Solar Panels:

The intensity of sunlight varies throughout the day.  The term "peak sun hours" is used to smooth out the variations. Early
morning and late-in-the-day the sun generates less power than the mid-day sun.  Cloudy days produce less power than
sunny days.  Geographical areas are rated in average peak sun hours per day based on yearly sun data.

The output of a solar panel is measured in watts.  Wattage is calculated by multiplying the voltage by amperage. The formula
for wattage is VOLTS times AMPS = WATTS.   For example, a 12 volt 60 watt solar panel measuring 20 X 44 inches has a
rated voltage of 17.1 and a rated 3.5 amperage.  V x A = W 17.1 volts times 3.5 amps equals 60 watts.  If an average of 6
hours of peak sun per day is available in an area, then this solar panel can produce an average 360 watt hours of power
per day; 60w times 6 hrs = 360 watt-hours.

Series or Parallel

Solar panels can be wired in series or in parallel to increase voltage or amperage. Series wiring connects the positive
terminal of one panel to the negative terminal of another. The outer positive and negative terminals will produce voltage the
sum of the two panels, but the amperage remains the same as one panel. So two 12 volt/3.5 amp panels wired in series
produce 24 volts at 3.5 amps. Four of these wired in series produce 48 volts at 3.5 amps. Parallel wiring refers to
connecting positive terminals to positive terminals and negative to negative. The result is that voltage stays the same, but
amperage becomes the sum of the number of panels. So two 12 volt/3.5 amp panels wired in parallel would produce 12
volts at 7 amps. Four panels would produce 12 volts at 14 amps.  Series/parallel wiring refers to doing both of the above -
increasing volts and amps to achieve the desired voltage as in 24 or 48 volt systems.

Charge Controller:

  • A charge controller monitors the battery's state-of-charge to ensure that when the battery needs charge-current it gets
    it, and also prevents overcharging.

  • Connecting a solar panel to a battery without a controller risks damage to the battery.  Charge controllers (or charge
    regulators) are rated on the amount of amperage they process from a solar array. If a controller is rated at 20 amps it
    means that you can connect up to 20 amps of solar panel output current to this one controller.

  • PWM - The most advanced charge controllers utilize Pulse-Width-Modulation (PWM) - which ensures the most efficient
    battery charging and extends the life of the battery.

  • MPPT - Even more advanced controllers also include Maximum Power Point Tracking (MPPT) which maximizes the
    amount of current going into the battery from the solar array by lowering the panel's output voltage, which increases
    the charging amps to the battery.

  • LVD - Many charge controllers also offer Low Voltage Disconnect (LVD) and Battery Temperature Compensation
    (BTC) as an optional feature. The LVD feature permits connecting loads to the LVD terminals which are then voltage
    sensitive. If the battery voltage drops too far the loads are disconnected - preventing potential damage to both the
    battery and the loads. BTC adjusts the charge rate based on the temperature of the battery since batteries are sensitive
    to temperature variations above and below about 75 F degrees.


Deep Cycle batteries are designed to be discharged and then re-charged hundreds or thousands of times. These batteries
are rated in Amp Hours (ah) - usually at 20 hours and 100 hours.  Amp hours refer to the amount of current which can be
supplied by the battery over the period of hours. For example, a 350ah battery could supply 17.5 continuous amps over 20
hours or 35 continuous amps for 10 hours.

Batteries can be wired in series and/or parallel to increase voltage and amp hours.  The battery should have sufficient amp
hour capacity to supply needed power during the longest expected period "no sun" or extremely cloudy conditions.

Lead-Acid batteries are the most common in PV systems because of low cost and availability.  It is important that they are
Deep Cycle batteries.  Lead-Acid batteries are available in Wet-Cell, and sealed versions such as AGM and Gel-Cell.

A lead-acid battery should be sized at least 20% above the anticipated power needs. If there is a backup power source,
such as a standby generator along with a battery charger, the battery bank does not have to be sized for worst case
weather conditions.  The size of the battery bank required will depend on a number of factors, including: storage capacity,
maximum discharge rate, maximum charge rate, and the minimum temperature at which the batteries will be used.

It is important to understand the relationship between amps and amp-hour requirements of 120 volt AC items versus the
effects on their DC low voltage batteries.  For example, if you have a 24 volt nominal system and an inverter powering a
load of 3 amps, 120VAC, with a duty cycle of 4 hours per day, your load is 12 amp hours (3A X 4 hrs=12 ah). In order to
determine the true drain on the battery, you divide the nominal battery voltage (24v) into the voltage of the load (120v),
which is 5, and then multiply this by 120vac amp hours (5 x 12 ah). In this example the calculation would equal 60 amp
hours. Another method is to take the total watt-hours of the 120VAC device and divide by nominal system voltage. In this
example, 3 amps x 120 volts x 4 hours = 1440 watt-hours divided by 24 DC volts = 60 amp hours.


  • An inverter changes the DC power stored in a battery to 120/240 VAC electricity (or 110/220).
  • Most solar power systems generate DC current which is stored in batteries for later use.
  • Most lighting, appliances, motors, etc., are designed to use AC power, so it takes an inverter to switch from DC to 120
    VAC, 60 Hz).  
  • In an inverter, direct current (DC) is switched back and forth to produce alternating current (AC). Then it is transformed,
    filtered, stepped  to get it to an acceptable output waveform. The more processing, the cleaner and quieter the output,
    but the lower the efficiency of the conversion. The goal is to produce a waveform that works for all loads without
    sacrificing power in the conversion process.

