Circuit diagram for grid tie to backfeed for net metering

[nc-diyer]

very noob here, still in process of learning things. I am trying to put together what is needed to build a grid tied solar system for a single detached home here in ottawa where there is snow for 5 months.

Our monthly average is 600kwh in winter, and 1200kwh in summer months where we have AC turned on.

However I could not find a circuit diagram or similar so I can come up with a list of everything that I need for my install to compute a estimate.

Also for my use case, can y'all advise if it is best to go with string or micro inverter setup?

 

[Hedges]

Microinverter lets you have any number of panels oriented any which way, so it is popular with installers.
Depending on inverter and panel wattage, may clip production under ideal conditions.
Doesn't require working with hazardous voltages beyond single PV panel and final grid connection.

String inverters are lower cost and higher efficiency, although if RSD is required that adds complexity and cost so not as much savings.
Each string should have all panels oriented same direction.
You can size strings to avoid clipping, or to clip for flatter production.
You can parallel multiple strings of same length, optionally of different orientation for flatter production with less clipping.
Although MC4 connectors are touch safe, you can have access to wires or screws with hazardous DC voltages and AC voltages.

The stuff needed is brand dependent. Microinverters I think have support electronics and custom cables, see block diagrams on their data sheets.
String inverters, data sheet should show block diagram. Combiner box for multiple strings will have documentation.
Your utility may show typical one-line diagrams from meter to PV panels, with a disconnect switch that may be optional or required.

 

[nc-diyer]

Sounds good, thank you very much for the intro.

A few followups,
1) what is RSD, how can I find out if RSD is required?
2) Any name brands for string inverters?
3) what is a good reliable kwh inverter output range to look at 4/6/10/12 kwh?
4) Also if a inverter is 10KWH, does it mean, it can output 10kw per hour (if produced from panel), then is my calculation correct? 10kw /hour x 5 sun hours x 30 days = 1500kwh return to grid per month?
5) if going with string, does the setup need mppt for grid tied?

 

[Hedges]

RSD is "Rapid Shutdown", PV module level electronics which disconnects each panel so a fireman won't encounter more than 80V within the array, 30V outside. In the US, I think it is required by recent versions of NEC, and each state adopts some version. For you, check with city permit office. But start by googling.

String inverters include SMA (they've discontinued PV only inverters in US, have a hybrid just now available.) Depending on what grid-support features, Rule-21, RSD, etc. are required, an older model may be acceptable.

Fronius, another premium brand.

EG4, GroWatt, probably more.

After you have grid tied PV, you may want battery backup. Maybe a hybrid inverter is the way to go. Or a separate battery inverter. Enphase offers their own (expensive) battery, but can work with battery or hybrid inverters including Schneider and SolArk.

Have you done the math comparing utility rates to PV, and considered what net metering plans are available, determined PV is a good investment?

Inverters are kW, not kWh. Batteries are kWh.

12 kWh/month / 30 days / 6 hours effective sun = 6.7 kW inverter suggested (check an insolation calculator, 6 hours is my location not yours.)
PV panels put out 75% to 85% of rating, so about 9kW of PV panels minimum.

Winter will be much worse. If you have net metering, oversize system so surplus in summer can provide credits meeting winter consumption.

A hybrid inverter like SolArk or Sunny Boy Smart Energy can process more PV than its AC capacity. So 12 to 15kW of PV panels on a 7.7 to 12kW inverter can be good.

You have an AC. How many kW does it draw running? Include air handler fan as well as compressor. What "LRA" or starting surge (about 5x running watts if not listed.) Inverter surge capability has to start motor.

Likely SMA SBSE won't do it, unless a small compressor. SolArk, Schneider, Midnight, some others are good at starting motors.

Inverter is 10kW, can produce 10kWh/hour, otherwise your math is correct.

String inverters and microinverters have MPPT function built in. So do hybrids.
Battery inverters like Midnight Rosie, SMA Sunny Island, Schneider don't have MPPT, can be used with MPPT SCC, those and some others can also AC couple to string inverters or microinverters.

My setup is older model Sunny Boy and Sunny Island. Works very well. Normally quite expensive, but old stock can be had cheaper. Only usable by you if your utility and city don't require some newer features.

SolArk is an all in one people are happy with, and fewer details to install.

Midnight is run by pioneers of the industry. Their Classic SCC is expensive but good. They just introduced Rosie inverter, not much history on it yet but it is powerful (can surge 15 or 20 kW). Midnight has an AIO, even newer.

 

[nc-diyer]

Wow, thank you very much for the insights. I am going to call the city energy office regarding the RSD

0) What is SMA?

1) The whole process of this is to find out a cost for installation (DIY) and find out the ROI, and then decide if it is a good investment. Is there a better way to assess if solar will be a good investment for me.

2) This is what I was able to find about the solar insolation for my location (Ottawa, Canada)

With this insolation, do you think 6.8KW inverter is still good? Any initial thoughts?

3) I will check into the power rating of my AC. What do you mean by "Inverter surge capability has to start motor." ?

4) Why would I need a battery backup, if I am tied into the grid, here the power doesn't go often. May be max of 2 days in a year. Do you still think hybrid inverter is the way to go for me?

5) what happens if my PV (ex: 10kw) overpowers the inverter (6KW)? Would the inverter die or there is some overage protection by default?

Much thanks in advance.

 

[robbob2112]

Have you looked at your power bill? Mine lists the kW used by month which I can then divide by 28/30/31 to get the per day use. If I log in and view their online stats they clearly list my average daily use which I can use to figure size needed for the solar array and battery bank.

