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How accurately does Charge HQ track excess solar?

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Charge HQ enables solar EV charging, by monitoring excess solar production over the Internet, then remotely controlling the rate of charge to match it. How well does this work? We think it's highly effective but you don't have to take out word for it. This article shares some detailed time series graphs showing it in action.

Before diving into the results, we'll start with a quick run through of the data we use and the core controls that are involved in making it work.

Deciding when to charge and at what speed

Solar data

Charge HQ uses data from home solar inverters and batteries to discover how much excess solar is available. The update frequency of the data for most of the inverters we integrate with is around 1 minute, with a small number of them being slower at 5+ minutes.

Rate of charge

Each time we get updated data, we adjust the rate of charge on the EV, but never more frequently than once per minute.

For single-phase chargers, the rate of charge can only be adjusted in 1 Amp increments (0.23 kW @ 230 V).

So if we see an excess of 0.4kW available, we’ll increase the charge rate by 1 A / 0.23 kW, leaving 0.17 kW of excess. If we see an excess of 0.2kW, we won’t change the rate of charge.

For 3-phase chargers, the rate of charge can still only be adjusted in 1 Amp increments but now across all 3 phases. So the rate of charge moves in 3 Amp / 0.69 kW increments.

Variable excess

The amount of excess solar varies every second, both due to solar production rates and changes in home energy consumption. Some appliances in the home have steady power consumption rates, not turning on and off very often - e.g. televisions, lights & pool pumps. Others constantly vary their consumption as part of their normal operation - e.g. air conditioners & washing machines.

In practice, this often means the available excess changes constantly. With the EV charge rate only being updated once per minute, there will be small amounts of import and export occurring between each update.

Starts and stops

Finally, there are times when there is no excess solar generation, or the excess is less than the minimum charge rate for the EV (typically 5A / 1.15 kW). When Charge HQ detects insufficient excess it keeps charging for 6 minutes to see if the sun will come out again or home loads will drop. If it doesn’t it stops charging. To avoid too much stopping and starting it then waits 15 minutes before starting again.

From a solar utilisation perspective, this means there will be a deliberate period where we use some grid energy for charging, and often a subsequent period where some solar generation is deliberately exported to the grid.

Solar Charging Performance

Putting all of the inputs, controls and operating rules together we get a constantly adjusted rate of EV charging. To see how this works in practice we’ll look at charging performance on three different days.

A sunny day with a single-phase EV charger

This first example shows a site with a home battery (with a 5 kW max charge rate) and a single-phase 7kW EV charger.

Charge HQ is configured in battery priority mode so the home battery charges to completion until shortly after lunch. As it ramps down around 12:30 the EV starts charging, then quite accurately tracks solar generation as it reduces through the afternoon. Notice the total consumption (yellow line) closely tracks the solar generation (green line).

Looking at energy production & consumption across the day:

Most of the unused solar energy that was exported to the grid occurred in two periods. First, early in the day when generation exceeded the home battery max charge rate, but where the excess was not above the minimum charge rate for the EV to start. Secondly late in the afternoon when the EV was unplugged (to drive).

A sunny day with a 3-phase EV charger

On a similarly clear sunny day to the first example, this example varies in that it’s configured with a 3-phase Tesla Wall Charger. The rate of charge on this charger can only be adjusted in 1 Amp increments across all 3 phases. This results in larger amount of energy constantly being exported to the grid and more notable steps in EV charge rate as solar production falls away through the afternoon.

Looking at energy consumption and production across the day:

Most of the unused solar was exported after 3 pm when the EV was unplugged (~2.5 kWh), with the remainder during the period of EV charging due to the larger 3 A steps from 3-phase charging.

The larger export volume compared with the single-phase site is a good example of why the choice between single and 3 phase EV chargers needs careful consideration, especially when solar charging.

Intermittent Sunshine

The first two days were perfect for home battery and EV charging - clear blue skies with uninterrupted sunshine! The next example was a relatively sunny day, but one with constant cloud interruption - the most challenging type for EV solar charging.

On this day there was a also pool heat pump running creating a substantial base load in the home and a 3 phase EV charger was used.

Generation and consumption are so variable it’s a little hard to read the graph so let’s break it down into individual series.

Solar production - after a rainy morning becomes highly variable throughout the day.

Home energy consumption - includes EV charging, but excludes home battery charging.

Home battery charging - the battery can be seen charging through to its max rate of 5 kW, then swinging through periods of charge and discharge as solar generation and home energy consumption vary.

Grid import - the net result is a lot of variation in power drawn from the grid, but not much total energy imported during the solar generation hours.

Lastly, the resulting EV charging profile in response to the available generation. Across the day there were 8 separate periods of charging, with 8 start and stop cycles.

The effects of the 6 and 15-minute timer delays for stops and starts can be seen both on the charging periods and grid imports/exports. On a highly variable day like this, they provided a buffer against what otherwise would have had a very high stop/start frequency - a bit like a 3-year-old playing with a light switch unsupervised. We think this is important.

Looking at the day as a whole:

Even with significant volatility in solar generation, the home battery and EV were able to make use of more than 90% of the energy produced.


Across our user base, for users with a solar data feed that updates every 1 minute, when solar charging is active, it’s common to see EVs charging with 90-100% of the energy coming from solar and less than 10% unused solar being exported.

How efficiently excess solar is utilised across the year is less about how quickly the rate of charge is adjusted, and more about:

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