Electrical Power and Energy Basics

Often the term Watts and kilo-Watt-hours (kWh) are used interchangeably when describing a renewable energy system; although related, they are not the same. Watts (W) is a measure of power, while Watt-hours (Wh) is a measure of energy. For example, for a 100W light bulb to work, it needs a constant 100W of power. If this light bulb is on for 5 hours, then it has consumed 500Wh or 0.5kWh of energy (Energy = Power x Time).

Electrical demand refers to power; and electrical consumption, to energy. Utility companies charge customers by their electric consumption in kW-h (1kW = 1000W). The table below shows a list of appliances with their electrical demand and for how long they are used during a day.

Appliance	Demand	Time
Five Light bulbs	100W	5 hours
One 15″ Laptop	50W	5 hours
One 32″ LCD TV	125W	3 hours
One Power drill	1000W	30 min

If all the appliances ran at the same time the total electric demand would be 1.675kW. The total consumption is then found by multiplying the demand of each appliance by its run-time. Doing so results in 3.625kWh. In other words, to run these appliances one would need a minimum of 3.625kWh of energy and the capacity to supply 1.675kW of power.

If considering using a renewable energy system we recommend having a similar power consumption chart with your own electrical needs. This will be essential for determining the type and size of the system.

 What is Renewable Energy?

Renewable or Alternative energy is an energy that can be replenished naturally, such as solar, wind, waves, geothermal, etc. Non-renewables are the traditional hydrocarbon based sources such as fossil fuels, coal, etc.

Renewable energy is sustainable with minimal environmental impact; produces little or no waste products (such as carbon dioxide or other chemical pollutants); and requires less maintenance than traditional fossil fuel based technology.

 Renewable Energy systems

These systems use renewable energy sources to generate electricity.  There are two main types of systems: Off-grid and On-grid.

Off-grid energy systems use batteries to store the electrical energy generated by the renewable energy source.  These types of systems are used in emergency situations, such as power outages and disaster response; remote locations where there is no access to shore line power; locations with unreliable power grids; or for overall electrical independence.

The energy source — either through solar panels, wind/hydro turbines, etc.– generates electricity in the form of direct current VDC which then goes through a charge controller and then to the battery bank. Charge controllers are used to regulate voltage and current levels to efficiently charge the batteries. Most electrical devises run on alternate current, AC. To use the energy stored in the battery bank, an inverter is needed to convert from DC to 120V AC. Electrical devices that run on DC current can be directly connected to the battery bank without using the inverter.

In On-grid systems the electricity produced by the energy source is used in your home or business, and the excess energy is routed to a local electrical grid. For example, a household consumes 5kW-h of energy in a day, but produces 3kW-h in renewable energy; the net consumption that will be charged by the utility company is 2kW-h. A second example would be that the energy produced by the renewables is 6kW-h, meaning that you produce more than you consume. By agreeing to a Net Metering policy, the utility company buys the excess energy.

These systems need a working electrical grid and an agreement with the local utility company. If power outages occur, the energy cannot be used since it is going to the electrical grid. For more versatility, an On-Grid system can have a battery backup.

 Solar Power and photovoltaic cells

Solar systems use photovoltaic cells to capture the energy of the sun and transform it into electricity. Solar panels are made out of individual photovoltaic cells connected together and then framed with a protective glass/plastic surface. Flexible solar panels are also available; these are rollable and weigh less than standard rigid panels. Enerdex solar systems use rigid and flexible solar panels, contact us for more information.

How much energy will solar panels generate during a day will mostly depend on how many hours of useful sunlight is available. Solar insolation is a measure of solar radiation energy over a given area at a given time, normally measured in watt-hours per square meter (Wh/m²). Simply, it means how much solar energy can be collected in 1hour over a one meter square area. The figures below were taken from the National Renewable Energy Laboratory (NREL)  and show insolation maps for the U.S., Central and South America (click on image to enlarge).  For example, if the solar insolation is 5.0 wh/m², you can expect 5 hours of full usable sunlight per day. Ignoring panel efficiency (about 12%), a 200W solar panel would generate 200W x 5H = 1000Wh (1.0kWh) of energy per day. Energy output also depends on other factors such as shading, panel inclination angle, time of day, weather, etc.

 Wind Power

In renewable electric systems wind turbines are used to convert the kinetic energy from wind currents to electricity. Wind passing through the turbine blades (rotor) spins a shaft that is connected to an electric generator.  While solar power is limited by useful sunlight, wind turbines can generate electricity for longer periods of time if placed in the correct location. 

How much electricity can a wind turbine generate depends on the size of the turbine and wind speeds. Small wind turbines normally range from 300W to 10,000W and have approximate rotor diameters of 8ft to 25ft (2m-7m), where the larger the rotor diameter the more electrical power can be generated. 

Each turbine has a range of operating wind speeds. The cut-in speed is the lowest speed needed to generate electricity, and the cut-off speed is the fastest operational speed. Power curves show the relationship between a turbine power output and an operating wind speed. For the smaller 300W-600W rated turbines, the cut-in speed is typically from 7mph to 10mph (3m/s – 5m/s). On average, at 15mph (~7m/s) one could expect an electrical power output of 70W – 200W. 

Small wind turbines can be mounted on towers, roofs and boats. If located on a roof or tower we recommend that it be placed 20ft (6m) higher than any other object around it to ensure minimal wind obstruction.