Can we get a solar system that doesn't feed back to the grid + sizing advice needed

[Krisz&Nuno]

Hi Everyone,

we moved into a new house and we’re planning our first solar system and we would be very grateful if you could help us out with any advice.

We have an East - West facing roof with some shade on one side. (The building next to us casts a triangular shaped shade on the center of our East side roof, that’s disappearing by 11:30-is.)

We would like to have a system that supports a 15kW/day usage in case of an outage on the grid on a winter day. (Our summer, normal usage is around 8-9kW currently, but we just moved in, we don’t have enough data points yet, we’ve been living on ready meals for the last 2 weeks, didn’t get new electrics etc.)

We’re in the UK (Northamptonshire), where we get 8-9hrs of daylight during winter.
Peak insolation hours are: January: 0.6 / February: 1.17 / March: 2.07 / April: 3.15 / May: 3.83 / June: 4.31 / July: 4.28 / August: 3.62 / September: 2.53 / October: 1.51 / November: 0.83 /December: 0.49

We had 3 surveys so far and talked to several companies, but we’re getting conflicting information. We would like to have an oversized system and can easily accommodate 20 panels (8kW system) on our roof (16 on the West side, no shade, and 8 on the East side, probably also safe from shade, but there might be some shady corners - we have no way to know yet; we just moved in and have trouble seeing the roof and calculating the shade-free area.) Therefore, the 8 panels on the shady side will have either optimisers or microinverters.


So far we got 3 suitable offers:

Option 1: this is the most expensive. We understand this would allow us to use “island mode”, which for us, ignorant people means pretty much being independent from the grid and provide enough electricity for our needs, winter/summer and turn the incoming grid power off, if we llike.

We were told we cannot NOT feed electricity back to the grid, because once we’re not using more, and our batteries are topped up, the produced energy NEEDS to go to somewhere, and that’s the grid (we can either sell it or give it away for free, but it needs to go there.) Is this correct?

This system would have:

• 20 Longi Hi-MO 5m monocrystalline panels
• 8 optimisers
• 2x3.6kW GivEnergy hybrid inverter
• 2x9.5Wh GivEnergy batteries. (If we want to reduce costs, we can have 2x5.2kWh batteries instead.)

We’ll need power diverter and PV protect but no voltage optimiser. We were told this is a big work, the electrician would change our system so much they will issue a new NICEIC certificate. (Bear in mind it’s a new build.) We understand they will rewire all the electrics with a secondary feed to the solar system.

We would have a changeover switch - in case of blackout, we would be able to manually switch to full solar mode/separate ourselves from the incoming grid electricity. Our electrics will still shut own, but we can just switch to use our batteries instead of the grid.


Option 2: this would need us to choose a couple of slots from the fusebox to be connected to our solar system. We won’t be able to operate all our equipment from solar in case of a blackout, just the ones that are wired to our solar system (e.g. boiler, fridge, micro, wifi, computers or whatever we pick).

This would have:

• 16 Longi Hi-MO 5m monocrystalline panels
• 8 optimisers
• one 5KW GivEnergy hybrid inverter
• one GivEnergy 9.5kWh battery

Option 3:

this company said they don’t even deal with connecting us to the grid - what we want is easy and possible without feeding electricity back to the grid and it’s possible. (Is this true?) No visit from the grid people is needed, no permission, no power diverter, no PV protect is needed.


They gave us a quote for:

• 20 Longi Hi-MO 5m monocrystalline panels
• 8 optimisers
• 3 GroWatt 6.5kWh batteries

Which option do you think is best from the above? Option 1 is almost double the price of option 3.

Meanwhile, another company keeps telling us, we’re really oversizing our system, it doesn’t make any sense; we should get 10-12 panels max.

What do you guys think of the above options?

Since we’re in the UK, one of our worries are that during winter we won’t produce enough V to startup the inverter in the morning, unless it’s very sunny, so we might be spending a lot of money on something that might not work. (Stupid side question: isn’t it possible to start up the inverter from batteries?)

Our main objective is not to save money, but to be able to use our essential equipment if there’s an outage, especially during the winter (last year there were outages that lasted for 2-3 weeks in some areas). (Combi boiler with a 40W pump, 900W micro, 3 computers, maybe a little electric heater etc.)

If we could avoid feeding electricity back to the grid, we would like that - it would indeed save us some money on equipment and hassle, getting permissions, visit from the grid people etc. We got conflicting info about it, so we don’t even know if this is possible or not.

We would be very grateful for any advice and confirmation about our inverters (needing 120 V or 150V startup voltage) to start working early on a winter day… I couldn’t calculate for how long it would take for them to start up in a cloudy, winter day.

Again, we don’t care too much about the return of our investment - we want backup power supply in case there are weeks of outage or issues with electricity (and potentially gas) supply.


Thank you!


PS: You guys rock - we read through so much of your advice on this forum and learnt so much from you already.

 

[sunshine_eggo]

December Insolation = 0.49 hours

maximum stated solar array = 8kW

Maximum daily usage in December = 8kW * 0.49 = 4kWh - about 1/4th of your stated need.

