Configure separate solar & grid charge limits

One of the special features of Charge HQ when used with a Tesla EV is the ability to set a different charge limit when charging from the grid vs charging from solar.

We highly recommend all users make use of these limits as it greatly increases how much charging occurs from solar whilst also offering a set-and-forget solution.

First up, configure a scheduled charge period (normally overnight) with the charge limit being the minimum amount of charge you’re comfortable starting each day with. Say 50%.

If you plug in your vehicle every night, it will always charge up to 50% from the grid but not more.

Then, set the vehicle charge limit to the maximum suitable level for your car battery. Say 90%. You can set this limit either in the Charge HQ app or your Tesla EV, it’s the same control and remains in sync.

With this higher upper limit, whenever there’s excess solar available, your EV will charge up to 90%. As long as you stay above your lower grid charge limit of 50%, all of your charging will come from your rooftop. But, if you do a lot of driving or experience a few days of cloudy weather in a row, your EV will keep topping up from the grid (e.g. overnight), but only to 50%. When the sun next comes out, you’ll have at least 40% of your battery capacity available to soak up your excess solar and avoid exporting it to the grid.

The following chart gives an example of how Charge HQ can make use of the two charge limits to avoid charging from the grid when not required.

• Monday: the EV charged overnight from the grid to 50% and then drove during the day using most of this charge
• Tuesday: an overnight grid top up to 50% then solar charged to 70% before being driven again dropping back to 60%
• Wednesday: no overnight charging required with the battery still above 50% but there was enough solar to fill it before the EV was driven again taking it back to 60%
• Thursday: a small top up from solar before driving drops the battery to around 30%.
• Friday: overnight top up from the grid stops at 50% before daytime solar again fills the battery to 90%

By using different limits for grid vs solar, the system automatically minimised overnight grid charging, avoiding it entirely on two of the days whilst still ensuring the car always had a minimum of 50% charge each morning.

How is this different to other solar EV chargers?

Regular wall chargers currently don’t communicate with the vehicle about the state of charge (SOC) of the battery. They just send power to the car until the car reaches its configured charge limit and the car tells the charger that it can’t accept any more power.

This means that charging from the grid or your rooftop solar will charge up to the same limit that’s set in the vehicle.

Controlling how much you charge from the grid and leaving space for solar charging requires either:

• Adjusting the duration of your scheduled charge periods
• Setting a desired number of kWh of charging (supported by some chargers)
• Choosing not to plug in on days when you know the car already has sufficient charge
• Setting your charger to only charge from solar then manually starting grid charging when you notice the battery being low

These methods work, but with variable excess solar availability and variable vehicle use it requires active management on a day to day basis.

This is the beauty of Charge HQ: by knowing the SOC of the EV, you get a set-and-forget system that maximises how much charging you achieve from your home solar.

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