 

[nc-diyer]

Have you looked at your power bill? Mine lists the kW used by month which I can then divide by 28/30/31 to get the per day use. If I log in and view their online stats they clearly list my average daily use which I can use to figure size needed for the solar array and battery bank.

Our monthly average is 600kwh in winter, and 1200kwh in summer months where we have AC turned on.

 

[Hedges]

SMA is a German company, inventor of grid-tied PV and transformerless grid-tie inverters.
They were once the market leader, but have lots of competition now.
Most of their products should be among the best quality available.
Features offered may not be as extensive as some other brands.

I think Growatt has low priced GT PV inverters. Don't know about quality and track record, think it is relatively new.

No need for backup unless power goes out. If you have time of use rates, battery can provide peak shaving or time shifting. But you have to consider cost of batteries vs. your utility rates. Small critical loads back is nice. In San Jose we had way more than 1 or 2 outages per year recently.

Your insulation ranges from just under 5 hours summer to just under 1 hour winter, so you need a bit more capacity than what I estimated.

7.7kW is a common size, because it can be connected to most 200A panels with a 40A PV breaker. 11.4kW works for 225A panel, 200A main breaker, 70A PV breaker. Look up "120% rule" and see what size busbar your panel has, and what size main breaker. There are other ways to connect to get around that limit.

Consider turnkey cost and DIY cost (which treats your labor as not worth dollars.)
I think turnkey GT PV costs about $2 to $4 per watt, and hardware for DIY about $0.80 to $1.25 per watt. Amortized over 20 years (ignoring interest or time value of money), around $0.10/kWh for turnkey and $0.025 or $0.03 for DIY.

Batteries may be around $0.05/kWh over 16 years, 6000 daily cycles. But some are much more expensive.

A motor that draws 2500W operating probably needs 12,500W for about 1 second to start. An inverter like Sunny Island can put out 11kW, two of them (for 120/240V split phase) 22kW. Midnight Rosie about 15kW or 20kW from one inverter (and they can be paralleled.) Sunny Boy Smart Energy 7.7 can only surge to 9kW, would not start the motor.

Those only matter if operating with grid down. Grid tie, grid handles loads and GT PV just spins meter backwards, or reduces draw from grid.

 

[nc-diyer]

Thank you very much for the detailed summary, we are on time of use rate plan.

I will spend some time on more research and get back with questions if any.

What is a sweet spot ROI for solar systems?

 

[nc-diyer]

One more clarification on the last part, since I am looking for grid tied, are you saying that I dont have to worry about upping the inverter to handle the peak load of the AC since for the peak load at the start, it will be drawing from the grid instead of PV system. Can you clarify?

 

[robbob2112]

Our monthly average is 600kwh in winter, and 1200kwh in summer months where we have AC turned on.

I should have seen that at the top, sorry for the dumb question -

Is your goal total replacement or just peak shaving and some critical load backup?

 

[Hedges]

One more clarification on the last part, since I am looking for grid tied, are you saying that I dont have to worry about upping the inverter to handle the peak load of the AC since for the peak load at the start, it will be drawing from the grid instead of PV system. Can you clarify?

If grid is down, inverter (or generator) has to start motor load.
If grid is up, the grid (which can put out massive thousand or so amps) has plenty of power. Circuit breaker and wire resistance is all that gets in the way.

Whether you have GT PV or not, whether the sun is out or it is night time, has almost nothing to do with starting a motor on-grid. GT PV does reduce current coming from grid, so reduces voltage drop a bit.

About all GT PV does is change how much current through your meter.

 

[Hedges]

Thank you very much for the detailed summary, we are on time of use rate plan.

Your rate per kWh and what hours each rate applies is the key. Please provide that.

What is a sweet spot ROI for solar systems?

Depends on what else you can do with your money.
During the decade of "Quantitative easing", we earned 0% but could bet on stocks.
Today, interest rates are much higher.

Inverters may fail, cost money to replace. PV panels shouldn't but faulty brands sometimes do degrade rapidly. Equipment can get damaged or stolen. So you want faster payback.

With my $0.40 to $0.60/kWh rates, and GT PV costing $0.10 (turnkey) or $0.03 (DIY) per kWh (amortized over 20 years), my break even could be 5 years, even 1 year. So quite compelling.
(therefore, net metering has been taken away from us.)

I'm going to install an oversize system (15kW) because I got the last of net-metering to be grandfathered for 20 years. May spend $5k to $10k on it. It will almost be competitive with natural gas at today's prices but I have a suspicion that will go up so electric will save me money. (If you shop hard you can find bargains. We're paying $0.20 +/- per watt of PV panel.)

Obviously you'd like a short ROI, something 3 to 7 years would be nice. But there is the risk that rules change. On the other hand, you're hedging your bets on rates. I think you have hydro, may have low rates today and in the future.

 

[Ledsam]

From My Understanding You use 20kwh each day in the winter and 40kwh in the summer. You need to find out your peak kw so you can choose the correct inverter. EG if your peak kw is 9.5kw (cooker, Heating and hairdryer all items on at once) Then you will need at least 10kwh Inverter.
As for battery's you would need at least 40kwh reserve but doubling it would be better. Then comes your solar panels, you state 0.86kwh of sun in dec per m^2, I think you would need at least 26kw of solar panels (how ever I could be wrong)
This Info is best guess, as for PV panels you really need average sunlight hours per day/month to work out storage capacity and production to fulfil your needs. Don't forget you always need a backup when being of grid.