This also assume an optimally positioned array with perfect single year round tilt (optimized for Spring/Fall) and no shading.

Your poor solar conditions (UK, E-W facing panels, limited space and presence of shade) + 15kWh/day need = you can't get there from here.

Can't comment on either option given the above.

Can comment on:

Since we’re in the UK, one of our worries are that during winter we won’t produce enough V to startup the inverter in the morning, unless it’s very sunny, so we might be spending a lot of money on something that might not work. (Stupid side question: isn’t it possible to start up the inverter from batteries?)

this concern is unfounded. AMBIENT light is enough to provide voltage. your problem will be a lack of solar intensity as demonstrated by your atrocious Insolation numbers. Think of it this way:

The presence of ambient light give Volts.
The intensity of sunshine on the panels gives Amps.

You'll have gobs of voltage, just no Amps.

Our main objective is not to save money, but to be able to use our essential equipment if there’s an outage, especially during the winter (last year there were outages that lasted for 2-3 weeks in some areas). (Combi boiler with a 40W pump, 900W micro, 3 computers, maybe a little electric heater etc.)

Suggest you pare that down to survival level stuff. electric heaters are a massive demand on electric power. A heat pump is 5-6X more efficient than a heater.

If we could avoid feeding electricity back to the grid, we would like that - it would indeed save us some money on equipment and hassle, getting permissions, visit from the grid people etc. We got conflicting info about it, so we don’t even know if this is possible or not.

The equipment must monitor the incoming grid and ensure that it never feeds back to the grid. Some equipment has this feature. Some don't.

We would be very grateful for any advice and confirmation about our inverters (needing 120 V or 150V startup voltage) to start working early on a winter day… I couldn’t calculate for how long it would take for them to start up in a cloudy, winter day.

Hopefully, you can answer this yourself now.

Again, we don’t care too much about the return of our investment - we want backup power supply in case there are weeks of outage or issues with electricity (and potentially gas) supply.

IMHO, solar in winter is a very poor option for you. March through September might work with 8kW.

 

[timselectric]

because once we’re not using more, and our batteries are topped up, the produced energy NEEDS to go to somewhere, and that’s the grid (we can either sell it or give it away for free, but it needs to go there.) Is this correct?

No

 

[timselectric]

You want an off grid system.
This can be done with an AIO (all in one).
Or with individual inverter, solar charger, and transfer switch.

 

[DThames]

"the produced energy NEEDS to go to somewhere". The energy is not produced until a load demands it. Look at the power line to your house. It is exploding with lightening bolts everywhere because the power has to go somewhere? (no) But if demanded, it will come into your house and do work.

 

[Krisz&Nuno]

December Insolation = 0.49 hours

maximum stated solar array = 8kW

Maximum daily usage in December = 8kW * 0.49 = 4kWh - about 1/4th of your stated need.

This also assume an optimally positioned array with perfect single year round tilt (optimized for Spring/Fall) and no shading.

Your poor solar conditions (UK, E-W facing panels, limited space and presence of shade) + 15kWh/day need = you can't get there from here.

Can't comment on either option given the above.

Can comment on:



this concern is unfounded. AMBIENT light is enough to provide voltage. your problem will be a lack of solar intensity as demonstrated by your atrocious Insolation numbers. Think of it this way:

The presence of ambient light give Volts.
The intensity of sunshine on the panels gives Amps.

You'll have gobs of voltage, just no Amps.



Suggest you pare that down to survival level stuff. electric heaters are a massive demand on electric power. A heat pump is 5-6X more efficient than a heater.



The equipment must monitor the incoming grid and ensure that it never feeds back to the grid. Some equipment has this feature. Some don't.



Hopefully, you can answer this yourself now.



IMHO, solar in winter is a very poor option for you. March through September might work with 8kW.

Thank you so much for your detailed answer; we clearly have a lot to learn. We can definitely survive on 4kWh if we have to. If heat pump wasn't an option, what would you do given the circumstances we explained? Would you still invest in an 8kW system? Maybe with more batteries that we could keep charged up to a certain level for emergency use? Or add more panels?

 

[Krisz&Nuno]

You want an off grid system.
This can be done with an AIO (all in one).
Or with individual inverter, solar charger, and transfer switch.

that would be amazing, but considering where we are and how low the number of sunny hours here is, I'm afraid we will always need support from the grid during the winter.

 

[sunshine_eggo]

Thank you so much for your detailed answer; we clearly have a lot to learn. We can definitely survive on 4kWh if we have to.

With so little solar, could you survive without heat?

If heat pump wasn't an option, what would you do given the circumstances we explained?

Fire.

Would you still invest in an 8kW system?

I live in an area of year-round abundant sun. I'm the wrong guy to ask.

Maybe with more batteries that we could keep charged up to a certain level for emergency use?

All this would do is provide more energy between charges at massive expense.

Solar determines how much you can use daily.
Batteries determine how long you can go between charges.

Or add more panels?

