How big does my off grid solar system need to be?

An off grid solar system will generate you free electricity by converting the rays of the sun into usable current, since it’s off grid you will not need to be connected to the national power grid in any way. Instead of drawing from the mains when the sun doesn’t shine, batteries are used for retrieval and storage of excess power.

A large enough off grid solar system could be making you free from both the national power grid and thus your electricity bill, would you wish it. But even a small amount of solar cells could be practical, actually this is the ideal solution for such places as a gazebo, RV or other places where no permanent power connection is present.

In the following steps, we will determine the right solar panel capacity and battery size for your needs, so keep on reading.

How to Calculate the necessary size for an off grid solar system

To determine how big of an off grid solar system you need it is important to know how much energy you use, or will use. When this is known, the correct number of solar panels can be determined and then the capacity of the required battery can be calculated.

Because electricity is measured in thousand watts per hour (kWh) we must first calculate how many watt-hours will be used per day. This is done as follows:

1) Make a list of all devices that will be connected to the off grid solar power system

2) Make a note of how many watt each devices uses . You can also measure this by using a simple wattmeter . Finally write down and how many hours per day the unit is turned on.

3) Multiply each appliances watt by the number of hours that the device is turned on and add the results together. The number you have right now is the number of watt-hours that the off grid solar system must be able to provide a day.

4) Determine how  powerful the  batteries  will need to be.

In step 3, we calculated how many watt-hours we will use during a day. So we need a battery to store at least that as a minimum capacity line. The capacity of a battery is specified in the number of Ah. (Ampere hours). The number of ampere-hour is the amount of current a battery can deliver for an hour.

To be able to calculate how many  Ah (Ampere hours) of battery capacity will be needed for the expected wh (watt hour) usage, the voltage of the battery is important. The voltage of batteries generally is 12V (volts) The conversion is as follows:

 Wh(watt-hours) / 12V (voltage v / d battery) = number of ampere hours.

TIP: It is practical to assume that the actual battery capacity needed is at least 2x as high asone days power consumption. This way you will never be without power.

5) How many solar panels will I need?

The efficiency of solar panels is specified in pW (peak Watts).The number of peak Watts indicates how many watts can be delivered under ideal conditionsi.e. when maximum amount of sunlight hits the panel.

Since the sun doesn’t shine equally strong and equally long in all parts of the world , the sunlight conditions of each specific location on earth is calculated by the number of full sun hours per day.The number of full sun hours equals the number of hours per day that the light conditions have been ideal. This figure is an average value of all the measured incoming sunlight throughout a whole day.

For example In Miami, Florida the number of full sun hours averaged over a year is about 5,5 hours /day, but in the UK that number is about half that i.e. 2,5-3 hours /day. The number of full sun hours in summer is of course higher than in the winter. If you only want to use the off grid solar system during a certain time of the year the average full sun hours could be a bit more or less than the total yearly average.

The calculation for how powerful solar panels you will need for your off grid solar system to be able to produce at least as much energy as is consumed per day goes as follows:

Number of watt-hours required per day / number of full sun hours per day = total pW (peak Watts) of the photovoltaic panels.

TIP: It is wise to multiply a bit due to losses that could occur in the system. To be sure that the panels produce enough power to replenish the batteries multiply the abovepW with 1,3

Attention! PV Panels should always have an output voltage which is approximately equal to the voltage of the batteries used. The panels for an off grid solar system are therefore smaller than the panels that are used in a grid-tied one. The bigger the panel, the more voltage it will produce andsince the voltage of the grid is much higher than that of a battery the off grid solar panels will need to be smaller.

Example calculation

1) List of devices connected to the system:

• Fridge
• Tv
• Tv standby
• Energy saving lamp
• Oven
• Laptop

2) Number of watts and the number of hours that they are used every day:

• Fridge: 20W, 24 hours
• Tvlcd 32 inch: 55W, 4 hours
• Tv standby: 6W, 20 hours
• Energy saving lamp: 8W, 8 hours
• Oven: 2000W, 1hour
• Laptop: 50W, 3 hours

3) Calculate the total watt-hours

• Fridge: 20*24=480
• Tv: 55*4= 220
• Tv standby: 6*20= 120
• Energy saving lamp: 8*8= 64
• Oven: 2000*1= 2000
• Laptop: 50*3= 150

Total: 3034 Watt-hours per day

4) Determine the correct battery capacity:

• Required: 3034Wh / 12V = 253 Ampere Hour
• 2 times the capacity: 86 x 2 = ca 500 Ah.

Minimum required battery: 12V, 500 Ah

5) Determine the needed amount of PV panels if total peak solar hours are for example 4h:

• Solar Panels needed: 3034Wh/4 = 759 pW
• 1.3 times for losses etc =  986pW

Minimum required panels ca: 1000 pW

Final conclusion:

To put together the off grid solar system in the above example the following will be needed:

• Battery 12V, 500 Ah
• Solar panels with a total of 1000 pW (for example, 5 solar panels a´200 pW)
• A battery charge controller
• The wiring between the Photovoltaic panels and the battery.
• An optional inverter to turn the 12V DC to 230V AC

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