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