Adding solar to inverter-battery systems 101

[impracticaldogg]

I have some pretty beefy lithium batteries and a basic (Mecer 2400) inverter that works well. Now that loadshedding has gone up so much I'm looking at integrating some solar panels to top up the batteries. As I don't trust the grid to be able to do that before the power goes down again.

What are the basic principles here? I need a more sophisticated inverter (estimate 10k) to handle input from the panels. The panels need to generate enough power to make up for the battery drain. From what I've seen at a friend's place you use batteries to store energy from the panels so the main batteries can also be topped up at night. So they balance out the supply when the panels can't generate energy.

The existing inverter has been connected into the DB board already - so the bulk of the cost should be the purchase and installation of the panels, the new inverter and balancing batteries.

Are there some rules of thumb to calculate the capacity of panels and balancing batteries needed to top up an existing battery installation?

 

[SY Stevenson]

would also like an answer if anyone has one

 

[TheChamp]

I need a more sophisticated inverter (estimate 10k) to handle input from the panels.

No you don't, solar panels have nothing to do with the inverter, they charge batteries. What you need is a charge controller that is capable of handling the charging current required for the batteries.

The amount of panels you need, determine how current you draw from the system, say your total power draw is 500W, to get current draw you divide that by the voltage of your system, 24V in this case, that gives you 21A, so your panels should at a minimum be good for 21A to cover your load, if they produce anything more that's a bonus.

Remember that your charge controller should have specs that are adequate to handle the panels.

 

[impracticaldogg]

No you don't, solar panels have nothing to do with the inverter, they charge batteries. What you need is a charge controller that is capable of handling the charging current required for the batteries.

Thanks for the explanation of the calcs for the panels. So you're saying I can add solar panels as input to the batteries via a charge controller? How would this allow for charging via the grid when it does happen to be up?

 

[Adeptus Mechanicus]

Start by changing your inverter. You will not reap all the benefits of solar simply adding an MPPT charger to keep your batteries topped up when there's no AC.

What you want is something capable of blending power between battery/solar/AC.

What/how many batteries have you got?

 

[TheChamp]

Thanks for the explanation of the calcs for the panels. So you're saying I can add solar panels as input to the batteries via a charge controller? How would this allow for charging via the grid when it does happen to be up?

Charging via the grid still happens through the inverter, the solar connection is a source that is connected in parallel to your inverter at the batteries.

Solar and grid can charge the inverter at the same time, so the inverter will continue doing what it was doing before you added solar. If solar is good enough you can even switch AC off and run solely on solar, that is what I do on weekends when I am home, my system can run all day without electricity as long as the sun is there. I also disconnect the power to the inverter during the day so that it charges from solar.

 

[TheChamp]

Start by changing your inverter. You will not reap all the benefits of solar simply adding an MPPT charger to keep your batteries topped up when there's no AC.

What you want is something capable of blending power between battery/solar/AC.

What/how many batteries have you got?

If he wants to, but he doesn't need to as he thought.

 

[wingnut771]

Start by changing your inverter. You will not reap all the benefits of solar simply adding an MPPT charger to keep your batteries topped up when there's no AC.

What you want is something capable of blending power between battery/solar/AC.

What/how many batteries have you got?

For starters, there is no AC as there is loadshedding.

If you set the solar charge controller's voltage slightly higher than the inverter's charge voltage then solar will take preference over the inverter when both connected to the battery in parallel.

Then it all works automagically when the sun is shining.

When power comes back, the battery is still full because solar covered the load.

 

[wingnut771]

Charging via the grid still happens through the inverter, the solar connection is a source that is connected in parallel to your inverter at the batteries.

Solar and grid can charge the inverter at the same time, so the inverter will continue doing what it was doing before you added solar. If solar is good enough you can even switch AC off and run solely on solar, that is what I do on weekends when I am home, my system can run all day without electricity as long as the sun is there. I also disconnect the power to the inverter during the day so that it charges from solar.

You have to increase solar charge voltage higher than inverter settings in order for solar to have priority when the sun is out over the inverter charger then you don't have to switch ac off.

 

[xefor]

Probably looking at a new system, otherwise I think you're just wasting your time and money.

 

[TheChamp]

Probably looking at a new system, otherwise I think you're just wasting your time and money.

How is he wasting time and money? Looks like a very affordable solution to me?

 

[Dimpie (JIGSAW)]

Probably looking at a new system, otherwise I think you're just wasting your time and money.

Why ... how .... explain ?

 

[impracticaldogg]

Start by changing your inverter. You will not reap all the benefits of solar simply adding an MPPT charger to keep your batteries topped up when there's no AC.

What you want is something capable of blending power between battery/solar/AC.

What/how many batteries have you got?

What/how many batteries have you got?

I have two Blue Nova 218Ah batteries in parallel (24V)

 

[Adeptus Mechanicus]

I have two Blue Nova 218Ah batteries in parallel (24V)

So my advice would be as follows having gone through the effort of doing a "just good enough" setup and regretting not being more forward thinking:

• You already want to go through the effort of installing some panels: Install more than you need for just charging the battery. Nothing silly, but a good few kw.
• Change your inverter to something a bit more user friendly in terms of solar and battery management.
  • Any decent off-grid/hybrid system will do. Ensure it has enough pv capacity, e.g. a 5kw system with at least 6000w of pv capacity
  • Moving from modified sine to pure sine gives the option of running motors/fridges/etc off the system even if you do not do so immediately
• Review your DB config
  • Move all lights to the inverter (if not already done)
  • Move selected (or all) plug circuits to the inverter (your preference)

You now run what you would run on your current setup purely on solar while the sun is out, while there is enough pv generation available to ensure the batteries are fully charged when the sun goes down.

