What size 12V solar panel do I need?
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 leadacid deepcycle battery discharged to 50% (600 watthours). A 150 watt 12 volt solar panel will generate 675 watthours with an average irradiance of 4.5 peaksunhours 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 leadacid 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
100 Watt Solar Panel Manufacturer 
Open Circuit Voltage (Ocv) 
Short Circuit Current (Isc) 
Maximum Power Voltage (Mpv) 
Maximum Power Current (Mpi) 
Renogy 100W 
22.5 volts 
5.75 amps 
18.5 volts 
5.29 amps 
HQST 100W 
21.3 
5.83 
18 
5.58 
RICH SOLAR 100 W 
22.6 
5.86 
18.5 
5.41 
WindyNation 100 W 
21.6 
6.32 
17.4 
5.75 
Newpowa 100 W 
20.23 
6.25 
17 
5.89 
ECOWORTHY 100 W 
21.6 
6.13 
17.4 
5.78 
21.4 
6.30 
17.7 
5.7 

ROCKPALS SP003 100W (Folding) 
21.5 
5.9 
18 
5.4 
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.
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 kilowatthours = 500 watts x 8 hours = 4kWh
A 100 watt panel in a geographic location with irradiance of 4 peaksunhours per day will generate:
Solar energy estimated = 100 watts x 4 peaksunhours = 400 watthours
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 leadacid deepcycle (leisure batteries) and lithium iron phosphate, which is inherently deepcycle.
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
12 volt battery capacity in Ah 
Time to fully charge with 100 watt solar (4 peaksunhours/day)) 

Car battery DoD 20% 
Deepcycle DoD 50% 
Lithium Iron Phosphate DoD 80% 

40 
1.4 hours 
4.2 hours 
5.6 hours 
60 
2.5 
6.3 
10 
80 
3.4 
8.4 
13.6 
100 
4.2 
10.5 
16.8 
120 
5 
12.6 
10 
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 deepcycle.
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
Location 
Glasgow, UK 
Chicago, Il 
Lancaster, Ca 

Peaksunhours/day 
1.86 
4.03 
8.07 

Solar panel rating in watts needed to fully charge each battery type in one day – full capacity 100Ah 
Car Battery – 20Ah discharged (20%) – 240 Wh required 
130 watts solar panel rating required 
60 watts solar panel rating required 
30 watts solar panel rating required 
Deepcycle – 50Ah discharged (50%) – 600Wh required 
322 watts solar panel rating rerquired 
149 watts solar panel rating required 
75 watts solar panel rating required 
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 mediumsized refrigerator would use about 2.184 kilowatthours of electrical energy per day. With an irradiance of 4 peaksunhours per day, a 100 watt solar panel will produce and estimated 400 watthours. (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 nighttime 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 watthours (1kW)
A 100 watt solar panel will produce about 400 watthours with an irradiance of 4 peaksunhours/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 long will a 12V 100Ah run fridges – annual energy consumption 200kWh to 1000kWh 

Annual Fridge Energy Consumption (kWh) 
Leadacid deepcycle (discharge to 80% of capacity) – hours run time 
Lithium iron phosphate (discharge to 95%) – hours run time 
200kWh/yr 
42 hours 
50 hours 
300 
28 
33.3 
400 
21 
25 
500 
17 
20 
600 
14.5 
17.2 
700 
12 
14.3 
800 
10.5 
12.5 
900 
9.4 
11.2 
1000 
8.4 
10 
How many amps does a 100 watt solar panel put out?
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:
 The irradiance (sun’s energy) in the geographic location
 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 peaksunhours value is sufficiently strong i.e. the panel is under full sunshine.
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