Basic queation - What kind of circuit allows me to throttle the power draw from two different supplies? i.e. 80/20 from a battery bank and utility power, respectively? Or 60/40, or 100/0?

Main idea - My utility company has off-peak ($0.048) and on-peak ($0.283) rates for electricity. I want to build something like a Tesla powerwall to capture the low cost power for use during on-peak times.

Specific problem - The battery bank I have in mind could provide 3KW if necessary, but I want the option to "throttle" the maximum output of the battery supply and supplement with mains power as necessary.

Example - litmiting to 2,800 watts:

Demand is 3,500 watts | Battery supplies 2,800 watts | Mains supplies 700 watts

Demand is 2,500 watts | Battery supplies 2,500 watts | Mains supplies 0 watts

Summary - I want to draw from my battery bank as much as possible (while limiting the output a bit to increase their lifespan / cycle count) and supplement with power from utility as needed when demand exceeds what the batteries can supply.

My research:

-Load Balancing- Seems appropriate if I want to draw equal power from 2 or more power supplies. Does not work if I want 80 amps from one and 20 amps from the other..

-Current monitoring- Using hall effect and placing some variable resistor in the line to adjust the voltage on the mains supply to limit current. But can this allow me to throttle the mains current down to 0?

  • \$\begingroup\$ I'd just run off batteries as long as possible, what's the point of saving your cheap electricity and using the expensive stuff? \$\endgroup\$
    – Finbarr
    Aug 15, 2022 at 12:29
  • \$\begingroup\$ Assuming I dont buy 5 - $700 batteries all at once. But one at a time, and add to it as time passes. I dont want to run batteries down to fast or past 50% capacity. \$\endgroup\$ Aug 15, 2022 at 19:11
  • \$\begingroup\$ There's little benefit in stopping at 50%, 25 to 30% is more realistic. Electric car manufacturers generally suggest down to 20% and up to 80%. Do some calculations to see if the expected increase in lifetime justifies the extra cost of peak electricity vs the stored off-peak electricity you're not using. \$\endgroup\$
    – Finbarr
    Aug 16, 2022 at 8:14
  • \$\begingroup\$ That's a fair point. And 30% isn't unreasonable. [$699.99, 2560Wh, 25.6V, 100A, 4000 cycles minimum, 80%-20%] = $0.114/kWh. Plus, $0.048 makes $0.162/kWh. Still, far cheaper than $0.283. The reason to throttle back the batteries is because at full load they would drain in 1 hour. On-peak lasts 4 hours. Drawing the same total energy over 4 hours at 1/4th the power provides the same total savings, but keeps the batteries cooler and extends the life. Even if it is not completely practical, I'd still like to understand the solution from a theoretical perspective. Intro to circuits didnt cover it \$\endgroup\$ Aug 16, 2022 at 11:53

1 Answer 1


When you are running from both battery and grid, the grid will control the voltage at the inverter terminals. It is up to the inverter programming/control how much power it delivers at that voltage.

How to choose what power to set on the inverter?

You could monitor the power being delivered by the grid, and choose an inverter power that keeps the grid difference small and positive, up to a defined maximum inverter power. When the load increases, the grid will automatically top up the difference, until your monitor/control system has responded.

The flip side of this is that if the load decreases suddenly, your inverter may try to export power to the grid for a few moments. If you're not set up for that mode of operation, then it may not be permitted, and get you into trouble with your supply company.


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