How? I thought 8K was your max?

 

[timselectric]

that would be amazing, but considering where we are and how low the number of sunny hours here is, I'm afraid we will always need support from the grid during the winter.

An off grid AIO can use the grid as a backup.
They have an AC in connection that can be connected to the grid or a generator.

 

[Warpspeed]

Another trap in all this.
"Average" daily insolation 0.49 x 8kW of installed solar capacity in worst month.
That does not guarantee you can rely absolutely on getting 4kw every single day.
Some days maybe only a fraction of that, essentially nothing.

So you need an absolutely HUGE battery to average out to that 4kW over several, maybe many consecutive bad days.
Even then, you are going to have to restrict your lifestyle to that 4kW per day, and 15kW per day is just not going to be possible in mid winter.

All you can do is cover every available surface in solar panels, and base everything on that, providing a battery large enough to see you through several days of dark grey sky. By the time you cost it all out, grid power starts to look like a real bargain.

Unless you are in the tropics, its not really possible to save money on power bills by installing solar.

 

[timselectric]

For your situation I would recommend that you install as much solar as possible. Use the grid to cover the rest. And just get enough battery for grid outages.
This will lower your electricity bill and give you a little emergency backup. For the least amount of cost.
Batteries are the most expensive part of a system.

 

[45North]

Forget it!
As if your latitude isn't bad enough there's all those rainy overcast days.
Suggest you talk to someone local who has solar.
And mebbe consider moving to Spain.

 

[squowse]

that would be amazing, but considering where we are and how low the number of sunny hours here is, I'm afraid we will always need support from the grid during the winter.

Any particular reason why you're determined not to feed any spare electricity into the grid?

 

[Warpspeed]

There are really two ways to go.
Have a professionally installed grid tie system, that must comply with all the legal requirements and you then become known to the power utility.

They keep changing their rules and the amount of feed in tariff, in such a way that you can never "win" and they can stay in business and keep their shareholders happy with regular dividends.

Or you can go completely outlaw, give the power utilities the middle finger, and go partially off grid, only relying on grid power as a backup on gloomy days.
Going outlaw places no limit on the amount of solar panels, so you can go really nuts.

Best bang for your buck will be a system without a battery.
Run your inverter straight from solar, supplementing that at night (and gloomy days) with grid power fed into your inverter as a dc voltage from a grid powered rectifier. It works like a grid tie system, but feeds nothing back. The grid only provides any shortfall of solar, and any momentary surges.

The advantage is the power utility do not know you are using solar, and you do not need to comply with any restrictions such as the amount of solar panels. Not having a battery will be a huge saving, the down side is that if the grid craps out you are sitting in the dark, but so are the majority of the grid tie people.

You could of course have a small battery not normally connected, with just a few hours of reserve power for typical short term grid down situations.
It could be kept fully charged and ready. An alternative might be a gasoline generator for very for short term grid down use.

 

[timselectric]

The Growatt SPF-5000-ES (or any AIO with the SUB mode) can run without battery. And still give you power during a power outage, as long as the sun is shining. And you can add a battery for backup, at any time in the future.

 

[OffGridForGood]

I would suggest the AIO units (as others have suggested) and max out the PV you can, look into some south facing PV (either on a wall, given your latitude, or building a support like a porch on the south facing direction to pick up what ever PV you can in this direction)
Since grid down is a primary concern I will part with others and suggest you have batteries sized to suit a frugal use of power during grid offline during winter events, consider what you really need during a winter power outage and be careful with use. OR see about a back up generator (small natural gas unit may be best for your case) to provide the power you need with the grid is down during winter.
Good luck with your project.

 

[Quattrohead]

The Growatt SPF-5000-ES (or any AIO with the SUB mode) can run without battery. And still give you power during a power outage, as long as the sun is shining. And you can add a battery for backup, at any time in the future.

I would start with the opposite, as much battery as you want to afford and add panels when you want.
The AIO will charge your batteries from the mains and act as a super UPS to whatever you want to connect to it, as this seems to be your main concern.
Then later you can add the panels to try and remove your mains charging requirements. The AIO can blend both charge sources.

 

[Solar Guppy]

Solis 5G ( UK version ) is common option in the AIO unit

 

[OffGridForGood]

I would start with the opposite, as much battery as you want to afford and add panels when you want.
The AIO will charge your batteries from the mains and act as a super UPS to whatever you want to connect to it, as this seems to be your main concern.
Then later you can add the panels to try and remove your mains charging requirements. The AIO can blend both charge sources.

This is a good suggestion, In fact I started my system with AIO and Batteries as a learning process, adding the PV the next year.
For the OP installing the PV may be the biggest expense of a complete system, and by starting with the AIO & Batteries, they essentially get a large UPS to carry them through those winter power outages for low cost for now, and they can learn how to carefully use battery power during grid-down periods. Later plan and build (or have built for them) the PV to go with their sustem, perhaps finding a space for some south facing panels.

 

[Cheap 4-life]

No

Or an inverter that uses CTs on the homes mains to limit its production to what the home uses