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The art (and a bit of science) of sizing your solar system: A primer

Updated: Aug 10, 2021

Sizing your solar components can sound really complex, but if you break it down into basics, it really is fairly straightforward. Also, if you have endless money, size really is not an issue, as bigger is (almost) always better if you don't mind the cost and you have enough space. However, most people get into solar to save money in the long run, and for that reason you want to make sure you are at the right spot on the cost / benefit curve:

  • Go too small and you will struggle with getting through a power outage, be restricted on what appliances you can run on your system, or just not save any significant costs.

  • Go too big, and you will have a massive investment that you can never pay off or get a return on.

  • In the middle is a sweet spot where you've spent the minimum amount of money to get a well functioning system that meet your consumption needs, and have 80% of the capabilities or "value" of a more expensive system.

Solar is all about being on the right spot on the value curve for your specific needs, and luckily the curve is not linear. There is a "just right" range where you are getting most of the benefits, but at half (or less) of the cost of a big system. This "just right" spot is usually fairly easy to determine. Interestingly, I see most people who eventually take the plunge into buying a solar system for their home, spending too much. They typically get convinced by the sales guys that they need a very big system. This results in a general misconception that you need to spend a big fortune before making solar worthwhile.

When doing sizing, you need to think about the size of these below components. They need to be in sync in terms of size... no use you have dozens of solar panels but can't convert or store the electricity they produce.

  • Load sizing (how much electricity do you use on your essentials per day),

  • Solar panel array sizing, (for generating electricity)

  • Inverter sizing, (for converting electricity to the right format for use)

  • Charge controller sizing (for your batteries), and

  • Battery sizing (for storing electricity for use at night, or when there is no sunshine, or for spikes in demand).

Modern hybrid grid-tied systems make it a lot easier to get into solar, since the utility power can still carry some of your household load (when / if needed). The inverter can also be "smart" in the sense that it seamlessly integrates, so you don't even realise when the utility is supplementing your usage. This allows you to size a system that does not need to take your maximum possible peak loads into account. Only for very short periods and spikes in the day you have a high load, like when boiling water, using a blow-dryer, microwave, etc. If you size your system based on these spikes, then you are going to over-invest and never pay off your investment on savings.

So, in short, if you want to spend the minimum and still get most of the value out of solar, you need to:

  • Identify your absolute essential minimum load that needs to run on your system when the mains utility power is not available. This will include things such as TVs, lights, internet, computers (and maybe) a microwave.

  • Either have a smart hybrid inverter that can supplement the load with utility power if you need to draw more power for a short bit, or split out this essential circuit (electrician needed) if you are sure you will not exceed your essential load limit.

  • Install enough solar panels so that it produces slightly more power than what you typically use per day (24 hours) on this essential circuit.

  • Buy enough batteries so that your solar panels can fully charge the batteries in a single sunny day, but also be able to run this essential circuit for at least 10 hours.

And voila, you have an entry level small solar system, which will keep your household going when there is no mains power! Practically, this system may look like this:

  • For example, if essential load is calculated at an average of 500 watt per hour for the day (12,000 watts or 12 kilowatt-hours of power total in 24 hours).

  • Minimum inverter size will be 1600 watts (1.6 kilowatt) to carry this. (Above this get's supplemented by mains power.)

  • 8 panels gives you about 2,400 watts of power x 6 hours a day, so call it 14,000 watts of power. (Strictly speaking this is called 14 kilowatt-hours of power.)

  • Minimum battery that will last about 10 hours at 500 watts = 5 kW of usable battery storage, and your solar panels can easily produce this in a day.

The above of course is fairly small, but you can just up it slightly (increase costs with about 40%) to be the correct size for a medium household install. By over-spending and going much bigger with a system that is multiples more expensive, you will have very little additional benefit. Especially if the additional cost just allows you to handle heavy appliances, which you may actually very seldom use. You will effectively just generate and store power you don't use, or just have the luxury of running heavy load spikes (typically non-essential) when the mains are off.

A BIG factor in sizing is making sure your house is already energy efficient before going solar. If your primary goal is to save on electric costs, the absolute first thing you need to do is install a good solar geyser (or retrofit to your existing). A solar geyser can immediately remove 40% of your electric bill at a fraction of the cost of a solar system. The other important things to do include:

  • Only use energy efficient LED lighting,

  • Use gas for heating and cooking if possible,

  • Use heavy appliances such as pool pumps and air-con sparingly and buy energy efficient units. (Typically you don't want to put these on your solar circuit, unless you are going for a big install.)

In summary, for most households the following table summarises our suggested minimum sizing of the various components:

P.S. Some other Pro-tips:

  • You do not need the most expensive solar panels. As soon as you go above 340 watt panels, they start to cost more per watt. You also do not need the fancy frame-less ones. The poly-crystalline ones are just fine. Make sure they are durable and come with a high impact rating if you live in an area that is prone to hail, and have a 20 year guarantee. On your roof all these different panel types look good!

  • If you have space / budget for a couple extra panels above what you calculated, that is always a good investment. Panels are not that expensive, and sometimes you have multiple days of cloud cover and the extra panels really help.

  • 48 volt system (inverter, charger, batteries, wiring, etc) are more efficient. Always go for a 48 volt system over a 24 volt system if at all possible.

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