Solar Panel Sizing – How To Calculate Home Solar System Size

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Solar panel sizing for your home

Sizing Solar Panels – How many solar panels does it take to power your home?

Solar energy is the buzz at the moment and many homeowners want to take advantage of the Federal Solar Tax Incentive while it lasts, but how do you know how many solar panels you would need to power your house?

Let’s get sizing!

How Many Solar Panels Do I Need To Power My House

Whether interested in installing your own DIY solar power system or or just curious, the first step in calculating the size of a solar panel array to power your home always begins with energy consumption.

These are the things you need to know before sizing your home’s solar system:

  1. What is your home’s annual energy consumption in kWh (kilowatt-hours)?
  2. How much do you pay per kWh of electricity (use this to calculate payback period)
  3. What is the irradiance (sun’s energy) level in your location?
  4. Will your system be grid-tied or off-grid (off-grid needs batteries)
  5. If off-grid, what type of battery will you use (lead-acid or lithium iron phosphate)?
  6. What size solar panels in watts will you use?
  7. Inverters – will you use a central, string or micro-inverter topology (system design)
  8. What are the PV system losses?

As a general rule a home solar power system needs 6 solar panels each rated 300 watts with average irradiance of 4kWh/m2/day for every 5kWh of daily energy consumption. The average US home consumes about 30kWh per day and will need 36 solar panels rated at 300 watts.

Video – How many solar panels do you need for your home?

How to calculate home energy consumption

You could add up the wattage of all your appliances and estimate how many hours they are used each day, but this is very cumbersome and not accurate.

For example, some appliances, such as refrigerators, are difficult to estimate. This is because they have compressor motors on board and have variable instantaneous power values.

By far the easiest way is to take it from last year’s utility bill, which normally shows the full year’s energy consumption in kilowatt-hours (kWh).

Simply divide this figure by 12 to get the monthy consumption, or 365 for the average daily consumption.

The average amnount in the US is about 30kW per day, but your own will depend on where you live.

How do I calculate solar panels for my home?

Once you have found your daily energy consumption, it can be used to size your solar panels.

It needs to be said that solar panel power ratings are not an accurate reflection of the actual power generation in real-life conditions.

In fact, there are many losses associated with a working home solar system. As much as 23% is normally lost and must be deducted from the solar array power rating – see infographic below:

Energy losses for domestic solar panels sizing

Infographic showing solar panel losses for home power system

The calculator I provide later on in the post takes these losses into account, but for this example I’ll go through the process manually step-by-step.

How Many Solar Panels Do You Need To Power Your House?

Example – Sizing solar panel PV systems 

How many solar panels and their power rating would be needed for a fixed solar system to cover the energy consumption of an average U.S. home?

Let’s say the house is in Burns, Oregon USA, with an average electricity consumption per day of 33 kWh.

Checking the site Global Solar Atlas, we see that the irradiation in this location is 5.83kWh/m2/day.

Irradiance at Burns for solar panel sizing

Use historical irradiance data to find sun’s energy in your location

  • Geographic Location: Burns, Oregon, USA
  • Average daily energy consumption: 33kWh/day
  • Irradiation (peak-sun-hours) for Burns, Oregon is 5.83kWh/m2 per day

PV System Losses

I previously said that system losses can be 23%, but they can also be higher. I’ll use 30% as the overall system loss, to make sure I don’t underestimate the number of solar panels to cover the home’s energy needs.

With 30% losses in mind, the energy needs are:

33kWh/day x 1.44 = 47.5 kWh/day load.

Now to take into account the efficiency of inverter, which is about 96%.

The power to be supplied to the inverter = 47.5/0.96 = 49.5kWh/day.

The average daily irradiation is 5.83kWh/m2 (also known as peak-sun-hours)

49.5kWh/day of power can be produced by:

49.5/5.83kWh/m2= 8.5kW or 9kW of solar panels working at 100% capacity rating.

To find the number of solar panels needed, divide the wattage needed by wattage of each solar panel (say, 300 watts):

Number of panels needed = 9kW/300 watts per panel = 30 solar panels.

How Many Solar Panels Do I Need To Power My House Calculator

Use the calculator below to estimate the number of solar panels needed to run your home:

Solar panel sizing calculator

How many batteries are needed to power a house?

Hoe many home storage batteries?

How many batteries would you need to power the average home?

If your home consumes 30kWh every 24 hours and you want to have 24 hours autonomy, then you can easily work out how many batteries you would need.

There are two choices for home energy storage batteries – lead-acid deep-cycle or lithium iron phosphate. I prefer LiFeP04 but as lead-acid are still the mnost common, I’ll use them in an example.

Deep-cycle batteries can be discharge up to 80% of their capacity, but 50% is recommended in order to maximize battery life.

This means that we need to double the amount of energy required from the batteries, simply beacuse half of it isn’t normally available. 80% Depth of Discharge is OK from time to time.

The calculation looks like this:

Energy for 24 hours = 30kWh

30kWh in battery amp-hours (Ah) = 2500Ah (12V batteries)

Total battery capacity needed for 24 hours = 2500Ah x 2 = 5000Ah

25 batteries rated at 200Ah will be required to power this home for 24 hours.

How many solar panels do I need for a 2000 sq ft home?

An average 2,000 square foot home consumes about 950 kWh of energy per month or about 30 kWh per day.

The average electricity bill is 0.6 cents per square foot.

About 30 solar panels each rated at 300 watts would be needed to power a 2000 sq.ft. home.

How long does it take for solar panels to pay for themselves?

Solar panel payback period varies according to your location, as the sun’s energy is stronger in some locations. It also depends on energy consumption and your utility prices.

The example below shows the payback period for an installation in Houston, Tx:

Solar Payback Period Calculation For 5kW Solar Power In Houston, Texas (2021)

How long does it take for solar panels to payback?

How long before solar pays for itself in Houston, Texas?

  • Location: Houston, Tx
  • Solar system power rating: 5kW
  • Solar installation cost = $13700 – 26% tax credit = $10138
  • Peak-sun-hours in Houston = 1552/year
  • Power generated by solar system = 1552 x 5kw = 7760 kWhrs
  • Houston home electricity price = 10.98 cents/kWh
  • Yearly savings = solar generation x unit elec. cost = 7760 x 10.98 = $852
  • Time to pay back solar installation = install cost/annual savings = 10138/852 = 11.9 years

The payback time might be less than the above – 9 to 10 years is possible. Professional solar installers can provide very accurate estimates using historical data for the irradiance in your city and the orientation and tilt angle of your proposed installation.

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