grid tie inverter recomendations

[lordulrich]

I'm getting started in the planning of a ground mount solar system for my place. I've done design of off grid before but this is a bit different and is my first personal project. Most of the resource seem to want to sell me something so hard to tell what to trust and I've got slightly different needs than most discission I've seen.

I'm looking to do grid tie, total system of probably 5-10 kw, but can go to 20kw, final size will be driven by finances. I'm in the use with 120/240 split phase.

I'm not wanting microinverters as much limits and connection fees are based on total inverter nameplate so it seems to make better money sense to have a few string inverters run with max panels.

I'm not really interested in batteries, ability to use batteries might be nice if it extends my production by saving some peek sun, but I don't see it working out from a cost benefit perspective, at least up front. I don't need backup as my grid is very reliable and I do have a portable generator if I need it, also batteries have extra permitting where I am.

this will be ground mount so need something weather resistant but most of the ul listed units seem to be.

And obviously I'd need something I can actually buy...while I'm 100% legal to do this work it doesn't mean people will sell to me...like the local supply houses ...

thanks

 

[12VoltInstalls]

Are you looking to merely shave the daily Wh figure?
Or “sell” to the grid?

I'm not really interested in batteries, ability to use batteries might be nice if it extends my production by saving some peek sun

I’d do at least one battery- at today’s prices it’s not terrible, either. The battery will act as a buffer/capacitor for moment by moment variations in sun, use/demand, and store for a bit of sundown savings.

total system of probably 5-10 kw

5kW is a broader selection of usable products at all price points. 10kW has just as many options but a lot less items that you should consider imho.
5-10kW is a bit of a range-spread; imho, at the current excellent products offered. 5kW and 10kW are nearly not related to one another except perhaps at the top-shelf tier of things. My opinionated observations.

 

[DIYrich]

Inverter can be mounted inside. Cheapest is Growatt, but batteries are high voltage. Eg4 18kpv or sol-ark work well with the generator and give you cheap battery options. Even 5KWh of batteries can help smooth things out.

Build a solar shed for the ground mount may simplify the permit requirements for the battery.

 

[Hedges]

Does your utility offer good net-metering plan?
What is your cost to buy per kWh?

I use SMA, Sunny Boy. Now discontinued, but their Sunny Boy Smart Energy allows optional battery for time-shifting. In the future it will support battery backup (but limited surge capability.)

 

[lordulrich]

I'd like to do as much as possible, and export to "sell" and in a perfect world would be at 20kw (max for residential with my power company) but site constraints and budget don't allow for that.

Costs for battery system is less about the actual battery than the other costs and permitting issues. Battery systems require separate permitting and I don't want to deal with that right now, keeping the option open is ok but I'd like to keep it out of the scope for initial build. Also the way my property is wired using solar for backup will require significant extra work.

I buy and sell both at $0.1/kwh, but have a straight monthly fee based on solar inverter ratings, calculating that value is a bit complicated. taxes make it a little better to consume than export and buy back but just a few percent and I'm not worried about those few percent.

 

[12VoltInstalls]

So you want to ‘sell back’ or are going to?
$0.10 is pretty cheap electricity so ROI ain’t fantastic though.

What jurisdiction?
I’m still thinking some battery is a good idea.

Refine your goals a bit and then those parameters will help get more specific suggestions on equipment.

 

[zanydroid]

I’m still thinking some battery is a good idea.

What is your math to support this? Feels to me like buying one UL9540 system upfront is a strict loser on cost only terms when the buy/sell is $.10

IMO it’s well worth planning ahead for enough space to put in the battery stack since the positioning is heavily constrained by code, and it sucks to have to redo all the wiring and move inverters, to satisfy this down the line.

 

[zanydroid]

Also the way my property is wired using solar for backup will require significant extra work.

Can you elaborate on this? Because I can’t really visualize how this arises. As a strawman I think you can use 200A bypass after main disconnect or combo main with (disconnect case) the addition of a few conductors leading to bypass and hybrid inverter; (combo main case) addition of new subpanel and moving of all circuits to it. These mods are identical for majority of 200A houses and below, unless there is a problem with equipment clearance or maybe multiple service entries.

 

[12VoltInstalls]

What is your math to support this?

Ha

As I noted and you also: $0.10/kWh is cheap. Though not always practical to plan in dollars.
It wasn’t a frugality math statement, mostly- the math doesn’t add up at ten cents.

I suggested a battery was a good idea from a performance perspective: it’s a great neo-capacitor and smooths out the “partly sunny” daytimes.

 

[zanydroid]

Ha

As I noted and you also: $0.10/kWh is cheap. Though not always practical to plan in dollars.
It wasn’t a frugality math statement, mostly- the math doesn’t add up at ten cents.

