Struggling with AC vs DC coupled systems

[fish1534]

I've been getting quotes on solar systems and am struggling between an AC and DC coupled system. I've been reading through many threads and it appears most people prefer DC coupled but I think that depends on your situation. I live in FL and we have net metering, my goal is making money, the highest ROI. My situation is:

I have a two story roof over my house and a single story over the garage.
Both roofs face east/west with no southern exposure.
I have trees that will create shadows when the sun is at low angles but I'm clear directly overhead.
The power meter is over the house, and the electric panel is on the opposite side of the house in the garage.
I'm looking for a 25-30 kW system so my yearly Net metering is 0.
I can fit about 60 panels on the main house roof.
Main usage is to run three air conditions in the FL heat.
I do not care about being grid independent.

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.

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.

Questions:
Which system is better in my situation?
If I go AC am I wasting my money by not future proofing my investment? It appears this group prefers DC.
Why do people install their own batteries instead of use the grid as a battery.
Do I really need micro inverters to handle the possible shading?
Is it dumb to have an AC and DC system? Can these be made to work together or is that needless complication? Will the wiring two systems be more complicated with each system at opposite ends of the home?
Could I power the home from the EV or future DC batteries?
Am I missing any important factor?

 

[Warpspeed]

Its like buying a boat, or choosing a wife.....
Many different ideas, hopes, needs, criteria, and budgets. And risks.

The idea that you can actually make money from the power utilities is an old and attractive one.
But the sad truth is, they MUST run at a profit to survive, and everyone including the totally corrupt Government fully understand that.

So the game is, juggle tariffs to attract new customers, then juggle them again to get right back to even higher profitability once the suckers have signed supply contracts and cannot escape.
Keep constantly moving the goal posts and changing the rules and the laws.

Good luck with your plan, but expect to outlay a whole bunch of cash, only to discover the payback period and ROI after some rule change has vanished over the horizon.

May of us here are content with off grid power security, and if the grid goes down we are o/k. And are quite happy paying minimal connection charges just in case our off grid system goes down for a short period.
With your proposed system, if the grid ever goes down, you will be sitting in the dark.

 

[timselectric]

Too many contradictory statements. I think more research is necessary.

we have net metering, my goal is making money

I'm looking for a 25-30 kW system so my yearly Net metering is 0.

I don't think it's worth producing extra power and selling it back at the wholesale rate.

I don't see any reason to invest in batteries when I am effectively using the grid as my battery.

You can't get a yearly net metering to zero, without making and selling excess to the utility company.

 

[Quattrohead]

Let me ask you a simple question, how will you feel knowing you have a solar system on your roof capable of producing power but not able to use a single watt of it because your grid is down after a hurricane ?
This is why you need some battery in your system.

 

[Quattrohead]

Although it is not very sexy, your first thing is to find out what is gobbling all of your power and making energy efficiencies.
You can run around your house with a thermostat gun or a thermal camera to find hot spots and leaks. Connect an energy monitoring system like Emporia to your panel to find out where your highest usage is coming from and deal with that first. I found my garage beer fridge to be a massive consumer of electricity 24/7, in the winter it takes more than the AC over the course of a day.
I think nearly everyone that has set up energy monitoring in their home has found very useful efficiencies can be made.

 

[DIYrich]

Questions:
Which system is better in my situation?

I don't like electronics on the roof, so string inverter. WHEN a micro fails, you have to go on the roof to fix.

If I go AC am I wasting my money by not future proofing my investment? It appears this group prefers DC.

If you add batteries, you have to pay for the inverter anyway (battery to AC), so why pay for Micros if you have to pay for the Inverter anyway?

Why do people install their own batteries instead of use the grid as a battery.

If you want to "make money", then you need a battery to arbitrage the rates. Other reason is power when the grid is down. Finally, when 1-1 net metering goes away (or at least make it easy to add a battery).

Do I really need micro inverters to handle the possible shading?

No. Diodes work fine.

Is it dumb to have an AC and DC system?

You do the best you can. Micros -> battery is less efficient than DC from panels -> battery.

Can these be made to work together or is that needless complication?

Needless complication.

Will the wiring two systems be more complicated with each system at opposite ends of the home?

Yes.

