Bigger is better as you run the pump less, cell lasts longer. If in doubt get bigger. NEVER have to buy/transport or put CHLORINE in your pool, EVERY day,ever again. Better too as you have control on correct dose, use less chlorine/ reduce your carbon footprint. USA EXPORTS AVAILABLE from K-CHLOR as they use the ATB72 time clock and Transformer for 60 Hz. Enjoy the pleasure of swimming in smooth salt water, adds a little boyancy for learners but low salt level to prevent damage to metal fences. OFF PEAK USE CAN SAVE 66% OF YOUR ENERGY COST, THAT INCLUDES THE PUMP AND THE SALT CHLORINATOR, ALL DONE WITH A $39 UPGRADE TO THE SALT WATER SYSTEM, CALL BOB 0243690299 Australia wide or email bob@ozepool.com.