We're already off-grid. Do we need more panels and batteries for an EV? | The Driven
Off grid house with solar panels

We are encouraging questions from readers about electric vehicles, and charging, and whatever else you want to learn. So please send them through and we will get our experts to respond, and invite other people to contribute through the comments section.

Our fifth question comes from David, who asks:

Two years ago we built a new house that was planned to be off-grid. We have 11KW of panels and 1350 Ah of storage in 24 lead acid batteries. The system works well for our current needs. What would we need to add if we wanted to add an electric car to the mix?

Our batteries are often 100% charged by mid-morning, but we need some power from the 8kVA diesel generator during prolonged cloudy spells (once a month or so).

If we wanted to use the EV during the day, the normal option of recharging from the grid at night is not possible… It’s a lot of variables to juggle.

Can you please suggest some combination of equipment and usage/recharging strategies that off-grid people could use?

As with all off-grid systems, the dilemma of adding a new load to the electricity supply presents two related issues – but each having a separate solution.

First of all – a new load will add to the total current draw (in amps) to the existing installation. If the inverter, solar panels and/or battery supply are not up to supplying that high a current if everything turned on at once, then the system will either simply shut down or, if not properly designed, can be damaged.

However, EVs are easy to both ‘load shift’ (charge at otherwise low electricity use times) and ‘demand manage’ (reduce the current draw based on a set maximum) if using timers and an appropriate EVSE.

Therefore adding an EV to your off-grid system does not have to create the need for upgraded PV systems or inverters based on current draw alone.

Demand managing can either be manual – only plug it in when it is really sunny/windy depending on your alternate supply, or automatically by using an EVSE (such as the Zappi EVSE.

See https://www.evolutionaustralia.com.au/product-page/zappi-7kw-type-1-or-type-2 that can throttle back/turn off the EV charging when your solar/alternate supply is not great enough to support it. (See answer to previous question).

Secondly, a new load also adds to the total kWh used each day – so the system storage must be able to supply the new load AND the existing one AND still provide the buffer needed for normal reduced winter sun, cloudy days etc.

It is this second issue where EVs will most likey cause grief for an off-grid system. This because when it comes to off-grid systems – there are only so many kWh to go around and EVs can add 50 – 100% more load to what are usually VERY energy efficient set-ups!

Consequently, the storage buffer and/or PV or other energy supply system must be upgraded to meet this extra kWh need. There is no way around this one.

For daytime charging – extra PV equal to the EV’s calculated should be added or, for overnight charging, extra battery storage. (Or find a DC fast-charger nearby and use that instead of your household supply for the majority of your EV charging!)

BTW: In this situation, it may also be worth considering buying a plug-in hybrid EV (PHEV) rather than a straight battery electric EV (BEV). In that way you could use and recharge at home the BEV part of the PHEV when energy is plentiful and still have the petrol motor to fall back on when the home energy generation budget is tight.

If other readers have some suggestions on this, please add in the comments section below.

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