Solar Direct is dedicated to increasing our customer's awareness of
sustainable and renewable technology by providing an educational, yet highly engaging internet venue,
where they can discover how green alternatives can impact not only their lives, but the world future
generations will inherit! Click here first to
begin your sustainable living learning adventure! Learn the step-by-step details of how and why sustainable
and renewable technology works, and the science behind the functioning of each alternative energy process. Solar Direct offers unsurpassed, highly-personalized
product selection assistance, with experienced professionals providing toll free support -
creating a truly inspired purchasing experience for our customers. Click here next to start
the selection process, then use online tools to decide your proper type of technology and its matching product.
Product Selection Assistants are standing by to help you with toll free support. Solar Direct hosts the Energy
SuperMarket website, the Internet's top-ranked shopping destination for renewable energy technology. Enjoy
a secure shopping for all your alternative energy needs. Solar Direct offers thousands of green, sustainable
and renewable products, combined with toll free Product Selection Assistants dedicated to providing a quality
purchasing experience for all consumers. Click here
last to finish your alternative energy adventure! The Energy SuperMarket website provides the
most advanced, real world authoritative, user-friendly shopping destination for renewable energy
technology today.

Solar Electric System Sizing
Step 3 - Size Your Battery

Read "Characteristics of Batteries" and then choose the appropriate battery for your needs. Fill out the Battery Sizing Worksheet.

Characteristics of Batteries

text bullet

Sizing Your Battery Bank

The first decision you need to make is how much storage you would like your battery bank to provide. Often this is expressed as “days of autonomy,” because it is based on the number of days you expect your system to provide power without receiving an input charge from the solar array. In addition to the days of autonomy, you should also consider your usage pattern and the criticality of your application. If you are installing a system for a weekend home, you might want to consider a larger battery bank, because your system will have all week to charge and store energy. Alternatively, if you are adding a PV array as a supplement to a generator-based system, your battery bank can be slightly undersized since the generator can be operated if needed for recharging.

text bullet

Temperature Effects

Batteries are sensitive to temperature extremes, and you cannot take as much energy out of a cold battery as a warm one. Use the chart on the Battery-Sizing Worksheet to correct for temperature effects. Although you can get more than rated capacity from a hot battery, operation at hot temperatures will shorten battery life. Try to keep your batteries near room temperature. Charge controllers can be purchased with a temperature compensation option to optimize the charging cycle at various temperatures and lengthen your battery life.

text bullet

Depth of Discharge

Depth of Discharge is the percentage of the rated battery capacity that is withdrawn from the battery. The capability of a battery to withstand discharge depends on its construction. Two terms, shallow-cycle and deep-cycle, are commonly used to describe batteries. Shallow-cycle batteries are lighter, less expensive and have a short lifetime. For this reason, we do not sell shallow-cycle batteries. Deep-cycle batteries should always be used for stand-alone PV systems. These units have thicker plates and most will withstand daily discharges up to 80% of their rated capacity. Most deep-cycle batteries are flooded electrolyte which means the plates are covered with the electrolyte and the level of fluid must be monitored and distilled water added periodically to keep the plates fully covered. We also offer sealed, lead-acid batteries that do not require liquid refills. There are other types of deep-cycle batteries such as nickel cadmium used in special applications. The maximum depth of discharge value used for sizing should be the worst case discharge that the battery will experience. The system control should be set to prevent discharge below this level.

text bullet

Rated Battery Capacity

The ampere-hour capacity of a battery is usually specified together with some standard hour reference such as ten or twenty hours. For example, suppose the battery is rated at 100 ampere-hours and a 20-hour reference is specified. This means the battery is fully charged and will deliver a current of 5 amperes for 20 hours. If the discharge current is lower, for example 4.5 amperes, then the capacity will go to 110 ampere-hours. The relationship between the capacity of a battery and the load current can be found in the manufacturer’s literature.

