How to best harness solar energy to charge EVs

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Photo / Supplied

Photo / Supplied

For many EV (Electric Vehicle) owners, the motivation for switching to a zero-emission vehicle is to reduce environmental impact.

This objective is now extending to “whole of life” where not only are the raw components, manufacturing processes and second-life use of batteries under the microscope, but also how clean the EV’s “refuelling” energy is.

When it comes to clean energy, NZ is already better than most countries. According to NZTE (NZ Trade and Enterprise), we currently have the fourth-highest renewable electricity percentage in the OECD (84 per cent and growing) and we’re aiming to achieve 100 per cent renewable electricity by 2035.

NZ is also looking to transition to a fully carbon-neutral economy by 2050, so as we move away from carbon-intensive energy supplies, renewables will play a crucial part in where our electrical energy comes from - and arguably very few sources are cleaner or more relevant to EV owners than solar.

Locally, the installation of solar power, or photovoltaic (PV) solar panels is on the rise, with an estimated 35,000+ solar power systems installed in NZ as of mid-2021 and producing 186.7MW of grid-connected PV power as at the end of last year.

There are not any direct NZ government incentives for homes to add panels (although there are efficient home grants available), but the huge price drop of PV systems over the past decade has seen a dramatic growth in home installations.

Solar power is increasingly being seen as an attractive way for households to reduce energy costs, gain independence from the grid (and sell clean energy back to it), plus of course, that guarantee of sustainably-generated electricity.

But how do we best harness this sun-kissed energy to charge EVs? The home PV system transfers the energy from the roof panels through an inverter to a distribution board that manages the import and export of electricity from the home. The energy can flow to appliances, including an EV, before requiring additional power to be imported from the grid.

Without appliance demand or an additional wall-mounted battery installed to store the energy, any unused solar energy is “sold” via export back to the grid for use elsewhere. In most cases, charging an EV with solar can save more money than the revenue from supplying to the grid. For instance,

Genesis’s FiT (Feed in Tariffs) or buy-back rates will credit your exported electricity at 12c per kilowatt hour, excluding GST.

At peak performance, when hit by the midday summer sun, an average-size home solar installation (3-5kW with eight to twelve panels) could effectively charge a Tesla Model 3 at 25km of added range per hour. That’s enough for an average daily commute within 1-2 hours of charging.

This appears straightforward, but conditions are not always ideal, and solar output can fluctuate greatly from minute to minute with the weather. This can be a problem when it comes to charging an EV for a couple of reasons.

First, not all chargers can offer as much power as a PV system produces. Using an 8 amp portable plug charger (granny charger) while solar is available will limit you to around 1.8kW, effectively missing out on the charging potential of any solar energy generated above that threshold.

The second issue is caused by technical limitations when offering low rates of charge to an EV. If there is less than 1.5kW (6 amps) of PV output available, any EV charger will need to draw additional power from the grid to meet this threshold before it can offer a charge to the vehicle. So if a cloud passes overhead and PV output drops to 1kW, a charger will start importing the required 0.5kW from the grid in order to continue the charging session. That means you’ll no longer have a 100 per cent sustainable “free” charge.

This is where the use of a “smart charger” can help, such as one from Evnex. Its chargers use a feature called solar export diversion that actively monitors excess PV output via a power sensor and can automatically pause a solar charging session during times of low solar energy availability.

This threshold for pausing can either be set at the point where imported grid energy is required to continue charging (1.5kW) or can be easily configured to a lower cut-off point deemed as acceptable by the customer (where smaller amounts of solar power can be utilised but require a little additional imported grid power). You also won’t have to worry about your PV system producing more than its EV charger can offer, with either 7kW or 22kW power rating options.

In short, solar export diversion gives you the most control when managing your solar for EV charging. You can simply leave your EV plugged in during the day knowing it will only charge from the sun, rather than waiting until after 9pm or whatever your off-peak “cheaper” times are - so you get the best of both worlds.

Evnex’s system can be added either before or after you get solar installed, as it’s solar-ready, with the power sensor doing additional duties such as home overload protection. So you’re future-proofing should you choose to install solar further down the road.

Using solar means that you are ensuring a clean charge of your EV, but to make the most of your solar energy, evidently you have to be “smart”.

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