Weekend Read: The solar-battery-EV dream

From pv magazine 09/23

Charging an EV from a home solar battery is an attractive proposition as it can optimize individual consumption and boost the environmental credentials of car ownership. Working out the economics of home solar, batteries, and EVs, however, is a complex business. There are multiple factors to consider, most of which hinge on geography and electricity markets.

“EVs are typically unavailable to absorb excess electricity during peak hours of solar generation so you need a stationary battery to do that,” says Nelson Nsitem, an analyst on business data company BloombergNEF's (BNEF) energy storage team. “However, residential batteries are expensive so you would need quite a few things to make it work. This includes very high annual electricity consumption, a lot of excess solar generation to shift to the evening, low or no feed-in tariffs that pay for excess solar generation sent back to the grid, and time of use (ToU) retail electricity tariffs.”

Some markets appear to tick all the boxes. Researchers at the University of South Australia (UniSA) have calculated homeowners who charge EVs during the day, from solar and a battery energy storage system (BESS) can save up to 39.6% in annual energy costs, compared to petrol-car owners.

The researchers analyzed configurations featuring ToU tariffs and real loads, and PV-generation data from South Australian households, varying daily load demand, solar and BESS capacity and costs, and power export limits. “For motorists with private car spaces, home charging is the most convenient option but for those still totally reliant on the electricity grid for their energy, the costs could mount significantly,” said Professor Mahfuz Aziz when the research paper was published.

Energy savings

The UniSA researchers calculated that when solar panels were added, around 20% less energy was imported from the grid and, with batteries, the figure was around 83%. When EVs were added, the amount of energy consumed rose significantly but grid consumption could be reduced by around 89%.

“Our results demonstrate that households with petrol cars can reduce their annual energy costs by 6.71% using solar panels, and by 10.38% with the addition of a battery system,” said Aziz. “Replacing petrol-based cars with electric vehicles can reduce annual energy costs by 24% and 32%, respectively. The most significant reduction (39.6%) can be achieved with off-peak charging.”

In countries with cheap, EV-specific electricity tariffs, the equation changes.

“The cost of electricity at these times is significantly cheaper than normal-rate electricity and therefore reduces the benefit of using your own solar electricity,” says Ryan Fisher, BNEF’s lead EV charging analyst.

For instance, OVO Energy’s Anytime EV charging rate in the United Kingdom is GBP 0.10 ($0.13)/kWh. This compares to an average electricity rate of GBP 0.35/kWh to GBP 0.40/kWh, in addition to the energy provider’s promise to calculate the greenest time to charge from the grid, within limits set by customers.

“If we take an average of 3 MWh per year, and said all of that was avoided by using home solar, and that there were no losses, that would require a large portion of the yearly solar production for a 5 kW solar system, estimates say around 4.5 MWh a year,” says BNEF's Fisher. “It would therefore only avoid GBP 300 a year, or GBP 3,000 over ten years. A residential battery system is more like GBP 10,000 to install.”

Learning by doing

There are other reasons homeowners might want residential storage, however.

“Some people want to have backup power and others are just willing to pay extra to ensure they consume their own solar,” says Nsitem. Revenue stacking can also apply. The use of cheap solar for EV charging can supplement the use of battery-stored solar power during peak-grid-price periods. Throw in virtual power plant (VPP) participation payments – such as the $2/kWh paid to Tesla’s Californian VPP members – and bill savings can add up.

In 2021, a survey was undertaken of 8,000 households in Baden-Württemberg, in southwestern Germany, by the university Rheinisch-Westfälische Technische Hochschule (RWTH) Aachen. The results showed 30% to 35% of homes had solar, a stationary battery, and an EV. Since then, high electricity prices, rising EV adoption, and the trend for installing batteries alongside solar have likely raised the number.

Product announcements reflect the trend. In July, Tesla added a “charge on solar” feature to its Powerwall app enabling owners to automatically charge EVs from their solar battery.

