Thread by @solar_chase

Apologies for anyone who follows me for non-#solar stuff. Here is a short thread on Why PV Analysis Should Stick to DC Capacity and Capacity Factor, And Only Use AC in Certain Rare Circumstances.

The Direct Current (DC) capacity of a PV plant is the power output of all its modules under standard conditions, incl. benchmark insolation of 1000W per square metre.

The alternating current (AC) capacity is that of the inverter, and is the max instantaneous power output.

A PV plant only operates near its DC rating for a few hours per day, (per year in Germany/ UK), AC capacity should often be lower than DC, by design. You don't need an inverter than can handle full theoretical output. The AC:DC ratio is called the 'inverter loading ratio', ILR.

The energy lost when your DC output exceeds your inverter's ability to process it is called 'clipping'. Here's a decent diagram of how it works. In low-sun places, you may only get clipping at all for a few days per year.
bxhorn.com/wp-content/upl…

So in low-sun places you often design your solar plant to be 1.3-1.5MW (DC capacity) for each 1MW (AC capacity). (If your grid connection capacity, ie AC capacity, is limited but your tariff is good you may go higher!). In sunny places you may pick 1.0-1.2MW(DC) to 1MW(AC).

Capacity *factor* is the MWh per year your PV plant makes, divided by your capacity. So whether you use the AC capacity or the DC capacity in this calculation makes a huge difference to your reported capacity factor - but none at all to the output of your plant.

Remember the DC capacity is the installed module capacity, which is the main factor determining how much your plant costs to build and how much land it takes up. So if you need to estimate either of these things, you need DC capacity.

DC capacity factor is also largely determined by how sunny it is (because clipping is so minor).

AC capacity factor is significantly determined by plant design. You can nearly double an AC capacity factor by installing more modules - but that's expensive!

So: AC capacity tells you max instantaneous theoretical power output. AC CF is a function of both system design and climate data. AC capex is really hard to estimate.

From DC capacity, you can estimate land use, capex, and with publically available climate data, energy output.

1. FOR THE LOVE OF ALL THAT IS HOLY, DO NOT MIX THEM UP
2. If only recording one PV capacity, DC is much better.
3. Grid operators in the US, Thailand and Japan publish mostly AC capacity. Which is fair, because they are concerned with wire size. Nearly everyone else uses DC.

Approximate DC capacity factors: 10-13% Germany, 10-12% UK, 15-19% Spain, with the highest without tracking likely about 23% (Atacama Desert, Tibetan plateau).

CFs above this are likely to be either AC, or using tracker devices, or both. (Trackers are a topic for another day!).