Struggling with AC vs DC coupled systems

[zanydroid]

Personally, I like seeing panel level production details... provides early trouble warning, but enabling that on DC coupled system adds costs (cost already included with micro-inverter AC coupled approach).. BUT if installing DC power optimizer on panel, then the whole, 'get up onto roof' issue is now the same as with micro-inverter [joy of trade-offs].

The conundrum here is, if you get panel level monitoring on a DC system such as with TIGO, you suddenly get tempted to add optimizers based on the small cost delta (it’s rather costly to get the monitoring in place, vs basic RSD)

But that’s not free in terms of additions failure points.

Talk about scope creep

 

[Lawrence_SoCal]

The best ROI on solar happens with a system sized SMALL enough that it never sells power back. This way every kWh it produces is avoiding at your retail rate.

In my case, and probably many others, I sell a lot of kWh back during the shoulder months. Since those kWh at wholesale, from an ROI view point, that is wasted solar capacity.

Maybe from a general/simplistic perspective... but Umm.. I'd argue that 'it depends'.
And there can be positive ROI from giving away some solar during mid-day peak to produce more kW during later peak rate pricing hours... depending on rate structure

In my case, I got onto a favorable net metering plan (CAN NEM 2.0) that is (supposedly) locked in for 20 years. My system is a net producer every month of the year, so yea, way over produce for the year.
BUT, I did that knowing that adding more than 1kW solar production capability in future years would kick me out of existing tariff. So I planned for future EV & appliance electrification (lots of natural gas at the moment) loads. So for me doing a full 20 yr ROI calculation, over-provisioning now made the most sense in my case.

As for wasted solar, I'd say it depends on the local spread of sell vs buy kWh rates, and can be mitigated by use of a home battery
Or there are those with electrical appliances that use 'excess mid-day generation' to run (oversized) hot water heater, Air Conditioner to cool house (lower than would otherwise), run pool pump, etc.

 

[1201]

Not sure what you are saying here.

Grid tie with no intervening battery is most efficient. Which of your statements does this correspond to?

if he wants to grid tie without batteries a simple grid tie inverter will be most efficient. up to 98% efficient. a traditional dc coupled inverter, like a hybrid, is going to be less efficient.

If you have batteries, DC coupled is generally more efficient. And your strings can bypass the battery while inverting. AC coupling requires extra power conversion stage to charge, and it may leave some power on the table when running off-grid because of the more complex / flakier control loop compared to DC. BUT, we are talking about a primarily grid-tie system here.

my opinion is that with batteries a combination of ac coupled and dc coupled is most efficient. the AC coupled power can run the daytime loads directly at maximum efficiency, and dc coupled power can charge the batteries for later at much higher efficiency than AC coupled.

 

[zanydroid]

nt. a traditional dc coupled inverter, like a hybrid, is going to be less efficient.

I only see that if it does internal MPPT to 48V to inverter. That’s not how it works today except for like Victron SmartSolar which is a bunch of separates packaged together in a trenchcoat

the AC coupled power can run the daytime loads directly at maximum efficiency, and dc coupled power can charge the batteries for later at much higher efficiency than AC coupled.

But a hybrid with direct solar to inverter, which is standard today, can do all of that.

 

[Lawrence_SoCal]

The conundrum here is, if you get panel level monitoring on a DC system such as with TIGO, you suddenly get tempted to add optimizers based on the small cost delta (it’s rather costly to get the monitoring in place, vs basic RSD)

But that’s not free in terms of additions failure points.

Talk about scope creep

exactly... trade-offs not having panel level data has its own considerations (no free lunch, so to speak). However, I can't speak to what a homeowner can/should do when they don't have panel level production info? thermal camera? regular panel cleanings? I really don't know... maybe the trade-off is negligible, and don't worry (geek out) about it?

In my micro-inverter install, my installer failed to follow the permit layout on panels. As I had panel level monitoring, the material shade impact to production (panels installed on either side of chimney) was immediately apparent ... 21+ months later I still haven't paid a penny for my entire system (their breach of contract (numerous failures, including code, not just panel placement) for which they have yet to rectify, haven't heard from them in 9+ months, I consider the install abandoned at this point... they may be going out of business) ... granted a truly extreme scenario.. but in my case, that panel level detail was VERY valuable ;^)

 

[n2aws]

One thing I'll add..

