Choosing the Correct Solar System for Your House

How many solar panels do I need?

When it comes to choosing the size of your new solar system, you want to do it right. Undersized is a wasted opportunity, where oversizing can increase your payback period. This post will make show how we recommend solar systems – it’s not as complicated as you may think. It makes sense to first look at your restrictions.


The calculation is made easier by looking first at the maximum size system you can install.

  1. The first restriction you may say is your budget. However, if you purchase your solar system using a low-interest Green Loan, it is very realistic that you will save more on your power bill than you will make on your loan repayments. (we don’t usually recommend so-called “interest-free” scams. The “free interest” is just added to the price of your system and you’ll get stung by fees that will add up to much more than just an upfront low-interest loan.)
  2. The second restriction is set by your utility. Most homes in the Brisbane area are single-phase. On single phase home, the maximum inverter Energex allows is a 5kW inverter. With a 5kW inverter, you can install up to 6.5kW of panels. (If you have three phase it gets a little more complicated, however the logic behind this post will still apply.)
  3. The third restriction obviously is the size of your roof. The easiest way to work this out is by using an online program Nearmap. With Nearmap, we get updated high-resolution photos of your roofline, and we have the ability easily overlay panels to scale on your roof and very accurately size your maximum capacity. It’s a simple program to use but only with an expensive subscription. If you call us, we can size up your roof over the phone. Most 3-bedroom homes can fit 6.5kw of panels

Do I need a 6.5kW system?

If you have a $300 power bill, no. You probably don’t have an electric hot water system or pool. In fact, you probably don’t have a computer at home and you work by candlelight.
Even so, the standby power and base load of your fridge, plus the occasional use of appliances will often sit between 1kW and 2kW. For similar reasons to the logic below, there is usually not much point in getting a system below 3kW, as a 3kW system will operate between 1 and 2kW most of the day.

You could make use of a 3kW solar system reasons as pointed out below.
If you have a household with a $500 bill or more, then read on. More often than not, the calculation for the size solar system you need is done with an unclear understanding of how solar works. People look at the solution far too simply using this misguided formula:

  1. I use 25kWh a day (from my power bill).
  2. With lifestyle changes, I can use 70 per cent of my power during daylight hours.
  3. I need a solar system to produce 70 per cent of 25kWh or 17kWh a day.
  4. I need a 4kW solar system.

The flaw in this logic is that solar calculations are instantaneous. For example,

  1. At 8 am, your 4kW solar system is producing 1000w. You are getting the kids ready for school, and you are using 3000W. Your system is undersized.
  2. It’s a hot day on Sunday 11 am. Your 4kW system is producing about 3000W you are cooking lunch and the aircon is on and the kids are watching TV. You are using 4000W. Your system is undersized
  3. It’s a rainy day you are at work and your 4kW solar system is only producing 700w at lunchtime. You have your pool pump set to run at lunchtime, but with your standby power and pool, your house is consuming 1400w. Your system is undersized.
  4. You want to turn your 3.6kW hot water tank into a solar hot water system with your excess power (using the SunnyMate or the Fronius Smart Meter). Your system is undersized.
  5. It’s 2029, and batteries are affordable. You want to install the Tesla Powerwall Battery to get a step closer to energy independence, but you don’t have enough excess power most days to charge the battery. Your system is undersized.

Economy of Scale

But shouldn’t you aim to cover most of your power load, and don’t waste money on panels that will often be sending power back to the grid. Well, no, it comes down to economies of scale.

  • Each panel we install comes with a financial incentive or Renewable Energy Credits. After the incentive, panels cost under $100.
  • 5kW inverters are by far the most commonly used inverter today. It usually costs less than $100 extra to buy a 5kW inverter rather than a 4kW inverter.
  • The Electrician has already loaded up the panels, driven to your house, set up a ladder, ran the cable and worked on your switchboard. It doesn’t cost much more to install a few more panels while they are on the roof.

In summary, the first 4 kW of panels on your roof are expensive, and the next 2.5 kW of panels are less expensive. If a large portion of that power is sent back to the grid, it’s not a wasted investment, it’s a solar system that will cover almost all of your daytime load on most days. And it is a system that is ready for the battery revolution.

Leave a Reply

Your email address will not be published. Required fields are marked *

Mark Cavanagh

Mark is the Owner and Manager of MC Solar & Electrical. He’s an Electrician, accredited solar installer/designer and an electrical contractor.

Get A Free Product Tour

Visit our showroom in Eagle Farm, Brisbane and discover what’s possible for your property.