC ratio is to my understanding not a large effect. In the StorEdge version, does the central inverter arrange the string optimizer voltage to match the battery charging voltage exactly, so that excess current can go directly to the battery, or is there a separate DC/DC stage for battery charging? In the latter case, there would presumably be some DC/DC battery charging capacity that you could also simultaneously saturate and cause clipping.

Adding a significant amount of energy storage will raise an eyebrow from a savy local wiring inspector. There are sections in the International Fire Code that will require a 2 hour rated fire room, sprinklered, with a fire alarm and monitoring to a central dispatch. In my research, there seemed to be a threshold at 20 kWh under which these strict and expensive requirements were not not mandated.

My understanding is that buildings subject to the IRC are exempt from those sections of the IFC, and the IRC has its own, less onerous requirements. But some jurisdictions around here are acting as if they have amended away the IRC exemption (and if they have not actually does so, are acting illegally, in my opinion).

Cheers, Wayne