Introduction

• Hi, y’all,– I’m leaving in the morning, so I didn’t get to spend

as much time on this as I would have liked– Please let me know if you find it useful– Suggestions for improvement are welcome

Lesson 1

• Before beginning this lesson, please read “Electrical Needs Assessment” with the glossary, and “The Vocabulary of Electrical Power

Contents

• Review of terms and relationships• Sample Problems• Skill building problems• Answers to skill building problems

Review of terms and relationships

• Current is the flow of charges– Measured in amps (short for amperes)

• Voltage is the electrical potential– Measured in volts

• Power is the rate at which energy is used– Measured in watts– 1 watt = 1 amp * 1 volt

Mechanical Power vs.Electrical Power

• A gasoline engine’s power is measured in horsepower– 1 horsepower = 746 watts

Energy

• Energy is what a photovoltaic cell captures• Energy is what a pump, motor, or light uses• Energy = Power * time– Measured in watt-hours

AC vs. DC

• The power produced by batteries and PV panels is DC (for Direct Current)

• DC current flows in one direction• AC current changes direction tens of times a

second

AC vs. DC power

• In a solar power system, DC power goes in to an inverter; and AC power comes out of an inverter.

• The inverter has electrical losses.– When you know the AC watts, add 15% to find

how many DC watts will have to be supplied

Sample Problem 1

• A 23 watt CFL (compact fluorescent light) puts out as much light as a 100 watt incandescent bulb. How much energy does each bulb use if it burns for 8 hours a night?

Answer to Sample Problem 1

• A 23 watt CFL (compact fluorescent light) puts out as much light as a 100 watt incandescent bulb. Both bulbs use 120 volt AC power, how much energy does each bulb use if it burns for 8 hours a night?

• This is a straight “plug in the numbers” problem

• CFL: 23 watts * 8 hours = 184 watt-hours• Inc: 100 watts * 8 hours = 800 watt-hours

Sample Problem 2

• A CFL bulb uses 184 watt-hours a night• An incandescent bulb uses 800 watt-hours a

night• How many hours of sunlight are needed on a

300 watt PV array to provide the energy to light the bulbs for a night?

Answer to sample problem 2• PV panel watts are DC, the lights are AC– CFL: 184 watt-hours AC * 1.15 = 212 watt-hours DC– Inc: 800 watt-hours AC* 1.15 = 920 watt-hours DC

• (* 1.15 adds 15 %)

• Since Energy = Power * time, time = Energy/Power

• CFL: 212 watt-hours/300 watts = 0.71 hour, or 42 minutes

• Inc: 920 watt-hours/300 watts = 3.1 hours, or 3 hours and 4 minutes

More on problem 2

• It takes 42 minutes for a 300 watt panel to gather enough energy to run a 23-watt CFL bulb for 8 hours

• It takes 3 hours and 4 minutes for a 300 watt panel to gather enough energy to run a 100-watt incandescent bulb for 8 hours.

• Solar 1 grads will work with the OP’s to reduce their energy demands while meeting the OP’s needs.

Sample problem 3

• A CFL bulb uses 184 watt-hours a night• An incandescent bulb uses 800 watt-hours a

night• How many amp-hours will each bulb draw

from a 24-volt, 225 amp-hour battery in one night?

Answer to sample problem 3

• From sample problem 2, the CFL needs 212 watt-hours DC and

• The incandescent bulb needs 920 watt-hours DC• Since watts = volts*amps, amps= watts/volts• CFL: 212 watt-hours/24 volts = 8.8 amp-hours• Inc: 920 watt-hours/24 volts = 38.3 amp-hours• Another reason to use CFL’s!

Skill Building Problem 1

• If a 310 watt PV panel array puts out 5.5 amps, what voltage is the PV panel providing?

SBP 2

• An AC air conditioner uses 1,500 watts. If a site gets 5 hours of sunlight a day, how big should the PV array be to run the air conditioner for 3 hours?

SBP 3

• Conduct an “electrical needs assessment” in 1 room in your home– That is, find the number of watt-hours of energy

per week that is used in one room in your home

Answer to SBP 1

• If a 310 watt PV panel array puts out 5.5 amps, what voltage is the PV panel providing?

• Since watts = volts * amps, volts = watts/amps• 310 watts/ 5.5 amps = 56 volts

Answer to SBP 2

• An AC air conditioner uses 1,500 watts. If a site gets 5 hours of sunlight a day, how big should the PV array be to run the air conditioner for 3 hours?

• 1500 watts AC * 1.15 = 1725 watts DC• 1725 watts * 3 hours = 5175 watt-hours• 5175 watt-hours/ 5 hours = 1035 watts