On/Off-grid residential system

[asdrav]

Hello solar diyers! This my fist post and I am very glad to join this forum!

Here is a situation that puzzles me for some time now.

A friend of mine has installed a net-metering solar system (with battery) in his house. Unfortunately, his grid connection is 1-phase and the max PV power allowed by our grid administrator is only 5kW. This covers only a small fraction of his electriccal consumption, as there are 4 or 5 freezers in the house, working 24/7!!! Even during summer, his system will hardly export any energy to the grid, as air conditining steps in as well.

The option of going 3-phase is out of the question both technically and finacially. So, the only possible solution I thought is to install a second, off-grid solar system, whose sole purpose will be to charge the battery of the on-grid system, while the on-grid inverter will be configured to export all its surplus output to the grid. This way, the grid-connected system will store some energy in the grid and use it along the energy of the battery (charged by the off-grid system) during the night in order to keep the house running. Is this technically possible? Can I have two inverters connected at the same time on the same battery or should I use some kind of switching device to switch the battery between the two inverters?

I'd appreciate any suggestion.

 

[Hedges]

Is he limited to 5kW max PV, or 5kW max export to grid?

There are inverters offering zero or limited export. They could have much larger PV array, use it to charge battery and offset loads, but export 5kW max.

Most hybrid/AIO should support this, also some GT PV inverters.

A current transformer (CT) used at meter monitors import/export power.

 

[DIYrich]

Can I have two inverters connected at the same time on the same battery or should I use some kind of switching device to switch the battery between the two inverters?

Yes.

Inverter 1 is connected to Grid. Assuming it can read the Battery's SOC, it exports 5kW whenever the battery exceeds 90% SOC (or whatever level you want).

Load Connection of Invert 1 goes to Grid Connection of Inverter 2.

Load Connection of Inverter 2 goes to Load.

Put most of the PV on Inverter 1 (so Inverter 1 can export the excess).

Set Inverter 2 to Zero Export (excess to charge battery).

Make sure that max charge rate of the Battery is not exceed when both inverters are charging with Max PV and minimal loads

 

[asdrav]

Is he limited to 5kW max PV, or 5kW max export to grid?

There are inverters offering zero or limited export. They could have much larger PV array, use it to charge battery and offset loads, but export 5kW max.

Most hybrid/AIO should support this, also some GT PV inverters.

A current transformer (CT) used at meter monitors import/export power.

Thanks for your reply.
Unfortunately (and unreasonably!!), the limit is for max nominal power of solar modules installed and not for exported power. There is no limit on inverter power or battery capacity.
The inverter already installed is a 8kW Goodwe unit (it was cheaper than its 5kW unit!!). Installing extra panels AFTER the ispection and connection of the system to the grid will be risky as the meter installed by the grid administrator tracks the produced energy and NOT the exported. he inverter can supported up to 12kW of panels. This way, we could get some 25-30% more energy production (much like adding a tracker). So, the excess power will be immedietely detected and there is no way to trick it by limiting energy export to the grid.

I think the only feasible solution is the use of a separate, off-grid system to charge the battery while the grid-connected system covers the load during the day and exports any energy surplus to the grid and not to the battery, in order to store as much energy as possible for the night.
My question is: can hybrid inverters suppost such a combination?

 

[JungleTim]

Why is he worried about selling back power? His goal needs to be reducing his grid power USE. If he ever (not likely) achieves that goal, then he can think about selling back. A string of 6000XP inverters, a LOT of solar, and a BIG battery bank is what he should install. You have to be successful OFF GRID before you can give away any extra power.

 

[DIYrich]

Solar charge controller to charge battery. Make sure you do not exceed battery max charge rate.

 

[asdrav]

Why is he worried about selling back power? His goal needs to be reducing his grid power USE. If he ever (not likely) achieves that goal, then he can think about selling back. A string of 6000XP inverters, a LOT of solar, and a BIG battery bank is what he should install. You have to be successful OFF GRID before you can give away any extra power.

Oh, but he doesn't want to SELL energy to the grid. He has a net-metering contract with the grid administrator for 25 years. According to that, he can EXPORT surplus energy to the grid and use it later (during the night) to power his home. His final electricity bill will be the net energy used from the grid (kWh drawn minus kWh exported).

The problem is that his net-metering system is small, as the max allowed solar power he can connect to the grid is only 5kW and cannot cover his annual needs. To give you the numbers, his annual consumption is about 18.000 kWh and the expected annual production of a 5kW system in Greece is around 7.500kWh. That is why we are considering the idea of using a second 8kW system (inverter with solar panels) to produce the extra 11.000kWh needed off-grid and store them to the battery. Then, the main inverter will power the house during the day and export all surplus energy to the grid, without sending any to the battery, which will be charged exclusively by the second, off-grid system. During the night, he will use the energy from the battery as well the energy already exported to the grid. By properly sizing the secondary system and the battery, he can become almost 100% independent from grid but he will keep the grid connection as a last resort. Best of both worlds!

