I found this answer on the net, hope it helps:
Array-entered formulas are actually pretty easy when you get used to them. For example, if:
-- Daily high prices are in the range E4:E27
-- Daily low prices are in the range F4:F27
-- Desired Acceleration (or Step) Factor is in cell G1
-- Desired Max Acceleration (or Step) Factor is in cell G2
...then all you need to do is:
-- Highlight range G4:G27
-- Edit this formula into the active cell of the range:
-- Hit Ctrl+Shift+Enter instead of Enter to enter the formula
Cell G4 will contain #N/A (because you need more than one day's values to assign PSAR), but G5:G27 should contain valid PSAR values.
And you can change the parameter values in cells G1 and G2 to alter how PSAR is calculated.
And it is very fast. On over 10,000 data points, my recalculation time upon changing one of the input factors in under a second.