text bullet

Battery Life

The lifetime of any battery is difficult to predict, because it is dependent on a number of factors such as charge and discharge rate, depth of discharge, number of cycles and operating temperature extremes. It would be unusual for a lead-acid battery to last longer than fifteen years in a PV system but many last for five to eight years.

text bullet


Batteries require periodic maintenance. Even the sealed battery should be checked to make sure connections are tight and there is no indication of overcharging. For flooded batteries, the electrolyte level should be maintained well above the plates and the voltage and specific gravity of the cells should be checked for consistent values. Wide variations between readings may indicate cell problems. The specific gravity of the cells should be checked with a hydrometer particularly before the onset of winter. In cold environments, the electrolyte in lead-acid batteries may freeze. The freezing temperature is a function of a battery state of charge. When a battery is completely discharged, the electrolyte becomes water and the battery may freeze.

Battery Sizing Worksheet
1. Enter your daily amp-hour requirement. (From the Load Sizing Worksheet, line 4)
AH/Day ____

2. Enter the maximum number of consecutive cloudy weather days expected in your area, or the number of days of autonomy you would like your system to support.

3. Multiply the amp-hour requirement by the number of days. This is the amount of amp-hours your system will need to store.
AH ________

4. Enter the depth of discharge for the battery you have chosen. This provides a safety factor so that you can avoid over-draining your battery bank.
(Example: If the discharge limit is 20%, use 0.2.) This number should not exceed 0.8.

5. Divide line 3 by line 4.
AH _________

6. Select the multiplier below that corresponds to the average wintertime ambient temperature your battery bank will experience.

Ambient Temperature Multiplier
80ºF 26.7ºC 1.00
70ºF 21.2ºC 1.04
60ºF 15.6ºC 1.11
50ºF 10.0ºC 1.19
40ºF 4.4ºC 1.30
30ºF -1.1ºC 1.40
20ºF -6.7ºC 1.59


7. Multiply line 5 by line 6. This calculation ensures that your battery bank will have enough capacity to overcome cold weather effects. This number represents the total battery capacity you will need.

AH _________

8. Enter the amp-hour rating for the battery you have chosen (use the 20 or 24 hour rate from the battery manufacturer).

9. Divide the total battery capacity by the battery amp-hour rating and round off to the next highest number. This is the number of batteries wired in parallel required.

10. Divide the nominal system voltage (12V, 24V or 48V) by the battery voltage and round off to the next highest number. This is the number of batteries wired in series.

11. Multiply line 9 by line 10. This is the total number of batteries required.


Step 1 -
Determine Your Power Consumption Demands
Step 2 - Optimize Your Power System Demands
Step 3 - Size Your Battery Bank
Step 4 - Determine the Sun Hours Available Per Day
Step 5 - Size Your Array

Purchase Online

toll free assistance
Personal Assistance
Toll Free support
Product Selection

Federal, State and Local Utility Cash Rebates for Solar and Renewable Energy

Federal, State &
Local Utility Cash
Rebates for Solar
& Energy Efficient
» Cash Rebates

photo gallery

Grid-Tie Systems

GTS General Info
GTS Packages

GTS Components

GTS Modules
GTS Inverter
GTS Mount/Wiring

Grid-Tie Systems with Battery Backup

GTB General Info
GTB Packages

GTB Components

GTB Modules
GTB Inverters
GTB Combiner
GTB Mount

Individual Components

PV Modules
PV Inverters
PV Controllers
PV Batteries
PV Mounting
PV Hardware

 Other Products

Solar Pool Heaters
Heat Pumps
Gas Pool Heaters
Solar Hot Water
Tankless Heaters
Instant Hot Water
Space Heating
Solar Electric
Wind Power
Solar PV Lighting
Tubular Skylights
Pool Accessories
Spa Accessories
Solar Cooking
Energy Savings

10 things you can do to help solve the climate crisis
  Solar Direct    
  Bringing Renewable Technology Down to Earth!   Copyright Notice
© 1986-2016 Solar Direct
All Rights Reserved