Tesla Powerwall Battery Charging Options

How the Tesla Powerwall Battery Works with Solar Panels

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Solar Electricity and Battery Backup

The battery is the heart of a battery backup system, regardless of whether it is charged with solar panels, wind generators, utility power, or a generator. Battery backup systems have been around for a long time, but the Tesla Powerwall battery has sparked a huge amount of consumer interest. I have already written about what the Tesla Powerwall is and isn’t. Below we go into how any battery backup system works, the other required components, and some of the charging options with utility and solar power.

Mouse over the image hotspots to learn more about each component. (Note: on mobile devices, the hotspot text may appear below the images).

The Tesla Powerwall Battery

Inverter/Chargers – Power Conversion

Critical Loads

Solar Panels

Solar Charging Equipment

There are multiple ways that solar energy can get transferred to your Tesla Powerwall battery. Each has it’s advantages and disadvantages.

  1. The traditional way is to use a charge controller to maximize and regulate the solar energy delivered to batteries. Because batteries have maximum charging rates, too much solar power could damage the battery. Charge controllers work in tandem with inverter/chargers to deliver excess electricity to the utility grid. This is the most efficient way to charge batteries from solar panels, but the least efficient way to deliver excess energy to the electric grid.
  2. Grid-interconnected string inverters take the output of several solar panels and deliver power to your home and excess power to the electric grid. Through a process called AC coupling, the string inverter can also send electricity to your battery through the battery’s inverter/charger. This is an efficient way to deliver excess energy to the grid, but less efficient at charging batteries with solar energy.
  3. Microinverters take the output of a single solar panel, maximizing each panel’s output individually and monitoring performance. Like string inverters, microinverters can also be AC coupled to a battery’s inverter/charger. This is the most efficient way to deliver excess energy to the grid, but less efficient at charging batteries with solar energy.


  1. Are you saying that the Powerwall battery does not save energy?

    1. Author

      That’s right, Debbie. The Powerwall (or any battery) does not save energy. In fact, it consumes electricity. You need a source to charge the battery, and the charging efficiency is not 100%. There is some loss of usable electricity whenever you charge and discharge a battery.

      Now a Powerwall will store energy, but that is different than saving energy.

      In theory, you can save money, but only if you have time of use metering at your home. You could store excess energy when utility electricity is cheap (at night), and then draw upon that stored energy when electricity is more expensive (during the day). This requires sophisticated electronics to manage your energy use, and only works where Time of Use metering is required for residential utility customers.

  2. Tesla is all hype right now; their $800mill in reservations is impressive, but its a no-money down deal. Enphase Energy, Aquion Energy, and others all have batteries in their line-up. Enphase’s alliance with Eliiy is powerful, and rumor has it that Midnight Solar is helping them with the AC-coupled solution necessitating a secondary battery farm. Still, only critical loads will be able to be served — BeUtilityFree and IronEdison market the NiFe battery by Changhong, the successor of Thomas Edison’s NiFe battery which Exide bought and then went bankrupt and sold off the assets to the Chinese. The Edison-Varga NiFe battery @ 1000Ah, 48V is about $25K plus about $7K for the electronics. This is still way to expensive for the average green enthusiast. Bill Gates invested in Aquion, but their systems are very pricey, too. I am anxiously awaiting for the AC battery from Enphase and have been told that the AC battery has been part of Enphase’s energy concept since day #1. Enphase has told me that they expect shipment of the product Q1 2016. We shall see!!

    1. Author

      I am eagerly awaiting the Enphase solution as well. The initial specs report a very small battery at 1.2kWh, but they are taking a module approach, much like their microinverters for solar panels. This architecture will allow replacement of bad “cells” over time and each battery should work independently to it’s maximum ability. That’s an exciting development that has not really been explored. It does not sound like it is the most cost effective approach, but the benefits may be worth the extra cost of DC->AC conversion electronics at each battery, and Enphase has proven that their monitoring capability and customer service is world-class. They are focusing on a customer friendly solution, which is exactly what the industry needs right now. It is practical? That remains to be seen. This will definitely be a topic of a near-future post here.

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