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Also keep in mind that cycling a battery through its full capacity will likely damage it if done repeatedly. To increase battery lifespan, there should always be some energy left in it before recharging; for this reason, usually only 50% of the capacity is used. As a result, the energy a battery can actually deliver is better measured by looking at half its full capacity.

Energy = 0.5 x voltage x capacity


A 100Ah battery contains 1,200Wh:

100 x 12 = 1,200Wh

To increase its lifespan only 600Wh can be used. How long would a 40W light bulb last in continuous use?:

600Wh / 40W =15 hours

A 40W light bulb could run for 15 hours before the battery needed to be recharged.


A battery with a rated float life of 10 years at 20°C. How long will it last if the average temperature is 30°C?

10 / 2 = 5 Years

It will last 5 years if the average temperature of the battery room is 30°C and only 2.5 years if the average temperature of the battery room reaches 40°C.

Cycle life

In addition to Float life, Cycle life is the number of cycles that the battery can withstand during its service life. A battery cycle is defined as a battery being fully charged and then fully discharge, making one full “cycle.” Is common to have this information in the technical specifications, always buy batteries with a Cycle life number bigger than 400 cycles


Once the battery model has been identified, the number of batteries required must be calculated. This can be done with the following formula, rounding the number up.


A system analysis indicates a need for 12,880Wh. The available batteries are 220Ah / 12V, and require a 50% maximum depth of discharge. How many batteries are required?

12880 / (50% x 12 x 220) = 9.76

10 batteries are required.

Note that all the batteries used in a battery system must be exactly the same:


To be able to charge the batteries in a fixed duration, the formula to use is:

Power=Energy consumption/charge duration



An installation has an estimated energy consumption of 12,880Wh, and needs to reach a full charge in 6 hours. What Wattage must the charger be?:

12,880 / 6 = 2,150W

The charge power must be at least 2,150W.

Charger power is often rated in current (Amps) rather than in power (W). To calculate charge current from the charge power simply divide the charge power by the charger voltage (usually 12, 24 or 48V).

  • If 12V charger is used, the charge current must be: 2,150 / 12 = 180A
  • If 48V charger is used, the charge current must be: 2,150 / 48 = 45A

Additional considerations:


There are several ways to wire multiple batteries to achieve the correct battery voltage or capacity for a particular DC installation. Wiring multiple batteries together as one big bank, rather than having individual banks makes them more efficient and ensures maximum service life.

Series Connection

Wiring batteries together in series will increase the voltage while keeping the amp hour capacity the same. In this configuration, batteries are coupled in series to gain higher voltage, for instance 24 or even 48 Volt. The positive pole of each battery is connected to the negative pole of the following one, with the negative pole of the first battery and positive pole of the last battery connected to the system.

For example; 2 x 6V 150Ah batteries wired in series will give 12V, but only 150Ah capacity. 2 x 12V 150Ah batteries wired in series will give 24V, but still only 150Ah.

Parallel Connection

Wiring batteries together in parallel has the effect of doubling capacity while keeping the voltage the same. Parallel coupling involves connecting the positive poles and negative poles of multiple batteries to each other. The positive of the first battery and the negative of the last battery are then connected to the system.

For example; 2 x 12V 150Ah batteries wired in parallel will give only 12V, but increases capacity to 300Ah.

Series/Parallel Connection

A series/parallel connection is the combination of the above methods and is used for 2V, 6V or 12V batteries to achieve both a higher system voltage and capacity. A parallel connection is required if increased capacity is needed. The battery should then be cross-wired to the system using the positive pole of the first and negative pole of the last battery.


Solar modules are rated in Watt-peak, represented as nominal peak power (P max), derived from multiplying peak power voltage (Vmp) with its peak power current (Imp):

Pmax = Vmp x Imp


A 100Wp solar panel produces 100W under standards test conditions (STC). The STC exist only in laboratories, applying a solar irradiance to panels of 1,000W/m2 with a cell temperature of 25ºC. In a real installation, the actual production of electricity is usually far lower than the peak-power, but the measures remain useful as qualitative reference to compare sizes and capacities as every panel is rated under those conditions.


  • Orientation: angle of the solar panel with the north-south axis
  • Tilt: angle of the solar panel with the horizontal plan

Orientation is quite easy - solar panels must face the south in the northern hemisphere and the north in the southern hemisphere.

Tilt is more difficult to optimize. Latitude can be used as an approximation of the optimal tilt angle, as referenced in the guide below for panels with fixed angels. However, even on the equator panels should have a minimum tilt angle of 5 to 10° to avoid accumulation of water and dust on the panel.


The output of the solar panels is connected to the solar regulator, while the output of the solar regulator is connected to the batteries. The solar panel mounting frame is connected to the ground, and a grounding/earthing connection may be required for the regulator and surge protector as well.