I suggested a battery was a good idea from a performance perspective: it’s a great neo-capacitor and smooths out the “partly sunny” daytimes.

Sure, I think it's socially more polite to get batteries so as to not tax my neighbors (quite as much) with my net metering (I'm on $0.40-0.60/kWh here on the left coast so batteries are very close to making sense DIY). But I also don't want to feed money to the early UL9540 products. Can't feed the regulatory capture trolls.

And if anything I could use the batteries to abuse net metering even more :laugh:

 

[lordulrich]

Can you elaborate on this?

My property is a old farm site. The main panel is a a power pole with a yard light. From there I have a series of subs at various buildings or near former buildings. All of my loads are at my home or shop, the location that works for solars is at a former building 400 ft away from where there are actually loads. I do have some loads at the old building site but small loads like a trickle charger. trenching new lines just is too expensive to make sense.

A bit of a weird setup but I'd want to feed at the sub and then feed back to the main. From my reading of code as long as the breaker from the sub to the main is secured and labeled it is compliant.

 

[zanydroid]

My property is a old farm site. The main panel is a a power pole with a yard light. From there I have a series of subs at various buildings or near former buildings. All of my loads are at my home or shop, the location that works for solars is at a former building 400 ft away from where there are actually loads. I do have some loads at the old building site but small loads like a trickle charger. trenching new lines just is too expensive to make sense.

A bit of a weird setup but I'd want to feed at the sub and then feed back to the main. From my reading of code as long as the breaker from the sub to the main is secured and labeled it is compliant.

Ah ok, yeah you've definitely thought this through.

Couple of weird ideas, which sort of point at there being a fundamental tradeoff between whether you put the inverter 400ft away, and have more MPPTs for more flexibility, vs putting the inverter 0ft away from where you want to backfeed and have easier access to the advanced inverter features.

• If you can get away with a single MPPT, you can send DC string from the "former building" (which presumably has no loads) by repurposing the AC run. It has to be a single MPPT since you don't have enough wires in 4-wire service to do two strings. This allows you to put the inverter next to the place you were originally going to backfeed. You don't need to retrench but you will need to place subpanel with terminal blocks.
  • Note: some equipment can be configured to let you lock two MPPTs together, so that the voltage point is locked but the power is distributed across the transistors from the two MPPTs.
  • There may be some pitfalls with the cable type you're using underground not being approved for solar. I'm not 100% familiar with the code here.
  • You would get lower loss by sending the solar at 450VDC than the inverter output at 240VDC
• Some equipment allows you to use a remote transfer switch to grid form. However, if you put the inverter 400ft away from the transfer switch, you'll still have to run control cable.
• Another issue with 400ft distance is that you will have a tough time adding zero export CTs, etc. Because all that will have to go 400ft. Even with digital communications CTs 400 ft is quite a distance, and you'll have to either trench for twisted pair or go against installation guide and use wireless adapters for RS485. 400 ft is long for the cheap wireless.
• One way to try to get the best of both worlds is to install a grid tie inverter 400ft away to start with. And if you want advanced features later, install a hybrid AC couple-capable inverter 0ft away. This way you can sort of have the best of both worlds - as many MPPT/different DC layout you want 400 ft away, while having an easy time using some fancier inverter features via the hybrid. At present this requires a more expensive inverter (hybrids with this capability are $$$, but I fully expect the prices to go down), and AC coupling is less mature in DIY space than DC coupling (but AC coupling I believe is very well understood in utility scale & at some of the inverter companies like SMA so it's arguably a "us DIYers are temporarily under-educated problem").

You don't necessarily need to use a hold-down on the inverter feed, since grid tie inverters are interactive (IE turn themselves off in a fault). Within 5-10 seconds after an unsecured breaker flies off the busbar, the AC will turn off. If you use hold-down and terminal covers then this 5-10 second window of frying yourself goes away. However, hold-down and terminal covers are not available for all breakers sizes.

I don't think you'll have a huge problem finding someone to sell you stuff. Only thing I can think of off-hand that's not "virtually walk into a store and buy it" is if you want, say, dcbel or other early access hardware. Which nobody on this forum uses.

 

[zanydroid]

I'm not wanting microinverters as much limits and connection fees are based on total inverter nameplate so it seems to make better money sense to have a few string inverters run with max panels.

If you set up export limit, is it still total inverter nameplate? Context of asking the question is to better understand the parameter space you're working with.

Most useful situation this comes up for is with hybrid, though potentially you might have an easier time maxing out your desired export target with export limit rather than making AC nameplate add up.

