You can calculate wattage from horsepower: 1 HP = (just under) 746 watts. This assumes 100% efficiency and 1.0 as a power factor, which you won't get. Rule of thumb with motors is to assume about 70%. If the pump really is 1/3 HP (1/2 HP is common), that's 746 / 3 ~= 250 watts, divided by 0.7, which means you need about 360 VA (volt-amps) to run the pump. That's 120 volts at 3 amps (120 x 3 = 360 VA). Ordinary household wiring (and extension cords) are good to 15 amps so there's no problem there.
For home use in an area with moderately frequent blackouts (2 or 3 times a year, for a day or two, at most), you probably just want a stationary generator installation with a transfer switch. (The transfer switch keeps you from electrocuting the line workers, somewhat like the anti-islanding provision in UL1761, which—to mention Capstone :-) —the C30 and C65 meet.) Your goal here is "backup power": you don't actually plan to use the generator, it's just there for emergencies. So you want something that is as cheap to buy as possible (low capital cost, high operating cost). This is the opposite of a Capstone, where you want something that is as cheap to run as possible (higher capital cost, low operating cost) because you will be running it 24/7.