Could I power the home from the EV or future DC batteries?

Yes, but plan for it now.

Am I missing any important factor?

Cost of battery with Micro system.

 

[fish1534]

Its like buying a boat, or choosing a wife.....
Many different ideas, hopes, needs, criteria, and budgets. And risks.

The idea that you can actually make money from the power utilities is an old and attractive one.
But the sad truth is, they MUST run at a profit to survive, and everyone including the totally corrupt Government fully understand that.

So the game is, juggle tariffs to attract new customers, then juggle them again to get right back to even higher profitability once the suckers have signed supply contracts and cannot escape.
Keep constantly moving the goal posts and changing the rules and the laws.

Good luck with your plan, but expect to outlay a whole bunch of cash, only to discover the payback period and ROI after some rule change has vanished over the horizon.

May of us here are content with off grid power security, and if the grid goes down we are o/k. And are quite happy paying minimal connection charges just in case our off grid system goes down for a short period.
With your proposed system, if the grid ever goes down, you will be sitting in the dark.

Warpseed, I agree with everything you said. I suppose I'm looking for others people's opinion on what makes a good wife!

I have no issues with the power company making money, I enjoy having electricity. And I agree the whole endeavor is vulnerable to government intervention. All investments have regulatory risk. If you buy oil stock, laws may become unfavorable to fossil fuels, if you invest in pharmaceuticals companies, the FDA may not approve the company's new drug, if you buy real estate the government can change the mortgage interest tax deduction. I am happy, to get an ROI over 10% to match the 30 year S&P 500. It seems if I DIY the system, I can see that easily. The question becomes where is the best place to stick $20k?

 

[OzSolar]

The question becomes where is the best place to stick $20k?

The stock market

 

[zanydroid]

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.

Not sure how you're seeing one conversion for AC on micros and not string inverters.

AIOs go PV string -> high voltage bus -> inverter, if you don't need to discharge from 48V battery.

Microinverters go PV module (a 1s string) -> step up to HVDC -> inverter.

It's MORE efficient with the AIO string architecture because microinverters start at 1s module voltage and go to 240VAC, while AIO strings typically start at 5s or higher.

Perhaps OP read the pre-AIO / component architecture where MPPTs stepped down to battery voltage, to then be boosted up by a separate AC inverter.

 

[Warpspeed]

I look on the whole off grid solar thing as a retirement project.
An interesting challenge, something to do and think about, and it has the advantage of lowering my regular electricity bills, and makes me more self sufficient.

Over the last seven years or so, being off grid has cost me.
Grid power is still the cheapest electricity, but its been fun, a bit of an adventure, and I have also learned a great deal, and that has to be worth something.

Where to stick a surplus $20K ?
IMHO, the way things are going, precious metals.
But not paper promises, you MUST have the actual real physical metal in your possession.

 

[fish1534]

Let me ask you a simple question, how will you feel knowing you have a solar system on your roof capable of producing power but not able to use a single watt of it because your grid is down after a hurricane ?
This is why you need some battery in your system.

Thanks, for the comment. I'm not concerned about the power going down; it's kind of like camping. Heck sometimes I even pay to take the kids camping. I have been through many hurricanes in the past 20 years, I have preemptively evacuated to the center of the state twice and lost power for a total of 5 days in two different storms. I don't need air conditioning and the kids don't need tablets. After hurricanes, neighbors come together because no one is at work. We have massive neighborhood grill outs to eat perishable food, and share generators. Two years ago I gained power 2 days before the opposite side of my street. Every neighbor received an extension cord to power their fridge, and charge their phone. I'm not saying it wouldn't be nice to have power backup but it's not the goal. If it's the only reason for batteries, to me those 5 days without power are not worth $500 a day for a $2.5k battery. To each their own. My goal is diversified, stable investments so I can leave on a sailboat a year earlier. The market 30 year average is around 10%. I think I can beat this with self installed solar.

 

[zanydroid]

The power meter is over the house, and the electric panel is on the opposite side of the house in the garage.

I would recommend taking photos and drawing this, since I think most would find it highly unlikely that the first means of disconnect after the meter is not at the meter. Unless it is an old house or Florida has different code.