(There are several hybrid setups where the MPPT output kW is greater than inverter kW. In this case, the surplus energy is made available to both inverter and charging. So let's say you get the 18kpv with 18kW MPPT and 12kW-AC inverter. Then during the solar peak you can push out 10-12kW to the grid while using 8kW to charge the battery)

(It's still generally smarter to use string inverters with ground mount if you are willing to do the math & save money, instead of monkeying in microinverters to avoid having to do math)

 

[Hedges]

My property is a old farm site. The main panel is a a power pole with a yard light. From there I have a series of subs at various buildings or near former buildings. All of my loads are at my home or shop, the location that works for solars is at a former building 400 ft away from where there are actually loads. I do have some loads at the old building site but small loads like a trickle charger. trenching new lines just is too expensive to make sense.

A bit of a weird setup but I'd want to feed at the sub and then feed back to the main. From my reading of code as long as the breaker from the sub to the main is secured and labeled it is compliant.

You can put a battery inverter closer to utility service entrance, AC coupled to GT PV inverter downstream where there is good sun.
This would let you focus on net metering now, consider adding battery backup later.
Almost any Rule-21 frequency-watts inverter should work for that. "Should", it seems Enphase IQ8 has difficulty AC coupling with other brand battery inverters, but they have their own.

 

[zanydroid]

I wouldn’t be surprised if buying a GTI in the 400 ft location now and then a future (after the market matures), lower cost AC coupled hybrid if batteries are desired, would end up being way ahead in overall cost.

 

[Danke]

I'm getting started in the planning of a ground mount solar system for my place. I've done design of off grid before but this is a bit different and is my first personal project. Most of the resource seem to want to sell me something so hard to tell what to trust and I've got slightly different needs than most discission I've seen.

I'm looking to do grid tie, total system of probably 5-10 kw, but can go to 20kw, final size will be driven by finances. I'm in the use with 120/240 split phase.

I'm not wanting microinverters as much limits and connection fees are based on total inverter nameplate so it seems to make better money sense to have a few string inverters run with max panels.

I'm not really interested in batteries, ability to use batteries might be nice if it extends my production by saving some peek sun, but I don't see it working out from a cost benefit perspective, at least up front. I don't need backup as my grid is very reliable and I do have a portable generator if I need it, also batteries have extra permitting where I am.

this will be ground mount so need something weather resistant but most of the ul listed units seem to be.

And obviously I'd need something I can actually buy...while I'm 100% legal to do this work it doesn't mean people will sell to me...like the local supply houses ...

thanks

I know a guy who sets up systems to sell back in Minnesota. Basically you can use cheaper microinverters undersized for the panels. That way you can sell more electricity back to the grid with the limits the utility puts on AC. What is the name of your utility? I’m in Minnesota and usually the limit is 40k. 20k limit is for the REC (?) credit.

 

[lordulrich]

ok so a bunch of things to answer here:

The old building sub still has a few circuits I want to keep live, outlets that get used just not sizable loads but it gets used for things like block heaters on equipment. I'd like to just put my inverters there and run ac back to the main. I'm not worried about zero export.

The way I read the rules my permit is based on the ac nameplate of the equipment regardless of how it is configured. I'd like a string inverter and the math is not something that worries me. I'm an engineer in my day job and have done electrical work more complicated than this before.

The power coop doesn't have a hard limit at 20kw, but 20kw allows an abbreviated permit process under 20kw, I could do larger but my site would make an array larger than 20kw hard to fit and not with the extra permit paperwork.

 

[zanydroid]

Ok. I think running AC back is fine. Mostly wanted to spell out all the options.

Is the feeder big enough?

The thing with the math is that some large scale projects would rather use more costly self adjusting hardware like microinverters than to do the math and engineering. IMO if someone has the time then they can do this themselves to save some money.

 

[Scrugs]

Where is the common application /use for micro inverters? pros / cons vrs. centralized inverter ? Would appreciate short response or resources to read. thank you

 

[zanydroid]

I wish I had an article to share lol.

Microinverters are way easier to install and design but they cost quite a bit more than string even when factoring in the extra cost of Rapid Shutdown on strings.

AC wiring is less labor and balance of system stuff than DC wiring. Though my turnkey installer installed the AC on my roof to DC standards ?

Hybrid inverter with DC string and battery inverter is the easiest way to add storage since you don’t have to worry about AC coupling.

Microinverters are the most vendor neutral way to handle crazy shading and small roof planes, which would wreck strings. Optimizers are too niche IMO. Small roof plane is a reasonable thing to install on with ROI. Crazy shading, debatable.

The reason turnkey installers like microinverters is because they’re fast, easy, and make more money for them.