How Many Solar Panels Do I Need for 20kwh a Day?

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Most American homes consume 30kwh per day, but those who go off the grid typically use less, around 20kwh. So if you want to be energy independent, how many solar panels are you going to need? There are a lot of factors involved and math too, but we simplified things for you.

If you consume 20kwh a day, you need a 5kw solar system or about 13 x 400 watt solar panels. To calculate, multiply your hourly wattage usage by the number of peak sun hours available. The result is the watts your solar panels have to generate per hour. Add 15% for reserve power.

How Many Solar Panels For 20kwh?

To find out how many solar panels you need, we have to consider several factors. The most important are:

  • Your daily power consumption in watts
  • The average number of peak sun hours in your area
  • The output of the solar panels
  • The calculation can be summed up as:

Number of solar panels = hourly power consumption (W) x peak sun hours / solar panel wattage output

Example Calculation

Step 1. First you need to convert kilowatt hours into watts. A kilowatt is equal to 1000 watts, so 20kwh is 20000 watts.

Step 2. To find out your hourly watts consumption, divide 20000 watts by 24. The result is 833.3.

Step 3. Multiply the hourly power consumption by the number of peak sun hours in your area. You can find this information at the Renewable Resource Data Center.

Step 4. Suppose you get 5 peak sun hours on average. 833.3 x 5 = 4166.5

To produce 20kwh a day, your solar panels must produce at least 4166.5 watts in 5 sun hours. Because solar panel output fluctuates (cloudy skies, rain, etc.) it is a good idea to add 10-15% additional to the output.

4166.5+ 15% = 4790.9

With 5 peak sun hours, your solar system has to produce 4790.9 watts per day.

Step 5. Solar panels come in all shapes and sizes, but the HQST 400W solar panels is a good choice because of its high output and saves space.

Solar panels rarely produce their maximum output, so a 400W solar panel might generate 390W on average. Using that number:

4790.9 / 390 = 12.2

You need 12-13 x 400W solar panels to get 20kwh a day. This assumes you have 5 peak sun hours and each panel produces 390 watts. You can also run these examples with other solar panel sizes to see how many you would need.

From this example you can see how the number of peak sun hours affects the results. If you live in a sunny state like Arizona or Nevada, you need fewer solar panels. But if you live in Washington for instance, more solar panels are needed to produce the same amount of power.

If you are going to be off grid, it takes more than just solar panels. You need a battery bank to store the electricity and keep your home powered at night. And you will need those batteries for backup power during winter or in case of a power failure.

An inverter is also required to convert direct current (which solar panels produce) into alternating current (AC) so appliances can run. You also need a charge controller to prevent the solar panels from overloading the batteries.

Related. 5 Reasons Your Inverter Keeps Shutting Off

How Many Batteries Does a 20kwh Solar Panel Need?

If you are going to go off the grid, batteries will be required. Solar panels cannot produce electricity at night, so you need batteries to store solar power and run appliances.

The number of batteries you need depends on your power consumption and how much power backup you need. The more sun hours available, the less battery storage is required.

Example 1

You want to go off the grid in Illinois. Your average daily power usage is 20kwh and there is 3.14 peak sun hours on average.

You want back up power that is good for one week, in case the power gets cut off during winter. How many batteries would you need?

The calculation is: 20kwh x 7 = 140kwh

140kwh=140000 watts

Divide the watts by the battery voltage (12 or 24V):

140000 / 12 = 11666

140000 / 24 = 5833.3

Rounded off that would be 12000 and 6000ah respectively

Quality batteries like the LiTime 12V 300Ah LiFePO4 have an average depth discharge rate of 85%. Assuming you will use 300ah batteries:

300 x .85 = 255

Each 300ah battery has 255ah of usable capacity.

12000 / 255 = 47.0

6000 / 255 = 23.5

You need 47 x 300ah 12V or 24 x 300ah 24V batteries for a full 7 day backup.

Example 2

Suppose you want to go off the grid in New Mexico. On average you get about 6.8 hours a day. You need less battery for backup, but your power consumption rises due to the heat. Let us see how that works out.

In this case you only need backup power for three days. However, the heat has increased your power consumption up to 30kwh.

We use the same steps as in example 1:

30kwh x 3 = 90kwh

You need a 90kwh battery bank to back up 3 days’ worth of power supply. That is equal to 90000 watts. Divide that by 12 volts and you get 7500ah. Divide it by 24 and you get 3750.

Assume that you will be using 300ah 12V batteries with an 85% DOD. It means 255ah is usable.

7500 / 255 = 29.4

You need about 30 x 300ah 12V batteries to back up 90kwh of power for 3 days (30kwh a day).

If you are going to use 24V batteries:

3750 / 255 = 14.7

You need 15 x 300ah 24V batteries.

If you are using 150ah batteries, double the number. But if you have a 400ah battery bank, you would need less.

As you can tell, it takes a lot of batteries to be completely off the grid. You have to make allowances in the calculations because if the sun goes out, there has to be enough battery power to run your home. This is why those living off the grid try to be as energy efficient as possible.

Frequently Asked Questions

What is the best solar panel size for 20kwh a day?

400W solar panels are ideal because they take up less space. You only need 14 of them to generate 20kwh a day (under ideal conditions). If you opt for 200W solar panels, you need 28 of them.

Why are my solar panels not producing the stated output?

Solar panels rarely produce the power they are rated for consistently. Changes in the weather, where the panels are installed, objects shading the panels, etc. A lot of factors affect the output. In general though, solar panels can produce close to their stated output in good weather.

Can solar panels produce 20kwh a day without batteries?

If you install enough panels and there are sufficient sun hours, it is possible. But without batteries, you cannot power your home at night. If the sky turns cloudy, it rains or snows, solar production will drop. By storing the electricity in batteries, you can provide your home with a consistent power source anytime.

Is 20kwh enough to power a house?

US homes consume 30kwh a day on average, but if you are energy efficient, this can go down to 20kwh. It all depends on how many appliances you use, how many there are in your household etc. But yes, it is possible to power a house with 20kwh a day.

What batteries are the best for large solar systems?

Lifepo4 lithium batteries are the top choice. They have the best depth discharge rate, need little maintenance and last a long time. As far as size goes, 300ah is just about right. You can go smaller with 100ah or 200ah but they will take more space.