EV explainer: charging your electric vehicle with excess solar power

The cleanest, cheapest and most convenient way to charge your EV is with your own rooftop solar. Tim Eden provides some tips.

According to an Ausgrid customer survey from 2020, some 60 per cent of households with EVs also own solar systems, much higher than average solar adoption rates.

More recent data is hard to find, although it is safe to assume this trend has continued with EV sales growing exponentially and solar installations remaining strong over the past few years, and strong anecdotal evidence that the two are linked.

If you have a rooftop solar system, charging your electric vehicle with solar power can mean simply waiting for a sunny day to plug in or scheduling your charging sessions during the middle of the day.

Even if you don’t have solar, charging around midday is a good idea because electricity from the grid is mostly renewable with peaks of 50 % or more coming from solar.

OpenNEM graph showing solar power generated 50% of NEM electricity at 1 pm on 1st May (Source: OpenNEM)

This is especially true in some parts of the grid like South Australia, where solar power alone can account for more than 100 % of local electricity demand.

Charging during the solar peak also ensures your electric vehicle is using as much green electricity as possible, because peak renewable generation corresponds with a trough in carbon emissions. Some fast charging stations have been trialling new tariffs that include heavily discounted rates in the middle of the day.

OpenNEM graph showing carbon emission trough that coincides with the solar peak (Source: OpenNEM)

It is also worth mentioning that most public EV charging operators purchase 100 % accredited GreenPower, or have direct contracts with renewable energy operators, so charging your vehicle away from home does not undo all the effort of charging with solar.

Using this simple approach to charging does not necessarily guarantee your EV will be powered by sunshine alone. To make the most of your solar system and ensure the cleanest electricity supply, the best way to charge is using excess solar power.

What is excess solar power?

Excess solar power refers to electricity generated by your solar system that is not being directly consumed by the household. In most cases this excess power is fed back into the electricity grid and results in a small credit on your electricity bill, although feed-in tariffs have gradually been reduced over the years.

The following image from my custom electricity monitoring setup might help to illustrate the concept. In the example below, my solar system is generating 2.5 kW and the house is only using 278 W. The net result is nearly 2.3 kW is being sent back into the grid, this amount is excess solar.

Screenshot from my custom electricity monitoring dashboard showing excess solar (Image: Tim Eden)

I took inspiration from the power / regeneration bar in my Model 3, with the size of the yellow bar on the left indicating how much excess solar is available. The data is displayed on a tablet in my lounge room, so I can glance across the room and easily see if I have enough solar power to switch on large appliances like the dishwasher or washing machine.

If you are not an energy nerd like me, you’ll be happy to learn there are more advanced and automated solutions available that can do the hard work for you.

Are there any downsides?

The main downside to charging on excess solar is that it may take longer to charge your car, particularly if you have a small solar system or use a lot of electricity. To counteract this, you can adjust your charging habits to plug in every day rather than waiting until the battery gets too low.

If you ever need to charge quickly, all of the solutions below can be overridden to use grid electricity as well as solar power to top up your EV in a hurry.

Slow down your charging

If you do not have an energy monitoring system or cannot take advantage of the more advanced solutions described below, the simplest way to charge using excess solar power is to slow down your charge rate.

This will minimise the amount of grid electricity needed to charge, but it will not be perfect. If it gets cloudy or you start using power hungry appliances, the household may occasionally use more power than your solar system is generating.

Slowing down your charge rate can be achieved by adjusting the charging settings on your EV to draw less power. Alternatively, using a portable EVSE plugged into a normal 10 A powerpoint will draw less power than using a dedicated wallbox.

Third-party apps

A few third-party apps are available to optimise EV charging with excess solar power from your system. Compatibility with your EV and solar system will depend on the app, so start by reading through the getting started guides on the linked websites.

These apps also require access to control charge settings in your vehicle or EV charger along with electricity usage data to function. Some of them may still support limited functionality depending on compatibility with your solar and charging equipment.

The most popular app is Charge HQ, which works with a range of different chargers and solar, battery or energy monitoring systems. Even if your solar system is not supported, Charge HQ still works with compatible EVs or chargers to optimise charging based on renewable energy levels or wholesale grid electricity prices.

Another similar app is ev.energy, although this does not appear to support many chargers for its solar only feature. Some electricity retailers such as Origin Energy also offer smart charging features through their own apps, but these are less flexible than Charge HQ.

Wallboxes with smart solar features

Several wallboxes from different brands include features to charge your EV using excess solar energy. These wallboxes require some additional hardware to be installed in your electricity meter box to monitor the current flow between your house and the grid. The advantage is these chargers will work with any EV and type of solar system.

The most well known charger in this field is the Zappi by myenergi. Other examples that I’m aware of are Wallbox, Ocular IQ Solar and Evnex. Similar to the third-party apps above, these wallboxes are flexible and can charge your vehicle using excess solar alone or solar combined with grid power to charge quickly if necessary.

Depending on the brand of inverter your solar system uses, there are also smart EV chargers that integrate directly without the need for additional grid monitoring hardware. The SolarEdge EV Charger is an example of this.

Tesla Charge on Solar

If you are all-in on the Tesla energy ecosystem with one of their EVs and a Powerwall home battery then you can take advantage of the Charge on Solar feature. This feature can be combined with other features like scheduled charging if you have time of use tariffs or off-peak electricity rates.

Tesla charge on solar in operation (Source: Tesla)

Tesla’s Charge on Solar also allows you to set different charging limits, a lower one for minimum daily charging using grid and solar power and a higher limit that just uses excess solar. This way your vehicle will recharge to a guaranteed minimum level each day before topping up with as much excess solar power as possible.

The Charge HQ app also supports utilising two different charging limits with Tesla vehicles, so it can be used in the same way to maintain a minimum charge level from grid power before topping up with excess solar.

Tim Eden
EV & renewable energy enthusiast
May 26, 2024
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