If the system is 12 volt, dividing the total watts of panel by 12 will give you how many amp hours you have at your disposal to recharge your battery.
So for instance, if your pump pulls 2 amps, and your battery is a 100 amp hour battery, you can run it 24 hrs. withdrawing only 48 amps and not dropping the charge below 50%. (Never discharge your batteries below 50% capacity or you damage them and shorten their life and performance.)
If your panel is 240 watts, that divides by 12 to 20, which means the panel can (under optimal conditions) replenish 20 amps per hour to your battery. If the battery is only drawn 48 total amps per day and night, then in just over 2 hours of direct sunlight, that 240 watt panel should replenish the entire 24 hrs worth of draw.
But in that scenario, 1 day without sufficient sunlight could see the single 100 amp battery run down below 50% and get damaged as the pump continues drawing on a diminishing bank of power. So in this particular instance, two 100 amp batteries might be prudent, or more, depending on the weather pattern of your area.