Decoding Solar Capacity: What do those huge megawatt numbers actually mean for the grid?
The US 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 converting direct current (DC) to the alternating current (AC) our grid uses.
Two elements lower DC ratings to AC ratings. First, inverter losses are around 4% losses.
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 rarely hitting full MW ratings, so developers resort to “solar clipping.” A a 100 MWdc solar array might see inverters delivering a max 80 MW of AC power to the grid. Typical DC/AC ratios are1.1 to 1.25.
So, MWdc numbers must be translated to the real world MWac of the grid
But 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 100% capacity factor). An average panel capacity factor is 25%, compared to 60% for a combined cycle gas plant. So, it’s best to think in terms of energy generated. It also matters where panels are located. Massachusetts is 16.5%, while Arizona is 29%.
One way to compare is by energy output. Solar is now approaching 10% of total energy contributed on the grid. And you can put in solar arrays faster than new turbines. With data center demand, we need all the electricity we can get.
However, the solar is not dispatchable. It only shows up when the sun shines, while the gas plant can be called upon anytime, except sometimes in extreme weather.
In 2024 mid-Atlantic grid operator PJM down-rated combined cycle turbines 96% to 79% in terms of their ability to meet peak demand on the worst hour of the worst day, and recently lowered that rating to 74%. But PJM has solar at only 7%.
When you hear about solar in terms of MWdc, , it helps to reframe those values using the above information.
Nonetheless, solar has grown considerably. In 2009, about 1 GW – 1,000 MWs of solar was added iomn the U.S. That total is now 279 GWdc, and analyst Wood Mackenzie forecasts an increase of 490 GWdc over the next decade.