Powell top scored with 50 in West Indies' total of 147 for eight, an innings shortened to 40 overs by rain. He faced 71 balls and hit six fours for his seventh one-day international half-century.
South Africa, who were set a victory target of 169 runs from 40 overs using the Duckworth-Lewis method, were 15 for one after five overs when the rain returned for good.
Runs trickled rather than flowed for West Indies, a result of their own defensive approach, tight bowling by the South Africans and an outfield made heavy by two days of rain.
Chris Gayle scored a single from 16 balls before edging a delivery from fast bowler Andre Nel on to his pad and being caught by Herschelle Gibbs at backward point.
Shivnarine Chanderpaul and Powell added 49 for the second wicket, a stand that was ended in the 20th over when Chanderpaul, on 17, top-edged an attempted pull off medium pacer Lance Klusener to Jacques Rudolph on the fine leg boundary.
West Indies were 79 for three six overs later after Powell flicked a delivery from fast bowler Makhaya Ntini to deep mid-wicket, where Boeta Dippanaar took the catch.
South Africa squandered an opportunity to dismiss Brian Lara for four when wicketkeeper Mark Boucher missed a stumping chance off left-arm spinner Robin Peterson.
Ramnaresh Sarwan and Lara shared 39 runs for the fourth wicket before Sarwan cut uppishly at a ball from medium pacer Shaun Pollock and was caught by Dippenaar on the cover boundary for 29.
Two balls later Lara skied fast bowler Jacques Kallis's first ball of the match to Boucher to be dismissed for 15.
Dwayne Smith hit Klusener through mid-off for four and over square leg for six off consecutive deliveries, but sent the next ball arching to Pollock at long-on to go for 12.
Kallis bowled Ryan Hurley with a full toss with the third ball of the final over and Mervyn Dillon sliced the first ball he faced to deep extra cover, where Peterson took a catch.
Ridley Jacobs, who was seven not out, denied Kallis a hat-trick, digging out his final ball.
The only wicket to fall in the South African innings went down in the third over when Graeme Smith sent a looping catch to Lara at mid-on off a delivery from Dillon.
South Africa won the first two games in the five-match series with the last two scheduled for Sunday and Wednesday in Centurion and Johannesburg respectively.
C.Gayle c Gibbs b Nel 1
S.Chanderpaul c Rudolph b Klusener 17
R.Powell c Dippenaar b Ntini 50
R.Sarwan c Dippenaar b Pollock 29
B.Lara c Boucher b Kallis 15
D.Smith c Pollock b Klusener 12
R.Jacobs not out 7
R.Hurley b Kallis 6
M.Dillon c Peterson b Kallis 0
R.Rampaul not out 0
Extras (b-1 lb-6 w-3) 10
Total (for eight wickets, 40 overs) 147
Fall of wickets: 1-3 2-52 3-79 4-118 5-119 6-134 7-146 8-146
Did not bat: C.Colleymore
Pollock 8-4-16-1, Nel 8-2-25-1 (w-2), Ntini 6-0-23-1, Klusener 8-0-46-2 (w-1), Peterson 7-0-22-0, Kallis 3-0-8-3
G.Smith c Lara b Dillon 4
H.Gibbs not out 7
J.Kallis not out 2
Extras (b-1 lb-1) 2
Total (for one wicket, 5 overs) 15
Fall of wicket: 1-10
Did not bat: J.Rudolph, B.Dippenaar, M.Boucher, L.Klusener, S.Pollock, R.Peterson, M.Ntini, A.Nel.
Bowling: Dillon 3-1-4-1, Collymore 2-0-9-0.
Result: No result due to rain.
South Africa lead five-match series 2-0.
First match: January 25, Cape Town - South Africa won by 209 runs.
Second match: January 28, Port Elizabeth - South Africa won by 16 runs.
Fourth match: February 1, Centurion
Fifth match: February 4, Johannesburg (d/n)