Why is it rated 218 watts when the part number says 200?

Because the part number is a series designation and the 218 is a measurement. CSB rates the block at 218 watts per cell at the 15 minute rate to 1.67 volts per cell at 25°C, and 385 watts per cell at 5 minutes. Across six cells the 15 minute figure works out near 1,308 watts for the whole battery. Size against the measured number and the rate printed beside it.

Can I put one new HRL12200W into an existing string?

We would advise against it, and we will say so before taking the order. A new block in an aged string charges and discharges differently from its neighbors, the older blocks pull it down, and the string ends up limited by its weakest unit anyway. If one block has failed, the rest are usually close behind. Replace the string as a set and the warranty term runs from one date for all of them.

What charger settings does the HRL12200W need?

Float at 13.5 to 13.8 VDC per battery and equalize at 14.4 to 15.0 VDC per battery, both at 25°C. Keep charge current at or under 20.0 amps. If your rectifier is fixed at a higher float voltage than 13.8, tell us before you order, because running above the window is the most common reason a long life block does not reach its design life.

How much longer does a 12200W run than a 12150W at the same load?

Roughly 40 percent, on CSB's own published figures. At the 15 minute rate the 200 delivers 218 watts per cell against 156, and at the 20 hour rate 52 Ah against 37. The gain is not free: the block is 11.79 lbs heavier and 1.28 inches longer, so the shelf has to take it.