battery system with power output control for maximizing self consumption bypassing net metering.

[cristipurdel]

I am looking to add a grid tie battery system to fully utilize my solar production.
I have a couple of 80W sunpower panels hanged on my balcony, and daily I can produce a maximum of 0.6kWh during summer by feeding around 130W back into the grid via a grid tie inverter.
What I want is to find a grid-tie battery system which should store the 0.6 kWh battery during sunlight but output a constant power of around 100W which should cover my minimum consumption (vampire draw + etc.)
Basically should be some sort of "legion solar" type but without the hustle of playing around with the mains/breaker box cables.
I guess I would need a charge controller but I have not seen many videos on how to limit the power output and to oversize the battery in order to increase also its lifespan.
It would be really helpful if I could see a video detailing this or some diagrams of sort.

The end goal would be the following scenario.
6x80W panels producing a maximum of 400W.
The panels should be connected to a charge controller / hybrid inverter which gives:
100W back to the grid constant to cover my minimum consumption (or maximum of 100W without feeding the batteries)
0-300W to the battery system. When the sun is not shining then the batteries take over and discharge up to a defined % (e.g. 30 SoC). The batteries should be connected to a grid tie inverter (I already have one of 260-300W max ).

 

[DThames]

If you are doing this in the US, you are most likely breaking some rules and I would not advise you do that. However understanding how a system like this might work is a topic that some might find interesting.

First understand that in this context there is a difference between Controlling your inverter output and Limiting the inverter output. A Control would allow you to set "no output higher than 200 watts" for example. If the battery was holding up, the output would always be 200 watts. You could adjust to 150 watts, 250 watts, etc, under your control. A Limit would be, "Based on measuring in real time, don't output enough to make the meter go backwards". While this is also a control, it is automatic inside the inverter and you can't limit it to 100 watt for example. If you started your electric stove, water heater, etc, your battery/inverter would deliver huge output and run the battery down right away if it was only limited by "Don't put back to the grid".

On the small scale you described, I doubt you will find a single piece solution. There is available a 1000 watt inverter with a current transformer (sensor) that will limit output but will not put control the level of power in your hands. It will output whatever it can get its hands on up to the 1000 watt max which could run your battery down sooner than you desire.

Let's talk about control for a minute. Suppose you put a shade over one of your two solar panels. You could control the output level with cardboard. But I would guess you want something more automated than someone waving cardboard back and forth while looking at two meters. What you are doing with the cardboard is controlling the current that flow from the solar to the battery. The same affect could be obtained by putting a control on the current(amps) from the battery as it goes to the inverter. The inverter still converters to AC but only outputs what you allow based on current (amps) setting. You would need an adjustable constant current power supply to do that. You could set it to X number of amps on that power supply so your inverter output would be under your control. But this control is blind to your exact load/demand is.


Solution 1. Output controlled but not limited:
You could use one of these inverters and control the input current with an adjustable constant current DC to DC power supply between the battery and the inverter.

Solution 2. Output limited but not controlled
Video: Limited grid tie
Product: (note the "sensor" which is a current transformer that measures current flow)

https://www.amazon.com/dp/B076KBCHBM/ref=twister_B076KCYP4D?_encoding=UTF8&th=1

Solution 3. Output limited and controlled
Same as option 2 but add an adjustable constant current DC to DC power supply between the battery and the inverter.

 

[cristipurdel]

Thank you DThames for your solutions.
This is exactly wahat I wanted to know.

Solution 1. Output controlled but not limited:
Should be the cheapest way to reducing your "base" load

Solution 2. Output limited but not controlled
It has some disadvantages:
Don't Buy a SOLAR Grid Tie inverter with power limiter until you watch this video
Plus I would need to tinker with the main cables and bring the solar cables across the apartment.

Solution 3. Output limited and controlled
Although reasonable possible, for this level of complexity I would prefer to have a hybrid on-grid off-grid system with smth like Enphase IQ8.

Once again thank you for "grid tie inverter with power limiter" and "adjustable constant current DC to DC power supply"

For Solution 1, I could also use 2 victron charge controllers (first for controlling the battery voltage and charging speed) and the second for controlling the Amps sent to the inverter?

 

[DThames]

Thank you DThames for your solutions.
This is exactly wahat I wanted to know.