FPL specifically has 3 tiers of solar.
at "25-30kw" you'd be in tier2. tier2 requires that you add $1,000,000 of liability insurance which is a monthly premium that eats into your ROI calculations/projections.
tier1 (0-10kw) doesn't require additional insurance
tier2 (10kw-50kw) requires 1m in liability
tier3 (50kw+) I don't know the requirements, cuz I've never looked into it due to never expecting to be anywhere near that production level

 

[zanydroid]

In my micro-inverter install, my installer failed to follow the permit layout on panels. As I had panel level monitoring, the material shade impact to production (panels installed on either side of chimney) was immediately apparent ... 21+ months later I still haven't paid a penny for my entire system (their breach of contract (numerous failures, including code, not just panel placement) for which they have yet to rectify, haven't heard from them in 9+ months, I consider the install abandoned at this point... they may be going out of business) ... granted a truly extreme scenario.. but in my case, that panel level detail was VERY valuable ;^)

Sounds a lot like my main use case of panel level monitoring… I had to use those myself bc installer wasn’t using it to detect the fact that they forgot to plug a few in, clearly no one quality checks the installs.

The spoiler compared to your experience is that I actually paid them despite still having problems to this day

The company going out of business is a best case scenario . You can probably pay pennies on the dollar to whoever scoops up the remains to settle the account

 

[OzSolar]

This is a very confusing statement, and it seems for a non-net metered or aggressively non-lucrative net metering terms

What do you exactly mean by selling power back?

I have 1:1 net metering, I produce way more in the summer than I can consume (on a daily/real-time basis) and send it out via my grid-tie only system (no batteries). (I'm in a heating dominated climate and I only use 5kWh/day on AC maybe 20 days out of the summer, the rest is 0kWh/day).

It's compensated at TOU retail rate. But on an annual basis I don't produce more than I can use, so everything is at retail rate. We have annnual true-up which sounds like what OP has.

Not sure how I could have said in an easier way to understand. I'm confused how you have solar and not know what selling power back means?

I was just reinforcing what the OP said. The don't have TOU, etc so all of that talk is just wasted on this thread.

My thought is to put up an AC system on the main roof that feeds into the panel to achieve NET 0. FPL only has two rates, you purchase at the retail rate and sell at the wholesale rate. I don't think it's worth producing extra power and selling it back at the wholesale rate. Extra capacity to sell at the wholesale rate would just lower my ROI and I would be better invested elsewhere

 

[1201]

I only see that if it does internal MPPT to 48V to inverter. That’s not how it works today except for like Victron SmartSolar which is a bunch of separates packaged together in a trenchcoat

fair enough - do aios turn off at night?

 

[wpns]

My thought is to put up an AC system on the main roof that feeds into the panel to achieve NET 0. FPL only has two rates, you purchase at the retail rate and sell at the wholesale rate. I don't think it's worth producing extra power and selling it back at the wholesale rate. Extra capacity to sell at the wholesale rate would just lower my ROI and I would be better invested elsewhere. I don't see any reason to invest in batteries when I am effectively using the grid as my battery. I don't need the grid when the sun is out and when it isn't shinning the grid is my battery. All this points to AC. The microinverters will handle shading better, it's easier to connect these panels at the meter instead of the electric panel, and for efficiency I only have one conversion to AC.

Straight grid-tie sounds like the answer for your criteria, but I don't understand the above. If all you can do with solar is grid-offset moment by moment, you are never going to get yearly-grid-zero.

 

[timselectric]

I want my consumption and production to balance at the end of the year.

Impossible, if you don't send anything to the grid.

 

[Lawrence_SoCal]

The company going out of business is a best case scenario . You can probably pay pennies on the dollar to whoever scoops up the remains to settle the account

the contract called for 60% payment when day 1 install completed (in a professional and workmanlike manner), remaining 40% at system power up with local util co (which I threatened them into doing a couple of weeks after install).
Until install issues rectified, I may not owe them a single penny (lawyers in the family to back me up). TBD

Realistically, I'd like things fixed... but I suspect it will be a toss up as to whether whomever picks up the 'accounts receivable' decides that my case is worth the effort. In my case, pre-solar I re-roofed and had a full roof warranty which covered this company doing PC panel install, but with some panels needing to be moved, and some other mounting screws having missed rafters, I *may* have a case to require a full lift and lay, plus rafter repair and plywood replacement, etc. on a concrete Spanish S-tile roof on that side of the 2-story house, (ie remove all PV panels first, then re-install) i plus wiring up another sub-panel, and possibly doing a flush main panel upgrade, only then would contract amount be due, and at that point that was for a system with a production level warranty, for a company that possibly won't exist... so yea... interesting as to whether I'll be on the hook for even pennies.
Oh, and if lift and lay got approved, then I'll probably pay myself to consolidate (and re-position?) some roof vents, and improve the panel layout while I'm at it.

Fortunately my case not as bad as someone else in NorCal, where company sub-contracted and supposedly did multi US$100Ks in damage. In my case, if I don't pay anything at all, them I'm ahead, sort of/maybe, in the short-term. But others have it much worse than I.
Even so, in retrospect, I'm still glad I didn't try to DIY with a 2-story Spanish tile roof-mounted, grid-tie system.

