In  Solar

How many solar panels do I need to power my home?

By [email protected] | January 2, 2021

Your energy usage in kilowatt-hours (kWh) dictates the size of your system. Panels have a broad range of wattages (270W-495W is common as of late 2020), and other factors like local sun exposure, mount orientation and the presence of a battery bank also play a part.

We sometimes get asked: “How many solar panels do I need?”

The answer is pretty complex, and frankly, most people approach it from the wrong angle when they start to look into solar.

We’re often asked to quote a system to power a 3-bedroom home or support a family of 4. In these situations, it’s impossible to provide an accurate estimate until we know more about the household’s energy needs.

In reality, the best place to start is to evaluate your current energy use based on past electricity bills. Past usage data is the best baseline to figure out how many panels you will need.

And this is how we have chosen to model our solar sizing algorithm at

Lifestyle, inverter preferences, battery choices and panel efficiency all play a role in figuring out the size of your solar energy system. Here’s the process we use to make an accurate estimate.

But, how many panels do I need?

In order to figure out just how many panels you need, you’ll want to gather up 6 months to a year’s worth of electricity bills. Take peak periods into account as you estimate how many panels you’ll need to cover your energy usage. The winter months are usually heavy on electricity consumption in South Africa. You will need to at least work in that consumption to calculate your average monthly electricity consumption.

Some municipalities provide a 12-month summary of how much electricity you use on every bill. Depending on your utility provider, you may just need one bill to find an estimate for the year.

Got your paperwork in order? Great – you want to look for how many kilowatt hours (kWh) of electricity you use per month.

Wait, what’s a kilowatt hour?

A kilowatt hour (kWh) is a measurement of energy. If an appliance rated for 1 kilowatt (1000W) runs for an hour, then one kWh of energy has been used.

Your local electricity utility in your city or town (i.e. City Power in Johannesburg) measures total energy usage in kilowatt hours. Your total usage in kilowatt hours determines how much you are billed each month.

Example 1: A fridge rated at 250 watts runs for 4 hours per day. 250W x 4 = 1000W, or 1 kW. This fridge uses 1 kWh of energy over the course of a day.

Example 2: An oven is rated at 2000 watts (2 kW). Cooking in this oven for half an hour would consume 1 kWh of power (2kw x 0.5 hours = 1kWh).

Find how many kWh of electricity you use per month, this is usually displayed on your monthly statement from your local municipality if you have a post-paid account. That will give you a good starting point for estimating your energy needs – but you’re not there yet.

Divide that number by 31 days to get your daily energy usage in kWh. If you are lucky, your statement may already have your calculated daily average electricity consumption. And thats the number that is needed when you work out the sizing of your solar installation at

Once you have your daily energy usage, use this formula to estimate your total system size:

Daily Usage (kWh) ÷ Sun-Hours ÷ 0.9 inefficiency factor = Minimum Solar Array Output

Sun-hours refers to how much sun you get each day where you live. In South Africa, its generally accepted that maximum direct sunlight that can hit the solar panels in a standard installation is an average of 5 hours in a day, sometimes less in summer when there is cloud cover. But it is best to calculate at 5 hours.

The inefficiency factor simply accounts for circumstances that would make your system run below its optimal output, like shade, extreme temperatures, voltage drop and equipment inefficiencies.

Take your daily usage and divide it by these two numbers to get an estimate of the overall output of your system.

For example:

29.5 kWh per day ÷ 5.5 sun-hours ÷ 0.9 = 5.9595 kW capacity system.

That would give you an approximate system size of 5.96 kW, or 5959W (remember, 1 kilowatt = 1000 watts).

From there, the last step is to divide by the energy rating of each individual panel. Solar panels are graded by how much power they use. The panels you would use in a residential setting typically range from 270 to 440 watts per panel.

Let’s say we want to use ArtSolar 440W panels. Take your system size and divide by the panel wattage to figure out how many solar panels you need in your system:

5959W ÷ 440W = 13.54 panels

Round up the final number, since you can’t buy partial panels. In this scenario, we would need 14 panels rated at 440 watts apiece to cover our energy needs.

We can’t stress this enough: this calculation is a very rough estimate. It should only be used to ballpark system size and make some pricing estimates.

But don’t take this estimate as gospel – there are too many factors that can change the size of your system in practice.

This article offers some material guidance, however it may be best for you to chat to one of our solutionists who would able to work out what your requirements are and how to more accurately size up your solar installation.

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