Solution 1. Output controlled but not limited:
Should be the cheapest way to reducing your "base" load

Solution 2. Output limited but not controlled
It has some disadvantages:
Don't Buy a SOLAR Grid Tie inverter with power limiter until you watch this video
Plus I would need to tinker with the main cables and bring the solar cables across the apartment.

Solution 3. Output limited and controlled
Although reasonable possible, for this level of complexity I would prefer to have a hybrid on-grid off-grid system with smth like Enphase IQ8.

Once again thank you for "grid tie inverter with power limiter" and "adjustable constant current DC to DC power supply"

For Solution 1, I could also use 2 victron charge controllers (first for controlling the battery voltage and charging speed) and the second for controlling the Amps sent to the inverter?

I am not familar with the Victron.
A grid tie inverter should try to deliver its full rating back to the grid if it can get the power at its input terminals to do that work. I think you can short the output of a solar panel and it is okay with you doing that. So an inverter can load a panel down until the voltage drops and the inverter just gets what it gets (for power). Or when an inverter loads a battery, if the demand is high, you see a voltage drop on the battery as power is shifted to the battery's internal resistance, making heat. But when you connect one to a constant current power supply (charger), I am not really sure how the inverter will load it. I would guess (really a guess) that the inverter might load the supply until the voltage dropped to near the inverter mininum input voltage, at which point it would should not load it further.....I guess. It might be wise to get some small cheap supply and do some tests. That way if you cook it you will not be out a lot of cash.

 

[cristipurdel]

I think I will wait until I find a smart grid-tie inverter with amperage control which can directly smart manage the batteries since:
"grid tie inverter with power limiter" - is too complex for me since the mains are not so easily accessible
"adjustable constant current DC to DC power supply" - allthough they are cheap, I do not believe they will survive in a cold/wet environment

Cross link for GTI:
Grid Tie Inverter Load Controllable

 

[SCM]

A grid-tie inverter shall be UL listed, with net meter approved by the utility company.
Main reason: A grid-tie inverter must product the perfect sine wave AC power, and it must be synchronized with the grid power. Any offset sine wave will product DC content in the grid, result in lower efficiency appliances in the house, or worse affecting the power quality for other customers in the same grid.
A hybrid system would be good for the system described. The solar power system with off-grid inverter can power the appliances through the transfer switch. When the battery is low, the grid power will power the load through the transfer switch, and leave the battery charging until voltage reaches a set level. This is how you would maximize the solar power usage.

 

[Picasso]

This cute little product can be had for $1k USD if you ask the vendor nice. https://www.aliexpress.com/item/32995765790.html

 

[Jordi]

Thank you DThames for your solutions.
This is exactly wahat I wanted to know.

Solution 1. Output controlled but not limited:
Should be the cheapest way to reducing your "base" load

Solution 2. Output limited but not controlled
It has some disadvantages:
Don't Buy a SOLAR Grid Tie inverter with power limiter until you watch this video
Plus I would need to tinker with the main cables and bring the solar cables across the apartment.

Solution 3. Output limited and controlled
Although reasonable possible, for this level of complexity I would prefer to have a hybrid on-grid off-grid system with smth like Enphase IQ8.

Once again thank you for "grid tie inverter with power limiter" and "adjustable constant current DC to DC power supply"

For Solution 1, I could also use 2 victron charge controllers (first for controlling the battery voltage and charging speed) and the second for controlling the Amps sent to the inverter?

Cristi! I would die to know what you did at the end with your project here and how it looks.

For recreational purposes, I have been trying to do something similar (If not the same) in combination with a small 0,3 KWh battery and a 100W solar panel.

I have found and ordered the following on-grid inverter which provides MPPT for solar panels and battery mode without MPPT function + load regulation from 60W to 350W. (waiting arrival of product)
https://es.aliexpress.com/item/4001045482425.html?spm=a2g0s.9042311.0.0.10a963c09aZoms

I will test a few configurations to see which suits best to my needs or has the highest output.
Currently at my place: Vampire load min. = 5W, Vampire loads max. = 70W (fridge on)

Option 1) Panels connected to battery via PWM and inverter to battery in battery mode at 60W regulated load.
Option 2) Same as 1 but with DC-DC buck to limit load to 10-20W and so allow continuous operation.
Option 3) Panels direct to inverter in MPPT mode = less self-consumption compensated and more excess to net.

What did you do, what has been so far your results?