However, the majority of EV charging management systems cannot communicate directly with stationary batteries. On the other hand, solar inverter manufacturers such as SolarEdge, Fronius, Growatt, SMA, and Sungrow – as well as battery suppliers including Sonnen, Senec, and Solarwatt – offer platforms that recognize home batteries when they are paired with PV and their proprietary electric-vehicle chargers.

As households continue to electrify, integrating all energy endpoints is likely to become an industry standard. Given the importance of interoperability, it is advantageous to have devices provided by one manufacturer. Separate apps and disjointed hardware can make things complicated for homeowners.

“Over time, companies will likely discover ways to integrate more of the power electronics for the solar, residential storage, and EV charger, which may help with the business case,” says BNEF’s Fisher.

Efficiency conundrum

Considering the storage capacity of most home and EV batteries, it appears odd to use stationary devices with, on average, five times less capacity, to charge the ones in cars.

“The home battery doesn’t know whether it is giving to another battery or to your washing machine,” says Gautham Ram, assistant professor of electric mobility at Delft University of Technology (TU Delft). “As long as it does the same amount of power and energy and the same number of cycles and depth of discharge, the aging is going to be similar. At the same time, you have to accept that you buy a battery because you want to cycle it frequently, so battery degradation is part of the picture. However, since EVs and heat pumps are flexible loads, it would make more sense to use the home battery for less flexible loads and reduce this battery’s aging via intelligent power control.”

Some residential batteries on offer today promise around 10,000 charging cycles during a 10-year warranty period. The average EV travel distance of 40 km per day corresponds to 7 kWh of charging energy – less than what a large home battery could store on a sunny day.

“Let us assume the EV charging adds 50% of a full cycle per day to a 14 kWh system during summer. Over a year, this adds up to about 100 full cycles, added to the same cycle number for the house, this amounts to 200 full cycles,” explains Jan Figgener, head of grid integration and storage system analysis at RWTH Aachen. “Even if 300 instead of 200 cycles per year are reached, the 3,000 full cycles within 10 years is still well below most warranty conditions. You won’t need all the cycles that the warranty states, in most cases, because the calendar aging will determine the product lifetime. Batteries age even when not in operation, just as we age while we sleep.”

The limitation does not lie with the PV system, either. While it’s clear that an array should be as big as possible to maximize the benefits of self-consumption, a typical 10 kW system could be sufficient to charge both EV and home storage.

Differing loads

“A big house might need around 5 kWh overnight and the EV could need 7 kWh for the average daily commute, which adds up to 12 kWh,” says Figgener. “You just need a couple of hours to produce this on a sunny day. The efficiency of EV charging is dependent on the power electronics, which show high efficiency at high power and low efficiency at low power. Here, a conflict between the power demand of the EV and the house arises.”

EV charging requires higher power – in the kilowatt range. House consumption, though, is typically around a couple of hundred Watts overnight. If an EV is charged solely from the stationary battery, the large inverter needed would, consequently, lead to high losses while covering household energy consumption.

“The best outcome would be to charge the EV directly from the PV system and this should only be necessary once or twice a week,” says Figgener. “The larger PV power matches the EV demand well and round-trip efficiency losses of the home storage system are avoided.”

In such a case, the residential battery could be smaller and would only be used as backup to charge the EV. Its main purpose would still be to cover household loads.

There are other ways of making solar-battery-EV synergy more frictionless. According to TU Delft’s Ram, an optimal way to improve efficiency is to integrate everything on the DC side with only one inverter. In such a setup, when power goes from solar to the home battery, it doesn’t have to be converted to AC and then back to DC to charge the EV battery.

“The DC-coupling can improve the power conversion efficiency by anywhere between 5% and 10% depending on if you’re having low or high power,” says Ram.

With only one inverter needed in this configuration, instead of three, the capital cost investment of the whole system would be much lower.

“With the DC integration that we are working on at TU Delft, and newer power electronics technologies based on silicon carbide, we can push the round-trip efficiency to new highs, especially at lower power, as well as save on the capex [capital expenditure] side,” Ram says. “However, we also need to think about bidirectional charging, which can offset an investment in the extra battery pack and deliver the same, or more value to the home.”