One of the most common questions I get is: “How big should my solar system be?” It’s a simple question — but the answer depends on a few key things, like how much electricity you use, where you live, and what your future plans are.
If you're just starting to explore solar, it can be tempting to say, “I’ll just throw a few panels on the roof and see how it goes.” But solar isn’t one-size-fits-all. And sizing your system correctly is what separates a system that works okay from one that pays itself off and makes you smile every time the electricity bill hits zero.
So let’s talk through how to figure out what you actually need.
The first thing to understand is that your solar system needs to be sized around your energy usage, not just your roof size or budget. Take a look at your last 12 months of electricity bills. Most utility bills will show your monthly usage in kilowatt-hours (kWh). Add those up to get your annual usage — that’s your starting point.
Let’s say you use 6,000 kWh per year. That’s about 500 kWh per month.
Now, here’s where your location comes in. Someone in a sunny region like southern Spain or California gets more daily solar production than someone in Germany or the UK. That means two people with identical energy usage might need very different system sizes.
This is where peak sun hours — which I explained in the last post — come into play. If you live somewhere with an average of 4.5 peak sun hours per day, that’s 4.5 hours of usable sunlight your panels can count on daily.
To roughly calculate your needed system size, divide your daily energy usage by the average sun hours.
So, for 500 kWh per month, that’s about 16.7 kWh per day.
Divide that by 4.5 hours of sun per day = a system that generates ~3.7 kW.
But wait — there are losses to account for: inverter efficiency, shading, panel age, dirt, etc. A safe bet is to add 10–20% extra. So your 3.7 kW becomes something like 4.2 to 4.5 kW.
That’s the ballpark system size.
Now, some people oversize a little on purpose, especially if they plan to:
- Add an electric car in the next few years
- Switch from gas to electric heating
- Store energy in batteries and go semi-off-grid
Others undersize intentionally, just to offset a portion of their bill. That’s okay too — the key is to know what you’re trying to achieve.
There’s no perfect size for everyone. It’s about matching your goals with your real-world energy use and local sun exposure.
That’s why, in our app, we made it easy to enter your energy use and location to get an instant estimate. You can even play around with “what if” scenarios — like using more power in the future or living in a cloudier area.
A well-sized solar system doesn’t just save you money — it gives you peace of mind. You’ll know it’s doing exactly what it was designed for, and that feels good.