IEEE Spectrum reports on solar-powered data centers that turn otherwise wasted sunlight into AI compute.

As billions of dollars flow into building data centers, many projects face a constraint that goes beyond construction: securing enough electricity to power them. San Francisco-based Rune Energy is reversing the usual approach. Instead of building data centers and waiting for grid power, the startup places them where there is excess power that the grid cannot absorb.

The company's modular data centers, called Renewable Energy Linked Intelligent Compute (RELIC), connect directly to the direct current (DC) electricity generated by a solar array. Each RELIC module is a self-contained system with its own network equipment, cooling system, power electronics, and GPUs, all housed in a container about eight feet long, two feet wide, and five feet high. A module can draw up to 100 kilowatts of power and weighs about 2,000 pounds.

"It's our estimate that there's 50,000 gigawatt hours of energy wasted by utility-scale solar every year," says William Layden, Rune's co-founder and CEO. A solar farm can produce more electricity than the grid can absorb at certain times, especially when generation is high and demand is relatively low. Curtailing that generation means reducing output even though the panels could otherwise be producing electricity.

"We're already in an energy abundant future," says Varun Palivela, Rune's co-founder and CTO. "But the energy abundance is stranded in time and space." The power constraints facing large data centers, he adds, are "not a real bottleneck, but a resource allocation issue."

**The direct current advantage**

In addition to curtailment, electricity is wasted through equipment outages, conversion losses, and a phenomenon called inverter clipping. Inverters typically convert the DC electricity produced by solar panels into alternating current (AC) for delivery to the grid. When a solar array produces more DC power than its inverter can handle, the inverter clips the excess.

Because Rune taps into DC power before this conversion, it avoids some of these potential losses. Constance Crozier, an assistant professor at Georgia Tech's School of Industrial Systems Engineering who has studied data center energy demand, says the main advantage won't come from eliminating this conversion step. "DC to AC conversion is typically quite efficient, 95 to 97 percent, so I don't expect the efficiency savings to be exceptional," she says. "I would imagine the main advantage is reduced component cost."

Rune signs power-purchase agreements with solar-farm owners, buying electricity directly from them. This also gives the solar facility income for power that would otherwise go unused. Palivela says utility-scale solar has a standardized voltage of around 1500. Although Rune's technology doesn't need an inverter to convert DC power to AC, it does use DC-to-DC converters to step the voltage down to a lower range. "You need to get the power in a conditioned state to allow the GPUs to consume it," Layden says.

**A modular approach**

Layden emphasizes that Rune is a product company, not a construction company. RELIC modules are manufactured at Rune's facility in Mountain View, California, and are designed for ultra-fast, simple deployment once they reach a solar farm. "You literally take two wires, plug it into the DC link, and you're done," Layden says. He adds that Rune recently fulfilled a request to bring a data center online within a week. "We were able to do that," he says. "We are the fastest time to compute."

The modular design also sidesteps some of the supply-chain bottlenecks facing conventional data centers. A traditional facility can require large transformers, switchgear, cooling systems, and other specialized electrical equipment, some of which can take months or even years to procure.

A single RELIC module can contain anywhere from 128 Nvidia B300 GPUs to more than 2,000, and multiple modules can be interconnected to form a large GPU network. "The only difference [between Rune and a conventional data center] is that it's not in a centralized building connected to transformers and switchgear," Palivela says. "We sell this to inference customers and those doing small training runs or fine-tuning runs on state-of-the-art models."

Rune's approach is part of a broader push by data-center operators to go behind the meter, connecting directly to power generation rather than relying on the grid. The strategy has drawn attention as data centers struggle to secure new grid connections, with companies exploring direct links to nuclear plants, natural-gas generators, and other power sources.

**When the sun doesn't shine**

The central question for Rune's technology is the intermittency of solar power: what happens when it's cloudy for several days in a row? "We have batteries integrated at the RELIC level to make sure we always have compute 24/7," Layden says.

Sizing those batteries correctly is key, Crozier says, particularly because Rune modules don't have a backup grid connection. "They need to size the batteries to store enough energy for nighttime and cloudy-day operations, and generate enough solar to fill these batteries on 'good solar' days," she says. This means some of the available power at any given time goes toward charging batteries rather than directly powering compute.

Rune models extensively to determine the optimal data center and battery sizing. It uses historical data to estimate how much power is likely to be available under different conditions at each site. Current sites are in California, Texas, and Massachusetts.

"Far more significant flexibility can be obtained by co-locating batteries with a data center, especially if they're paired with a generation source," Crozier says. "This way the data center can remove part of the load from the grid without changing its operation."

Rune plans to scale its operations to 100 megawatts of compute over the next 12 months.