Designing a water boiler?
Water kettles often have a spiral resistance wire insulated with interlocking ceramic beads. The resistance determines the kettle's power:  

\$ P = \dfrac{V^2}{R} \$

Keep in mind that this resistance is low at room temperature, you can't measure it with a multimeter to calculate the power. See the wire's datasheet to find its resistance at a given voltage.  

All substances have a specific heat. The specific heat is the amount of heat per unit mass required to raise the temperature by one kelvin or °C. For water that's 4.186 J/(g K), that means you need 4.2kJ to get 1 liter of water 1°C hotter. The tap water here is more or less constant throughout the year at 15°C. To get that liter to boiling you'd need 4.2kJ \$\times\$ (100°C - 15°C) = 356kJ or 356kWs, that's 0.1kWh. So a 1kW kettle will boil that liter in 0.1 hour or 6 minutes.  

If that fits your needs you can fit this in the equation:  

\$ 1kW = \dfrac{(120V)^2}{R} \$  

so R is 14.4\$\Omega\$.   

You'll want to add a thermostat to prevent the heater from overheating should the kettle boil dry.

trivia
The specific heat of water is used to define the calorie (non-SI unit). A calorie is the amount of heat required to raise the temperature of 1g of water from 14.5°C to 15.5°C.  

This allows you to calculate the power your boiler (gas or electric) has to supply to give you hot tap water. For me that's at 60°C, and with it's capacity I can fill a 10L bucket in about a minute. Then P = 4.2J \$\times\$ 10 000g \$\times\$ 45°C / 60s = 31.5kW.