Pete Goes Off Grid – Part 2: Adding an EV

A couple of weeks ago I posted some analysis looking at how much battery storage I would need to add to my existing solar system at home to be fully self-sufficient & 100% carbon free with respect to my electricity consumption.

The post generated a bit of interest & some questions around the impact that electric vehicles, in particular EVs with bi-directional charging, might have on the challenge. So it was back to the Gridcognition software to run some more simulations.

This time around I’ve added an EV with V2H (vehicle-to-home) capability to the mix to see how that changes the picture. Does the EV provide sufficient storage capability courtesy of its own very sizeable battery such that stationary storage isn’t required? Or does it add a heap of additional energy requirement & make the 100% carbon free goal even harder? Well, both is the answer.

First, some modelling assumptions:

• Same underlying load shape & solar system yield as before
• My modelled EV has a 50kWh battery with a 7kW charger, which reflects a setup you can get your hands on today
• EV efficiency assumed to be 6km per kWh, which seems to be a widely accepted figure, but folks can correct me if I’ve got that wrong
• EV utilisation assumes 35km (5.81kWh) driven on weekdays & 120 km (20kWh) on weekend days
• Driving pattern, so when is the vehicle parked in the driveway or out on the road & how far does it drive, reflects the way I tend to live my life & drive my car e.g. lots of working from home. Needless to say that is a very important input.
• The EV battery gets first rights to any excess solar. If it doesn’t want it then the stationary battery can charge.

The V2H feature will draw energy from the battery to offset my grid-supplied energy to the house whilst still meeting the requirements for driving. There are a few occasions where it sails close to the wind. I could maybe run a more conservative optimisation that preserves a higher minimum EV battery SoC

And to the results:

• Adding an EV doubled my overall energy requirement for the year, taking it from about 3.5MWh to a tick over 7MWh for the house & car, but….
• …..in terms of grid-supplied energy the EV significantly reduced (by nearly 50%) my requirement, from 1.85MWh under BAU to just 0.97MWh. It achieved this by soaking up a lot of excess solar during the charge cycle so it often didn’t have to charge on-grid, & then feeding some of that back into the house load.
• So, I’ve doubled my overall energy requirement but halved by grid-supplied requirement. A neat trick.

Now for the not-so-good news: I’m no longer able to go 100% grid & carbon free.

The additional energy requirement shifts the limiting factor from being the amount of storage to the amount of solar generation, particularly through the winter months of Jun, Jul & Aug where I simply don’t have enough solar yield to cover the needs of the house & EV.