Solar Panels and Batteries

Estimating Quantities of Batteries and Solar Panels Required

In order to estimate how many solar panels and batteries you might need for a given requirement of daily energy, please fill in the relevant figures into the form below.

For more information about the various components normally included into a solar system, please read our Basic Overview of Solar System Components.

The calculation assumes the following:

• The solar panel will produce full power for at least 4.1 hours per day.
• System losses are about 20%.
• The battery nominal voltage is 12V.

Please note that you may have to adjust the number of batteries needed to match the DC system voltage selected.  For example if you decide on a 24V system, then the number of 12V batteries needs to be divisible by 2, if you decide on a 48V system, then the total number of 12V batteries needs to be divisible by 4 etc.

Its preferable to select a higher DC voltage because higher DC system voltage reduces cable losses, as well as reducing the price of DC cabling (because you can use thinner cables).

Please further note that you also may have to adjust the number of panels required if you need to form a number of equal length strings (a set of panels connected in series is referred to as a string).  Please see our panel array section below for more info.

Grid Tie System Calculations

In order to work out how many panels you need in a grid tie solution, you need to start with the maximum power required from the system at midday.  The form below will then estimate the minimum number of panels required assuming system losses of 15%.

PV Panel Array Design Using “Strings”

When you need more than two or so panels, then it is likely that you will have to start forming strings.  You connect PV panels in series to increase the voltage so that you reduce the current flowing in the cables that lead back from the solar panels to the MPPT or charge controller.  It is preferable to try and get as high a voltage as you can because that reduces cable losses caused by high currents. It also allows you to use thinner cables which brings down the cost of the installation.

However you cannot exceed the input voltage rating of the MPPT, so you are limited to how many PV panels you may connect together in series.

Your array will then be formed by a number of equal length strings.  Each string will produce the nominal current of a single panel.

For more information about how the panels are wired to form strings please read our Basic Overview of Solar System Components.

The calculator below can be used to work out the maximum number of panels you can connect together in series to form a string.  It will also estimate the minimum number of strings you will need in your array.  You will need to have worked out the minimum number of panels you require in your array using one of the calculators above.

So using the numbers calculated above you can now decide how many panels to wire together to form a string and then how many strings you will need (the total number of panels in the array must be at least equal to the minimum number of panels you need).

You can try vary the string length and thus number of strings required to minimise the number of panels you need to buy, as long as the total number of panels in the array remains at least equal to the minimum number of panels you calculated earlier.

Please note that all strings in the array must have the same number and types of panels wired in series.

Please note that these calculations are offered as a rough guide based on various assumptions which may not be applicable in your case.  You should do your own calculations to be sure.