What size 12V solar panel do I need?

What size 12V solar panel do you need to charge a 12 volt battery?

The most common use for 12 volt solar panels is to charge batteries.

You would need a 150W 12V solar panel using an MPPT solar charge controller to fully recharge a 100Ah lead-acid deep-cycle battery discharged to 50% (600 watt-hours). A 150 watt 12 volt solar panel will generate 675 watt-hours with an average irradiance of 4.5 peak-sun-hours per day.

Video – What can you run with a 100 watt 12 volt solar panel?

What is the nominal voltage of a solar panel?

The nominal voltage of any device gives an indication of it’s designed operating voltage.

An auto battery, for example, operates at a nominal 12 volts d.c., because auto equipment is designed to operate at this average voltage. The same auto equipment could operate at 11 volts or 13 volts if needed, but 12 volts is the design voltage.

A lead-acid battery can have a terminal voltage of between 11.5 volts or almost 13 volts, depending on it’s state of charge.

A solar panel operates in a completely different way, but we call it a 12 volt panel because it’s ideal for charging 12 volt batteries (or 14 volt batteries in the case of lithium technologies.)

Solar panel open circuit volts (Voc) is much higher than the normal operating volts. For a 12 volt solar panel it’s about 22 volts (see specs below.)

Table – 12 volt 100 watt solar panel specifications

Vmp and Imp are the values of voltage and current that create maximum power, and a typical output working voltage is about 17 volts.

So why is this called a 12 volt solar panel?

When a load such as a battery is connected to a solar panel, the voltage is pulled down to an unknown value.

The working volts is not known until the load is connected – the actual current and voltage is determined by the load resistance. For a battery the resistance may be around 1 ohm.

Solar charge controllers adjust the solar panel output voltage to a value needed to provide the right charging current. At the beginning of the charge, for example, this could be about 14 volts.

A PWM (Pulse Width Modulation) charger will use the voltage it needs for the correct charging current, wasting the rest. That’s why they are considered inefficient.

It’s inefficient because all of the available power that the solar panel generates is not being used. Remember this is when Vmp and Imp are at the right values – it’s called the Maximum Power Point.

MPPT devices match the internal resistance of a solar panel to extract maximum power

This point is achieved when the panel’s internal resistance (AKA Characteristic Resistance) is the same as the load resistance. A typical value for a 100 watt 12 volt solar panel is 3 ohms, compare to about 1 ohm for a battery.

An MPPT charge controller (Maximum Power Point Tracking) adjusts its internal resistance to that of the panel and draws maximum power (and charging current) for battery charging.

This is why MPPT controllers can be up to 40% more efficient than PWM type.

How do I calculate what size solar panel I need?

Solar panels are always sized with reference to the load i.e. how much energy do you need to generate every day? Once you know this you can work out the size of solar panel needed.

Let’s say you had a simple load of 500 watts running 8 hours a day. The energy needed to supply this load is:

Energy in kilowatt-hours = 500 watts x 8 hours = 4kWh

A 100 watt panel in a geographic location with irradiance of 4 peak-sun-hours per day will generate:

Solar energy estimated = 100 watts x 4 peak-sun-hours = 400 watt-hours

Solar panels required = 4000Wh/400Wh = 10 solar panels @100 watts rating each (1000 watts)

How long will a 100 watt solar panel take to charge a 12V battery?

The two most common solar storage batteries in common use are lead-acid deep-cycle (leisure batteries) and lithium iron phosphate, which is inherently deep-cycle.

Leisure batteries can be discharged up to 80%, but 50% is recommended for maximum life. LiFeP04 can be discharged to 80% with long life.

This means a 100Ah leisure battery only has 50Ah available and LiFeP04 has 80Ah available for use. A standard auto battery shouldn’t be used for solar storage as only 20% discharge is recommended.

Table – 100 watt solar panel charging time 12 volt batteries

What size solar panel do I need to charge a deep cycle battery?

The table below gives some idea how much solar is needed to charge two kinds of battery – auto and deep-cycle.

It also shows how the irradiance (sun’s energy) at a location effects the solar panel wattage needed and the charging time.

Table – Effect of irradiance on solar panels size for battery charging

Related questions:

Can a 100 watt solar panel run a refrigerator?

A 100 watt solar panel is too small to run an average size fridge.

A typical medium-sized refrigerator would use about 2.184 kilowatt-hours of electrical energy per day. With an irradiance of 4 peak-sun-hours per day, a 100 watt solar panel will produce and estimated 400 watt-hours. (100 watts x 4 peak sun hours = 400Wh).

If we divide 2184Wh by 400Wh we have 5.46 solar panels rated at 100 watts.

This means 600 watts of solar panels would be needed to continuously run the refrigerator together with a suitable sized battery to account for surge current and night-time operation.

Resource: Can a 100 watt solar panel power a refrigerator?

How many solar panels do I need to run a 12 volt refrigerator?

The average 12 volt fridge used in RVs is rated at about 60 watts and will pull about 5 amps when running continuously. This is called the ‘spot’ or instantaneous load.

This value isn’t much good when sizing solar panels because sometimes the compressor doesn’t run and when the fridge motor starts the inrush or surge current can be several times more than the running current.

Why is this important? If you size a solar panel just big enough to run the fridge, it may not have enough power to feed the motor as it starts.

This is one reason why a battery is essential – to act as a reservoir to supply motor inrush current. The second reason is to run the fridge through the night.

Energy required for 60 watt fridge = watts rating x 0.7 utility factor x hours running

Energy = 60 x 0.7 x 24 = 1008 watt-hours (1kW)

A 100 watt solar panel will produce about 400 watt-hours with an irradiance of 4 peak-sun-hours/day.

Number of solar panels required to run 12 volt fridge:

1008/400 = 252 watts, so 3 solar panels rated at 100 watts each.

How long will a 12 volt battery run a refrigerator?

Table – How long will a 12 volt battery run a fridge?

How many amps does a 100 watt solar panel put out?

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At the panel’s maximum power point a 100 watt solar panel can put out between 5 and 6 amps, so about 5.5 amps on average.

The actual current value depends on two things:

1. The irradiance (sun’s energy) in the geographic location
2. The load resistance connected to it

Vmp (about 17 volts) and Imp (about 5.5 amps) occur when the load internal resistance matches that of the panel, known as the Characteristic Resistance.

These maximum values will only occur when the peak-sun-hours value is sufficiently strong i.e. the panel is under full sunshine.

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