You can install a grid tie system at garage and it will push up to the grid no problem. Single story is easier to DIY

The challenge is laying it out properly for batteries

How long is the net metering scheme locked in for in your state?

 

[fish1534]

The stock market

Yes, that is honestly my biggest struggle here! Real estate has actually been better to me than the market but I am worried about corrections in both as boomers retire and die. Solar seem like a safe place to stick a small amount of money.

 

[fish1534]

Although it is not very sexy, your first thing is to find out what is gobbling all of your power and making energy efficiencies.
You can run around your house with a thermostat gun or a thermal camera to find hot spots and leaks. Connect an energy monitoring system like Emporia to your panel to find out where your highest usage is coming from and de

Quattrohead, I could not agree more! I'm not looking for sexy. We are spray foam insulating the house as part of the same project. I can tell you right now my two air conditioners are power hungry and we plan to add a third AC as we finish the attics. As far as appliances go, maybe the software with the solar can measure my individual usage. I'm not confident however that a new $1000 washing machine will outlast my 1980s models that are free and still ticking. They may use more energy but my luck with new appliances has left me jaded. Anyone else have this same problem.

 

[Quattrohead]

Okay well you and me had different goals then, I decided to go all out and be entirely independent of FPL so I am sitting on a mountain of batteries LOL.
This is for a new build so I was able to bury the cost of the solar within the general build budget and my missus said as long as the light turns on when I hit the switch you can do what you want, so I did

 

[1201]

no need to complicate it:

for grid-tie no batteries AC coupled is far more efficient.

if you have batteries then a combination of AC and DC coupled would be most efficient

 

[fish1534]

Too many contradictory statements. I think more research is necessary.






You can't get a yearly net metering to zero, without making and selling excess to the utility company.

Timselectric, this may be a terminology thing that differs from state to state. 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.

 

[1201]

Timselectric, this may be a terminology thing that differs from state to state. 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.

Do you have a link to the plan?

 

[OzSolar]

Yes, that is honestly my biggest struggle here! Real estate has actually been better to me than the market but I am worried about corrections in both as boomers retire and die. Solar seem like a safe place to stick a small amount of money.

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.

 

[zanydroid]

for grid-tie no batteries AC coupled is far more efficient.

if you have batteries then a combination of AC and DC coupled would be most efficient

Not sure what you are saying here.

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

String is probably more efficient over microinverters. BUT Enphase might have optimized their power electronics more than a less well capitalized string competitor.

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.

If we want to talk about max power conversion efficiency you would DC couple into HV batteries instead of 48V, but I doubt most people on this thread will want to recommend that for other reasons.

 

[zanydroid]

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.

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.

 

[Hedges]

But if you pay your utility $0.40 to $0.60/kWh (like I do with California PG&E),
and you can roll your own power for $0.025/kWh by DIY GT PV,

Simply installing a massively oversized system and wasting several kWh that cost you only $0.025 each some of the time, lets you avoid paying $0.60 some of the time. Will be diminishing returns at some point but probably not before you over-install 10x.

 

[Crowz]

I live in FL and we have net metering, my goal is making money, the highest ROI.

If you want to make money farm crypto with the solar. Trust me its more profitable than net metering

 

[Lawrence_SoCal]

My recommendation - take a look at what CA did with NEM 3.0 and the recently announced new fixed monthly fees. Ignore politics, the reality is that with lots of residential solar such a change in tariff/net metering plan was bound to have this outcome, especially if/when there isn't cheap, reliable, massive grid ESS systems.
So, the earlier point about not counting on whatever your current net metering agreement is to be the only financial costs 'plan' for electricity of your net metering tariff agreement timeframe is something to consider.
My install (late '22) was CA NEM 2.0, but I'm under the impression the new fixed monthly charges will impact me.... grumble, grumble, damn CPUC... but anyway... my point being (as was also made earlier) is that there will be tariff/net metering agreement changes after-the-fact , and these are likely to impact your ROI. For me, still does NOT make positive ROI to add a battery... yet. the time will come, I'm just hoping battery tech evolves to something much better than LFP when the time comes for me to purchase.
As for why battery vs using grid? modern net metering agreements is why. Old plans to incentivize installing solar, when residential solar systems were a lot more expensive, offered essentially free grid 'storage' but that has gone away in some places already is likely to go away elsewhere, as it is not reasonable/practical/sustainable at this point. So .. .it depends.. I did NOT get a battery with my solar install (almost 2 years) ago, as my grid circuit is very reliable locally, and there was NOT a good ROI for ESS at my house at the time (and LFP not prevalent at the time).
Or answered another way... in CA and AUS? there is so much residential solar being pumped onto the grid that wholesale electricity costs are negative on mid sunny days, ie too much electricity. So, you can pay utility to handle 'storage' (often actually is fresh generation later during peak time) or you can do it yourself. That is its own calculation, expect it to change over time, etc. basically, don't assume you won't add a battery at some point for ROI reasons.. [unless some new magic technology enables grid-level ESS way cheaper than individual homeowner can do for themselves]

