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What size solar system does the average house need? What is the right size solar system array for my home?These are common questions for people serious about installing solar panels in their homes.

To calculate how size solar system you need, you need to know:

1. the number of peak sun hours in your area
2. your home electricity usage
3. Roof conditions
4. System losses

1.the number of peak sun hours in your area

Peak sun hours are a really important thing to consider. Some people make a mistake thinking that peak sun hours (PSHs) are the hours of daylight. In fact, the PSH describes the intensity of sunlight in a specific area, defined as an hour of sunlight that reaches about 1,000 watts of power per square meter.

The peak sunlight hours for your particular location will have a direct impact on the energy you can expect your home solar system to produce. we see that sunny Tucson, Arizona gets 7.5 – 7.9 PSH. That is more than twice the sunlight that rainy Seattle, Washington gets – which is only 3.3 – 3.9 PSH. That doesn’t mean a Seattle homeowner can’t go solar; it just means the homeowner would need more panels.

Any method you choose to calculate solar system size will take your location’s peak sun hours into account.

2.your home electricity usage

Now, on to the big question – ‘What size solar panel system do I need?’ We cover the three ways to calculate your energy needs.

1. Calculate your daily load manually
2. Look at your electricity bill for average usage
3. Ask a solar installation company to do it for you

①Calculate your daily load manually

Checking the power ratings.Make a list of all the devices you use daily, find out their power ratings (Watt) and multiply by the time you normally use them a day and sum them up.So get your average energy consumption.

②Look at your electricity bill for average usage

Look at your electricity bill for average usage.You’ll need to find your utility bills for at least the past three months. The best-case scenario is to take the electricity bills for the past year (12 months) and find their arithmetic mean. The thing is that our energy consumption varies depending on the season. For example, in winter your home electricity usage may be higher because of room heaters, christmas lights, etc.

A small home in a temperate climate might use something like 200 kWh per month, and a larger home in the south where air conditioners account for the largest portion of home energy usage might use 2,000 kWh or more. The average U.S. home uses about 900 kWh per month. So that’s 30 kWh per day or 1.25 kWh per hour.

Your average daily energy usage is your target daily average to calculate your solar needs.It’s important to note that solar panels don’t operate at maximum efficiency .recommend adding a 20 percent “cushion” to your target daily average to ensure you can generate all the clean energy you need.

③Ask a solar installation company to do it for you

Solar companies can combine peak sunshine hours in your area with in-person roof assessments to develop the most accurate projection of solar panel production. A reputable solar company would be happy to calculate the right solar system for your energy needs and provide a free quote for you to consider. 

3.Roof conditions

The orientation (‘azimuth’) and pitch (‘tilt’) of your roof will determine the orientation and angle of your solar panels. Both factors, especially direction, can have a considerable impact on solar energy production.Read Can every family install a solar PV system? learn how roof conditions affect solar energy systems

A solar array facing east or west will produce approximately 15% less output than one facing south. If you can only install on an east or west-facing roof, you’ll have to increase the size of the solar system to account for that.

4.System losses

You will not have access to the full DC output of your solar panels; that’s because all solar power systems experience system losses.

There are typically wiring losses of about 2% during the transmission of DC electricity to the solar inverter. Then there are conversion losses that occur as the inverter changes the power from DC to AC electricity – these account for a further 3% of electricity produced. 

Overall

That means with a typical grid-tied solar power system, You need to consider the efficiency of the solar panel, the connection loss, the DC conversion loss and so on about 25%.If you can only install on an east or west-facing roof，You also need to increase your budget by 15%.For example, if your home consumes 30KW per day, you will need to install a 7.5kw solar PV system(For the US, the average PSH number is something between  5.)