Charging an EV With Solar: Timing Is the Problem
Generation peaks midday and most charging happens at night. The options are scheduling, a battery, or export and import under whatever tariff applies.
Pairing solar with an electric car is intuitively appealing and the appeal hides a scheduling conflict. The array generates when the sun is up. The car is usually charged when it is parked overnight. Those two facts do not align, and everything about doing this well is about resolving that.
The Conflict
Generation follows a daily curve peaking around midday. Household charging typically happens in the evening and overnight, when the vehicle has returned and is going to sit for hours.
Left alone, the array exports at midday and the car imports at night. Electricity is still being used and the array is still generating; they are simply not the same electricity.
Whether that matters depends entirely on how export and import are valued where you are, which is set by the utility and the regulator and varies by state and by utility. Where export credit approximately matches import cost, the timing barely matters economically. Where export is credited well below the import rate, it matters a great deal.
Check the arrangement that applies to your own account before choosing between the approaches below; see net metering and time of use.
The Options
| Approach | Suits | Limitation |
|---|---|---|
| Charge during the day | Home working, a second vehicle, weekends | Requires the car to be present |
| Solar-aware charging | A car parked during generating hours | Does nothing overnight |
| Battery storage | Evening and overnight charging | Cost, and the battery is sized for the load |
| Export by day, import at night | Where export is credited close to import value | Depends on the tariff arrangement |
| Shift charging to an off-peak window | Time-varying tariffs | Not solar directly; it is cheap grid energy |
The last row is worth stating plainly. Charging in an overnight off-peak window is often the cheapest option available, and it is not charging from solar. Conflating the two leads to buying equipment to solve a problem a schedule would have solved.
Solar-Aware Charging
A charger that monitors the site’s import and export can modulate its rate to absorb surplus generation rather than drawing from the grid.
Done well, the car takes what the array is producing above household load and backs off when a cloud passes or a large appliance starts. Many chargers offer a mode that charges only from surplus and another that tops up from the grid to guarantee a minimum charge.
It requires monitoring hardware at the meter and a charger that supports the function, and it delivers only while the car is there. For a household with a vehicle that is home during the day, it is the most direct answer available.
Storage
A battery charged from midday generation and discharged in the evening resolves the timing conflict without depending on the tariff.
The consideration is scale. A car’s daily consumption can be comparable to a household’s, so a battery intended to cover both needs to be sized for the sum rather than for the house alone. See battery bank sizing.
Where a battery is being considered for backup anyway, the ability to shift generation into the evening is an additional benefit rather than the whole justification.
Sizing the Array
An electric car is usually the largest single load a household adds.
Daily consumption depends on distance driven and on the vehicle’s efficiency, and it is worth estimating from actual driving rather than from a headline figure. An array sized for a house without a car, with a car added afterwards, is undersized for the new total.
Where a car is expected within a few years, sizing for it at the outset is generally cheaper than extending later, particularly if the extension would require inverter changes.
The Electrical Installation
The charger needs a dedicated circuit sized for its rating, and the panel needs capacity for it.
On older installations, panel capacity is frequently the binding constraint. Establish it before specifying a charger, because a panel upgrade is a significant piece of work and finding out late reorders the whole project.
Load management devices exist that allow a charger to share capacity with other loads by reducing its rate when the rest of the house is drawing heavily, which can avoid an upgrade.
What People Get Wrong About This
The first mistake is assuming the car will charge from the array without anything being arranged. Without control or storage, midday generation exports and the evening charge imports.
The second is buying solar-aware charging for a vehicle that is away every day. The function has nothing to act on.
A third is sizing the array before deciding about a car, and then adding one.
A fourth is overlooking panel capacity, which is the constraint that most often delays these projects.
And the last is confusing off-peak charging with solar charging. Both can be sensible, and they are different things achieving different results.
Related
Solar savings covers the economics this sits inside, and virtual power plants covers what a battery can do beyond serving the house.
Common questions
Can I charge an electric car directly from my solar panels?
Only while they are generating, which for most people is while the car is somewhere else. The technical connection is straightforward; the difficulty is that generation peaks in the middle of the day and most charging happens overnight.
What is solar-aware charging?
A charger that monitors what the array is exporting and modulates the charging rate to match, so the car absorbs surplus rather than drawing from the grid. It works well where the car is present during the day and does nothing where it is not.
Do I need a battery to charge from solar?
Not necessarily. A battery stores midday generation for evening charging, which addresses the timing directly. The alternative is exporting during the day and importing at night, which achieves a similar economic result depending on how export and import are valued where you are.
How much does adding an EV change the system size needed?
Substantially. An electric car is typically the largest single load a household adds, often comparable to a significant share of existing consumption. Sizing an array without accounting for it, and then adding the car, leaves the system undersized for the new total.
Does the charger need to be on a particular circuit?
It needs its own suitably rated circuit, and the electrical panel has to have capacity for it. On older installations the panel capacity, rather than the charger or the array, is frequently the constraint, and establishing that early avoids a surprise.
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