For my smaller 7kW system on a roof with tall chimney in the middle of largely south facing roof, a micro-inverter/AC coupled grid-tied system made for a better ROI. The main benefit to me of AC coupled... the VERY simple wiring with no battery involved ... For larger systems like yours, without shading concern, I'd seen/heard that DC makes better financial sense... though string inverter reliability is questionable (seen people reasonably argue either side of the issue)... so I'll not take a position on the matter.
(IF I was to go DC coupled,) make sure you plan for the expense of putting in a bypass switch/capability to enable grid-power to house when (not if) inverter fails (sort of like a hot water heater) [In re-reading, I will end up installing such a bypass as well when the time comes for installing AC coupled battery as I know I will want grid-down power (ie, require a grid-forming inverter setup, because for the extra cost, worth it to me to avoid the inconvenience). My recommendation, if going for max ROI over the long run, do the wiring now to make adding a whole house ESS a near plug 'n play when the time comes (vs my scenario where I'll have a bunch of expensive wiring to do in addition to battery cost when the time comes)

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].

Depending on your specific net metering agreement, realize there are grid-following battery setups (ie, do NOT provide house power when grid down, and this can save thousands of dollars). My understanding (I could easily be wrong) is that typically, if you have a DC/string inverter setup, and go to add a battery, you will usually (?? depends?) move the strings from the panels away from existing string inverter to direct connect to battery/ESS (assuming interest in avoiding AC/DC conversion losses). Something to think about in terms of leaving wiring loop length, system placement, etc [to avoid more expensive rewiring in the future].

As for DC conversion losses... I recommend doing some research on why home EV DC charging is not a thing, and possibly won't ever be (I'd love for that to change, from a theoretical perspective... but from practical consider, may be insignificant??). So yes, V2H [vehicle to home/vehicle to grid V2G] is coming (still waiting for UL/NEC/IRC, or ??) standards , but unless vehicle battery warranty is unlimited cycles, is likely something to only use for emergencies (vs TOU consumption shifting) as home batteries way cheaper to replace than vehicle battery...
- and be aware of cheaper portable power systems that only output 120V, so 1/2 your load center wouldn't be powered...

My $0.02
- I suggest AUS is ahead of CA which is ahead of most other US states when it comes to residential solar impact on grid in high sunshine environments .. so look what is happening/being done in those markets to see what is bound to happen (at some point) in your local electricity market
- from a grid perspective, Avoiding large variance from base to peak grid demand amounts (ie, adjusting consumption patterns via rates) could save a fortune (which, in the end, the consumer pay for. Meaning time of use rates are essentially required for long-term cost-effective maintenance of the grid with current technology ... it is counter-productive to NOT have TOU rates... . so as much as I'm not happy with my state utility commission, I do understand that basic economics (not politics) is driving changes in rate plans
- batteries can be positive ROI even for just TOU consumption shifting, if/when rates are high enough (like in CA, certainly not the case everywhere).
- if you don't need a battery now, fine. but make sure you truly understand your local net metering calculations to determine if that is the case.
but do yourself a favor, and make adding a house battery/ESS easy

 

[olivinelife]

Highest ROI vs NET 0 are kinda contradictory if you don't have a favorable net metering plan. Highest ROI completely depends on how your utility rate structure is and what changes might happen in the future.

Nobody here really knows what your plan is and just taking shots in the dark. Local installers honestly would understand your economics better for maximizing ROI.