• Deep-Cycle Gel Cell batteries are lead-
    acid batteries, but different in construction
    and chemistry than Wet Cell batteries. They
    offer unique features like: no maintenance,
    low self-discharge rate, and low internal

  • Gel Cell batteries use an electrolyte that has
    been immobilized using a gelling agent.

  • Gel cell batteries are sealed but the battery
    compartment still needs to be ventilated.

  • These batteries will have slightly shorter life
    span than wet cell batteries.

  • Gel cell batteries dimensional sizes and Ah
    capacities are more limited than wet cell.

  • Neither AGM or Gel cells will leak if inverted,
    pierced, etc. and will continue to operate
    even under water.

  • Gel Cells use a thickening agent like fumed
    silica to immobilize the electrolyte. Thus, if
    the battery container cracks or is breached,
    the cell will continue to function.

  • The thickening agent prevents stratification
    by preventing the movement of electrolyte.

  • As Gel cells are sealed and cannot be re-
    filled with electrolyte, controlling the rate of
    charge is very important or the battery will
    be ruined.  Gel cells use slightly lower
    charging voltages than flooded cells and
    thus the set-points for charging equipment
    have to be adjusted.

  • Virtually no gassing under normal operating
    conditions: Unlike flooded cells, gel cells and
    AGMs are hermetically sealed and
    operate under pressure to recombine the
    oxygen and hydrogen produced during the
    charge process back into water.

  • Gel cells require no maintenance once the
    charging system has been properly set up.
    No equalization charges (usually), no
    electrolyte to replenish, no specific gravity
    checks, no additional safety gear to carry on
    board in order to protect yourself.

  • VRLAs can be shipped anywhere by
    air. Flooded cells have to be bought
    locally or delivered by surface

  • The faster, more efficient bulk charging that
    AGMs and gel-cells allow will lead to
    reduced wear and tear on your charge
    source (engine, gen-set, etc.).

  • VRLAs: Gel and AGM batteries can
    dispense charge at a higher rate than
    flooded cells due to their lower Peukerts
Lead-Acid Batteries contain an electrolyte consisting of sulphuric acid and use lead grids to store electrical energy.  Some are made specifically
for Deep Cycle applications - meaning they can be discharged and re-charged thousands of times - ideal for solar systems.  VRLA batteries are
the most commonly used for solar systems.  VRLA stands for Valve-Regulated Lead Acid.  The three main types of lead-acid batteries are:
  • Gel Cells (VRLA)
  • AGM (VRLA)
  • Wet Cells (Flooded)
  • Absorbed Glass Mat (AGM) batteries are the
    latest step in the evolution of lead-acid batteries.
    Instead of using a gel, an AGM uses a fiberglass-
    like separator to hold the electrolyte in place.

  • The physical bond between the  separator fibers,
    the lead plates, and the container make AGMs spill-
    proof and the most vibration and impact
    resistant lead-acid batteries available.

  • AGMs use almost the same voltage set-points as
    flooded cells and thus can be used as drop-in
    replacements for flooded cells.

  • AGMs are hermetically sealed and operate under
    pressure to recombine the oxygen and hydrogen
    produced during the charge process back into

  • AGMs require no maintenance once the charging
    system has been properly set up. No equalization
    charges, no electrolyte to replenish, no specific
    gravity checks.

  • The charge acceptance of AGMs can burn up an
    alternator if the charging system is not adequate for
    extended runtimes at full power. The larger the
    battery bank and the harder the charger is made to
    work, the more important it is to ensure that the
    charging system can handle the currents for
    extended periods of time.

  • The higher charge efficiency of AGMs allows you
    to recharge with less energy.  As little as 4% of the
    electrical energy is converted into heat instead of
    potential power.

  • The higher charge efficiency of AGMs can
    contribute to significant savings when it
    comes to the use of expensive renewable
    energy sources (wind generators, solar
    panels, etc.) as your charging system can be
    15% smaller (or just charge faster).

  • AGMs can be used in environments where other
    batteries would literally fall to pieces. This is another
    reason why AGMs see broad use in the aviation
    and the RV industry.

  • AGMs offer the highest charge acceptance,
    efficiency, and a reasonably long life.

  • VRLAs can be shipped anywhere by air.

  • VRLAs: Gel and AGM batteries can dispense
    charge at a higher rate than flooded cells due to
    their lower Peukerts exponent.
  • Wet Cells or Flooded are the most common
    lead-acid battery-type in use today.  They
    offer the most size and design options and are
    built for many different uses.

  • They usually are not sealed so the user can
    replenish any electrolyte the battery vented
    while charging the battery.

  • Typically, the cells can be accessed via small
    ~1/2" holes in the top casing of the battery.
    The plastic container used for flooded cells will
    have one or more cells molded into it.

  • Each cell will feature a grid of lead plates along
    with an electrolyte based on sulphuric acid.
    Since the grid is not supported except at the
    edges, flooded lead-acid batteries are
    mechanically the weakest batteries.  

  • Since the container is not sealed, great care
    has to be taken to ensure that the electrolyte
    does not come into contact with you or
    seawater (chlorine gas).

  • The water needs of flooded cells can be
    reduced via the use of Hydrocaps, which
    facilitate the recombination of Oxygen and
    Hydrogen during the charging process.

  • Flooded cells have to be bought locally or
    delivered by surface transport.

  • Deep-Cycle Flooded cell battery
    manufacturers recommend a 4 to 1 ratio
    between battery bank size and the largest load
    encountered on board.

  • Flooded cells lose up to 1% per day due to self-
batteries - energy storage

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
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

  • 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

Power Plants

Electric power for residential use comes from power plants via a power
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

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
1 Microwave .5600300
2 Lights (x4) 640240
3 Hair Dryer .75750563
4 Television 4100400
5 Washing Machine 1375375
Total Daily Watt Hrs. Used = Add Lines 1-5 = 1,878

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 inAmp 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% >>>2,500
  • PV panels from 5 to 500 Watts
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