Result: Less wear and tear on your batteries and some degree of future proofing should you want to scale up on pv/batteries at a later stage (e.g. wanting to move all low wattage appliances to inverter side)

Alternatively if you dont want to make that kind of outlay or future proofing is not a concern:

• Get a 3kw Mecer/Axpert type inverter (again for the pv/battery management user friendlyness more than anything else)
• Stick up the maximum amount of pv that can do (usually 1200-2000w)
• Leave everything as is DB side.

 

[wingnut771]

I have two Blue Nova 218Ah batteries in parallel (24V)

24V is 2 x 12V in series.

 

[wingnut771]

So my advice would be as follows having gone through the effort of doing a "just good enough" setup and regretting not being more forward thinking:

• You already want to go through the effort of installing some panels: Install more than you need for just charging the battery. Nothing silly, but a good few kw.
• Change your inverter to something a bit more user friendly in terms of solar and battery management.
  • Any decent off-grid/hybrid system will do. Ensure it has enough pv capacity, e.g. a 5kw system with at least 6000w of pv capacity
  • Moving from modified sine to pure sine gives the option of running motors/fridges/etc off the system even if you do not do so immediately
• Review your DB config
  • Move all lights to the inverter (if not already done)
  • Move selected (or all) plug circuits to the inverter (your preference)

You now run what you would run on your current setup purely on solar while the sun is out, while there is enough pv generation available to ensure the batteries are fully charged when the sun goes down.

Result: Less wear and tear on your batteries and some degree of future proofing should you want to scale up on pv/batteries at a later stage (e.g. wanting to move all low wattage appliances to inverter side)

Alternatively if you dont want to make that kind of outlay or future proofing is not a concern:

• Get a 3kw Mecer/Axpert type inverter (again for the pv/battery management user friendlyness more than anything else)
• Stick up the maximum amount of pv that can do (usually 1200-2000w)
• Leave everything as is DB side.

 

[impracticaldogg]

24V is 2 x 12V in series.

Eish - thanks for that This is why I'm not an electrician..

 

[impracticaldogg]

So my advice would be as follows having gone through the effort of doing a "just good enough" setup and regretting not being more forward thinking:

• You already want to go through the effort of installing some panels: Install more than you need for just charging the battery. Nothing silly, but a good few kw.
• Change your inverter to something a bit more user friendly in terms of solar and battery management.
  • Any decent off-grid/hybrid system will do. Ensure it has enough pv capacity, e.g. a 5kw system with at least 6000w of pv capacity
  • Moving from modified sine to pure sine gives the option of running motors/fridges/etc off the system even if you do not do so immediately
• Review your DB config
  • Move all lights to the inverter (if not already done)
  • Move selected (or all) plug circuits to the inverter (your preference)

You now run what you would run on your current setup purely on solar while the sun is out, while there is enough pv generation available to ensure the batteries are fully charged when the sun goes down.

Result: Less wear and tear on your batteries and some degree of future proofing should you want to scale up on pv/batteries at a later stage (e.g. wanting to move all low wattage appliances to inverter side)

Alternatively if you dont want to make that kind of outlay or future proofing is not a concern:

• Get a 3kw Mecer/Axpert type inverter (again for the pv/battery management user friendlyness more than anything else)
• Stick up the maximum amount of pv that can do (usually 1200-2000w)
• Leave everything as is DB side.

Thanks, just what I was looking for! Now I need to find a good supplier and cost estimates

 

[Adeptus Mechanicus]

View attachment 1460065

It did, but the OP's initial post seems to indicate he's debating expanding on his current solution, not just simply adding solar.

 

[StefanB7]

I have some pretty beefy lithium batteries and a basic (Mecer 2400) inverter that works well. Now that loadshedding has gone up so much I'm looking at integrating some solar panels to top up the batteries. As I don't trust the grid to be able to do that before the power goes down again.

What are the basic principles here? I need a more sophisticated inverter (estimate 10k) to handle input from the panels. The panels need to generate enough power to make up for the battery drain. From what I've seen at a friend's place you use batteries to store energy from the panels so the main batteries can also be topped up at night. So they balance out the supply when the panels can't generate energy.

The existing inverter has been connected into the DB board already - so the bulk of the cost should be the purchase and installation of the panels, the new inverter and balancing batteries.

Are there some rules of thumb to calculate the capacity of panels and balancing batteries needed to top up an existing battery installation?

For a quick and dirty short-term solution that wouldn't break the bank, I would just install one 420W solar panel and a Victron 75/15 charge controller, as here. According to the Victron charge controller's specs, it can handle up to 440W at 24V.
That means during the day, your batteries will last much longer if you've got load shedding. The batteries will also wear less since you'll probably not drain them as much during daytime load shedding.
If you don't have load shedding, you'll have the added benefit of the batteries charging faster since they will be charged by the inverter as well as the solar panel. <- This is not true, as pointed out by wingnut771

The solar panel will cost about ~R3000 and the charge controller R2000, R500 for cables the whole affair will set you back about R5500. And it is super easy to set up, just connect the batteries and solar panel to the charge controller, and it'll do its thing. The charge controller I linked has the ability to connect to your phone via Bluetooth that way you could configure it for your lithium batteries.

If your base load is about 400W (that's the internet, a TV and a few lights), you'll basically use no battery power during daytime load shedding and your batteries will stay full for nighttime load shedding.