 

[zanydroid]

Not sure how I could have said in an easier way to understand. I'm confused how you have solar and not know what selling power back means?

Of course I know how sellback works, I have plenty of ROI spreadsheets. I was asking for your specific definition because I couldn't map it t o mine. I defined mine in detail, AFAICT.

I guess we can just drop this 1:1 debate.

 

[zanydroid]

fair enough - do aios turn off at night?

I'm not sure about the working mode details.

I suspect if you bought a hybrid and ran it battery-less like a grid-tie (basically like what a bunch of the Growatt MIN enjoyers on this forum have been doing, before ponying up for a battery), it has little choice but to turn off at night.

 

[zanydroid]

Even so, in retrospect, I'm still glad I didn't try to DIY with a 2-story Spanish tile roof-mounted, grid-tie system.

It’s funny how few people here talk about DIY on Spanish tile

 

[Lawrence_SoCal]

In Florida you pay your bill monthly but they reconcile your account at the end of the year. They add up all the energy you consumed and all the energy you produced. If you consumed more power than you produced you pay the retail rate for the difference. If you produced more power than you consumed, they pay you the miserable wholesale rate. There are no special time of use rates. I want my consumption and production to balance at the end of the year.

That is a very generous systems (as was CA NEM 1.0 & 2.0). With no TOU consumption rates, even easier/better for you (smaller system required to net out your annual kWh consumption)
BUT, as I mentioned previously, do NOT be surprised when your rate plans change... see other countries with silly policies that drive companies (in general, utilities for sake of this discussion) into the ground... That rate plan is shifting significant costs onto ever shrinking non-solar consumer base... and is not economically sustainable for multiple reasons (nothing to do with 'equity'). period. something will have to give.. and like most things in life, the longer the resistance to a required adjustment, the harder (costlier) the change/shift becomes.

this means, you can take a look at your annual kWh consumed, and plan for a system that produces that much, approximately. done.. easy peasy...
you wont' ever get exact, as no one can forecast the exact amount of sun you'll get next year, or the year after, etc. It will be your judgment call on expected kWh consumption as to whether to shoot to over-size the system or not
BUT, realize that net metering scheme will change (somehow) during your systems life expectancy/ tariff agreement timeframe. just be prepared for it.
One strategy would be to target your existing kWh... and then, typically, new tariff plans are announced in advance, with a time period to make adjustments before the new rules go in effect. That gives you time to get experience with your system, and then if you find you need more panels, you can add them just before new net metering goes into effect, get re-tariffed but no impact as still same plan as you have now, and be on your merry way (with an extended tariff timeframe) ... just a thought
in which case, do your mounting, panel layout, wiring, etc to make adding panels easier when the time comes

.. dang, that's a lot of kWh you are consuming annually .. As you've read in this thread already, the best ROI you are likely to get is reducing kWh consumption (more efficient appliances, insulation, etc typically create better return than trying to offset consumption with additional PV panel purchase)... in your case, spending $20K (or whatever) now on PV install, then optimizing consumption as makes sense?? Which means, some circuit level consumption monitoring/tracking likely to be a worthwhile invests (not a SPAN panel, or anything that expensive. I'm not a fan of Emporia Vue's cloud model, but something like that will assist in targeting the most band-for-your buck)

 

[Lawrence_SoCal]

The wrinkle is I may want to get an EV so i stop paying for gasoline and double down on my solar investment. I'm concerned about ROI and it seems that we will all drive an EV eventually. I guess it also wouldn't be bad to have the capability to power the house from the car battery at night or if the grid goes down but this is not the primary concern. I've been through plenty of hurricanes and lack of power has never been an issue. I also worry that batteries will improve greatly in the coming years and any DC batteries purchased now will be the equivalent of a 1970s $30,000 microwave. I could also install 10-15 kW of panels over the garage when I buy an EV and have a split system. This would also be an easy wiring job to the electric panel.

Planning for future EV and CA NEM rules and expected new plan is why I oversized upfront.
Car batteries are weight conscious, which drives a different chemistry make-up than house batteries. And batteries have lifecycles, with different chemistries having different standard charge cycle expected lifecycles. something to factor in, when thinking of using car as whole house ESS (though as noted previously, not primary consideration if only for emergency). However, V2H would require a grid-forming inverter, with its associated purchase and installation costs, including possible rapid shutdown/disconnect in some markets, etc. Others will now more than I but grid-forming setups usually costs thousands more than grid-following

Hopefully, solid-state or similar batterie aren't too far off (unlike Fusion power, 20 years for last 50 years ;^) If the hype is to be believed, their warrantied life should be around double, and order-of magnitude or much more in terms of charge lifecycles. But how soon to such batteries (or comparable) are value competitive for house market???
As for adding panels, see prior posts regarding implications from a change in net metering scheme and/or your PV system capacity. ie sure, just add panels until new plan is announced/comes into effect (whenever that may be)
On the other hand, if reducing consumption as I mentioned previously is viable, that may cover some of a new EV charging load?