  • Modified sine wave inverters make the conversion from DC to AC very efficiently. They are relatively inexpensive,
    and work for most household appliances.  Most 120VAC devices use modified sine wave.  Exceptions are devices
    such as laser printers which use triacs and/or silicon controlled rectifiers are damaged when provided mod-sine wave
    power.  Motors and power supplies usually run warmer and less efficiently on mod-sine wave power.  Some devices,
    like fans, amplifiers, and cheap fluorescent lights, give off an audible buzz on modified sine wave power.

  • Sine wave inverters can virtually operate anything. Your utility company provides sine wave power, so a sine wave
    inverter is equal to or even better than utility supplied power. A sine wave inverter can "clean up" utility or generator
    supplied power because of its internal processing.  

  • Inverters are made with various internal features and many permit external equipment interface.  Common internal
    features are internal battery chargers which can rapidly charge batteries when an AC source such as a generator
    or utility power is connected to the inverter's INPUT terminals.

  • Auto-transfer switching is also a common internal feature which enables switching from either one AC source to
    another and/or from utility power to inverter power for designated loads.

  • Battery temperature compensation, internal relays to control loads, automatic remote generator starting/stopping and
    many other programmable features are available.

  • Most inverters produce 120VAC, but can be equipped with a step-up transformer to produce 120/240VAC. Some
    inverters can be series or parallel "stacked-interfaced" to produce 120/240VAC or to increase the available amperage.

Efficiency Losses:

In all systems there are voltage losses as electricity is carried across the wires, batteries and inverters not being 100
percent efficient, and other factors. These efficiency losses vary from component to component, and from system to
system and can be as high as 25 percent.

Power Consumption

2.5KWh Inverter, 480Wh Solar array, will produce on average approximately 1.4kWh per day. Power is defined as the rate
at which work is done or energy is consumed.  The formula for average power is acquired by dividing work by the time
needed to perform work: P = W/t. Power has units of newton-meters per second or joules per second or watts.

Power Plants

Electric power for residential use comes from power plants via a power distribution grid. The power derives from a power
site within the power plant, consisting of a central mover like a turbine that is then pushed by water or steam to run a
system of generators.

Household Power Consumption

The amount of power that a household consumes depends on how many appliances there are and the amount of time they
are in use. Some appliances take a lot of energy to operate, so it will result in more use of power.  A kilowatt-hour is the
electrical energy consumed in one hour at the constant rate of one kilowatt. The average household uses 8,900 kilowatt-
hours of electricity each year.

Measuring Electricity

Watts describe the rate at which electricity is being used at a specific moment. The amount of electricity that a 100-watt
light bulb draws at any particular moment is of course 100W.

Watt-hours measure the total amount of electricity used over time.  Watt-hours are a combination of the how fast the
electricity is used (watts) and the length of time it is used (hours). For example, a 100-watt light bulb, which constantly
draws 100 watts, uses 100 watt-hours of electricity in one hour.

Kilowatts and kilowatt-hours are useful for measuring amounts of electricity used by large appliances.  One kilowatt
(kW) equals 1,000 watts, and one kilowatt-hour (kWh) is one hour of using electricity at a rate of 1,000 watts.  Energy-
efficient refrigerators use about 1.4 kilowatt-hours per day, and about 500 kilowatt-hours per year.

Megawatts are used to measure the output of a power plant or the amount of electricity required by an entire city. One
megawatt (MW) = 1,000 kilowatts = 1,000,000 watts.

Gigawatts measure the capacity of large power plants or of many plants. One gigawatt (GW) = 1,000 megawatts = 1
billion watts.

Step 1: Determine Daily Energy Needs
(Hours of use times watts equals daily watt hours used)
AC ApplianceHours of Daily Usage X Appliance Watts = Daily Watt Hours Used

Step 2 Determine Rough Battery Estimate
Multiply total daily watt hours used by number of anticipated days of autonomy (days between charging, usually beteen 1 to
5) to determine your Rough Battery Estimate.
Total Daily Watt Hrs. Used x days of autonomy >>>5,634

Step 3: Determine Safe Battery size in Watts Hours
Multiply Rough Battery Estimate x 2, to determine safe battery size in watt hours. (This allows for 50% maximum battery
discharge in normal operation and an additional 50% in emergency situations.)  Safe Battery Size in Watt Hrs.
Formula:Rough Battery Estimate x 2 >>>11,268

Step 4: Determine Safe Battery Size in Amp Hours
Convert safe battery size to amp hours by dividing the Safe Battery Size in Watt Hours by DC system voltage (i.e. 12, 24,
48 volts DC) Safe Battery Size in Amp Hrs. Formula: Safe Battery Size in Watt Hours / System Voltage >>>470

Step 5: Determine Inverter Wattage
To properly determine inverter size add together the appliances that must/will run at the same time, and add 25%. Then
roundup to the next inverter wattage size.  Properly sized inverter wattage Formula: Add Total Appliance Watts + 25% >>>2,
Only a few basic components are needed for a solar power system:    
  • Solar Panel - Collects energy from the sun to provide power to electrical devices.  
  • Energy Storage - To store energy for later use, Batteries are used as Energy Storage Devices.  
  • Charge Controller - A Charge Controller controls the charge to the battery and prevents overcharging.  
  • Inverter - An Inverter is needed if you want to switch stored DC power to AC power used for most household devices.
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Solar Power Components

The four primary components for producing
electricity using solar power are:

  • Solar panels charge the battery,
  • The charge controller ensures proper charging of battery,
  • The battery provides DC power to the inverter,
  • The inverter converts DC power to household AC current.