Decoding Solar Capacity: What do those huge megawatt numbers actually mean for the grid?

The U.S. solar industry installed 43.1 gigawatts-direct current (GWdc) of capacity in 2025, down 14% from 2024. GWdc is the nameplate rating of projects before they connect to the grid through inverters, which convert direct current (DC) to the alternating current (AC) our grid uses.

Two elements lower DC ratings to AC ratings. First, inverter losses account for around 4%.

More importantly, solar panels have specific output duration curves; there’s only a very small period when they produce maximum output, or even 80–90%. It’s uneconomical to buy an inverter that rarely hits full MW ratings, so developers resort to “solar clipping.” A 100 MWdc solar array might use inverters delivering a maximum of 80 MW of AC power to the grid. Typical DC/AC ratios are 1.1 to 1.25. You lose only a bit of energy on an MWh basis, but with significantly lower inverter costs. Therefore, MWdc numbers must be translated to the real-world MWac of the grid.

However, all capacity is not the same: a MW of solar capacity has two factors differentiating it from, say, a MW of gas-fired generation.

First, solar operates at a different capacity factor (a resource operating at 100% output all year would have a 100% capacity factor). An average panel capacity factor is 25%, compared to 60% for a combined-cycle gas plant. Because of this, it’s best to think in terms of energy generated. Location also matters; the capacity factor in Massachusetts is 16.5%, while in Arizona it is 29%.

One way to compare these is by energy output. Solar is now approaching 10% of total energy contributed to the grid. Additionally, solar arrays can be deployed faster than new turbines. With rising data center demand, we need all the electricity we can get.

(Source: https://www.eia.gov/todayinenergy/detail.php?id=67005)

Furthermore, solar is not dispatchable. It only generates power when the sun shines, while a gas plant can be called upon at any time, except during certain extreme weather events. In 2024, the mid-Atlantic grid operator PJM down-rated combined-cycle turbines from 96% to 79% in terms of their ability to meet peak demand during the worst hour of the worst day, and recently lowered that rating further to 74%. By comparison, PJM rates solar at only 7%.

When you hear about solar in terms of MWdc, it helps to reframe those values using the information above. Nonetheless, solar has grown considerably. In 2009, about 1 GW (1,000 MW) of solar was added in the U.S. That cumulative total is now 279 GWdc, and analyst Wood Mackenzie forecasts an increase of 490 GWdc over the next decade.

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