But consider wiring .. if DC (which probably makes sense in your case), presumably main panels and garage panels all need to be wired to same Inverter? For electrical safety, when grid-tied and no hybrid/grid-forming inverter in place, panels need to shutdown when grid fails. That means monitoring typical on meter to main load center connection, basically. Wiring can get complicated with multiple inverters/systems. But for a system of your size, multiple inverters may be required anyway? depends on what you current peak and sustained high load figures looks like on consumption side... then again you are talking well about 18k PV anyway, so multiple inverters regardless ??? but you'd want to avoid a 3rd inverter due to cost, presumably??

 

[Lawrence_SoCal]

Questions:

So.. you asked, lots of feedback provided..
Do you feel your questions answered? any new questions?

 

[OzSolar]

Of course I know how sellback works, I have plenty of ROI spreadsheets. I was asking for your specific definition because I couldn't map it t o mine. I defined mine in detail, AFAICT.

I guess we can just drop this 1:1 debate.

Oh, there's no dropping it now. Like most outside of CA, I have monthly 1:1 with annual true up. My excess at the end of the month is carried forward to next month at wholesale. EG : 100 kWH x .03 = $3.00. Maybe that helped?

It's clear that I know nothing about CA and you know even less about anything outside of CA. Sorry, couldn't resist. OP is still in FL with no TOU so why do people keep trying to derail this thread CA details?

 

[DIYrich]

The best ROI on solar happens with a system sized SMALL enough that it never sells power back. This way every kWh it produces is avoiding at your retail rate.

Not so fast there buddy. You have to look at the marginal cost of adding one more panel vs the marginal benefit. Even assuming $0 for exported power (or power left at the panel), there can be a positive benefit from using 90% of its output.

 

[OzSolar]

Not so fast there buddy. You have to look at the marginal cost of adding one more panel vs the marginal benefit. Even assuming $0 for exported power (or power left at the panel), there can be a positive benefit from using 90% of its output.

Let's look at it then. This is the place where you get to use data to back up your opinion.

 

[1201]

Let's look at it then. This is the place where you get to use data to back up your opinion.

View attachment 233509

Where's YOUR data?

 

[Citlalatonac]

Questions:
Am I missing any important factor?

Purely looking at ROI with net metering I'd go some of the hoymiles 4 panel micro inverters. RSD requirements + shading really tilt in that direction. Worst case you can find an inverter that will take your AC coupled PVs are a generator input.

If they break in 5 years and they've paid for themselves something better will exist to replaces them with.

String inverters can let you overpanel better, which might be a big benefit with your E/W slopes, esp if they're steep. Basically you can get better bang for your buck from a string inverter if you plug in an E and a W array because they'll produce at different times of day. You might be able to put 15k panels on a 10k inverter with little to no clippibg losses because the sun only shines on one set of panels at a time.

 

[fish1534]

Holy Wow! Thank you everyone for all the advice! I'm trying to reply but with every post if have bunch of research to do on t suggestion or help. I'm trying to sort through it all but many thank you!

 

[fish1534]

This is a very confusing statement, and it seems for a non-net metered or aggressively non-lucrative net metering terms

What do you exactly mean by selling power back?

I have 1:1 net metering, I produce way more in the summer than I can consume (on a daily/real-time basis) and send it out via my grid-tie only system (no batteries). (I'm in a heating dominated climate and I only use 5kWh/day on AC maybe 20 days out of the summer, the rest is 0kWh/day).

It's compensated at TOU retail rate. But on an annual basis I don't produce more than I can use, so everything is at retail rate. We have annnual true-up which sounds like what OP has.

I understood exactly what OzSoalr was saying and believe you two are saying the same thing. Some months you will be a consumer and some months you will be a producer but because the bill is annualized all that matters is the offset between to two. If I undersize the system, I am always earning 11c per kWh (by not paying the retail rate). Once the system is oversized and annually I produce more than I consume I am trying to pay of that last panel that oversized the system at the retail rate of 2c /kWh. The goal is to be exactly at 0 kWh or slightly under to earn the highest rate 11c per kWh for your power.

Zanydroid, you and I have exactly the same goal, except I am in a AC dominate climate! We do not have TOU rates but I think this will benefit me because I am always receiving the highest retail rate.