Of course, by staying connected, he cannot avoid the charges for using the grid, even if his net energy balance is 0. I think this is a small price to pay, as the cost to go full off-grid with such consumption would be extremely high.

 

[asdrav]

Solar charge controller to charge battery. Make sure you do not exceed battery max charge rate.

If I get it right, you suggest that he has the battery connected to both inverters. Set a very low (even 0%?) SOC limit to the on-grid inverter, so that all its surplus energy is directed to the grid. Then use the off-grid inverter (with its own separate solar modules) in zero feed-in mode, only for charging the battery. Must these iverters be the same model?

What happens on a cloudy day, when solar production of the on-grid system is insufficient for the load? Can the first inverter draw energy from the battery while the second one is charging it? Or should we configure the first inverter to NOT draw energy from the battery until it's fully charged and draw from grid till then? It starts to get complicated!

 

[JungleTim]

Oh, but he doesn't want to SELL energy to the grid. He has a net-metering contract with the grid administrator for 25 years. According to that, he can EXPORT surplus energy to the grid and use it later (during the night) to power his home. His final electricity bill will be the net energy used from the grid (kWh drawn minus kWh exported).

The problem is that his net-metering system is small, as the max allowed solar power he can connect to the grid is only 5kW and cannot cover his annual needs. To give you the numbers, his annual consumption is about 18.000 kWh and the expected annual production of a 5kW system in Greece is around 7.500kWh. That is why we are considering the idea of using a second 8kW system (inverter with solar panels) to produce the extra 11.000kWh needed off-grid and store them to the battery. Then, the main inverter will power the house during the day and export all surplus energy to the grid, without sending any to the battery, which will be charged exclusively by the second, off-grid system. During the night, he will use the energy from the battery as well the energy already exported to the grid. By properly sizing the secondary system and the battery, he can become almost 100% independent from grid but he will keep the grid connection as a last resort. Best of both worlds!

Of course, by staying connected, he cannot avoid the charges for using the grid, even if his net energy balance is 0. I think this is a small price to pay, as the cost to go full off-grid with such consumption would be extremely high.

You missed my point. He has such a high load that the goal should be to reduce his bill.

 

[DIYrich]

If I get it right, you suggest that he has the battery connected to both inverters.

yes

Set a very low (even 0%?) SOC limit to the on-grid inverter, so that all its surplus energy is directed to the grid.

Sure.

Then use the off-grid inverter (with its own separate solar modules) in zero feed-in mode, only for charging the battery.

yes. Or, can use a solar charge controller (does not invert).

Must these iverters be the same model?

no

What happens on a cloudy day, when solar production of the on-grid system is insufficient for the load? Can the first inverter draw energy from the battery while the second one is charging it?

yes

Or should we configure the first inverter to NOT draw energy from the battery until it's fully charged and draw from grid till then? It starts to get complicated!

No. You want to leave as much room in the battery for solar to put in.

You might want to set the on-grid inverter to export from battery at 6am to drain it for that day's production.

 

[asdrav]

yes


Sure.




yes. Or, can use a solar charge controller (does not invert).


no


yes


No. You want to leave as much room in the battery for solar to put in.

You might want to set the on-grid inverter to export from battery at 6am to drain it for that day's production.

Thanks for your valuable comments. We will try the solution of a solar sharger along with a bigger battery!

 

[asdrav]

You missed my point. He has such a high load that the goal should be to reduce his bill.

I think we say the same thing! He wants to reduce his bill by offsetting energy consuption (kWh for kWh) and not by selling energy to the grid (euros for euros), which makes no sense financially as the selling price is almost half the buying one!

The problem is that his on-grid system is too small and can only offset about 40% of his annual consumption. That's why we are considering the addition of a second, off-grid system to charge the battery of the first system and increase consumption offset during night as well.

 

[dopeassjackson]

My power company buys your solar back at whole sale and sells it back to you at retail price..so in reality you never want to make more than you use.

 

[Hedges]

My power company, under NEM 3.0, buys at $0.02/kWh wholesale and sells at $0.40 retail.

DIY PV costs $0.025/kWh

So in reality you want to make more than you use, to the extent you never import power.

Yes, you can store power in a battery (costs $0.05/kWh) to use later. But amount of sunshine and PV production varies so if sized to never make more than you use, often you will make less than you use and pay $0.40/kWh for it.

Better to make 3x what you use on the best days, so you make at least 1x what you use on more days.