How much does your home really use? How to measure before installing solar

The most expensive mistake in a solar system

Most people size their solar system from a single number: the total kWh shown on the bill. That is too little information. Two homes with the same annual consumption can need completely different systems — and one of them will waste half of what it produces.

The reason is simple. Panels produce during the day. If the house consumes at night, that energy goes to the grid and is sold at a fraction of the price you would pay to buy it back. What matters is not how much you use, but when you use it.

Before asking for quotes, it is worth investing two weeks and a few tens of euros in measuring. It is the step that separates a well-sized system from an expensive one.

What your bill already tells you — and what it hides

If you are on a two-rate or three-rate tariff, the bill already gives you a useful clue: the split between off-peak, mid-peak and peak. A home with more than 40% of its consumption off-peak (the early hours of the morning) is a home that will need a battery to make real use of solar.

If you are on a single-rate tariff, the bill tells you absolutely nothing about timing. In that case, the only way is to measure.

There is also one piece of information hardly anyone uses: the contracted power. If yours is 6.9 kVA and the breaker never trips, you are probably paying for more power than you need — and that can be fixed with a phone call, without installing anything.

How to measure for less than €40

The simplest way is a smart plug with energy monitoring. You plug the appliance into the smart plug and the smart plug into the wall, and after a week you know exactly how much that appliance uses and at what times.

Start with the usual suspects: electric water heater, dishwasher, tumble dryer, air conditioning and pool pump. These five typically account for more than half of the consumption of a Portuguese home.

If you want the whole house rather than one appliance at a time, there are DIN-rail meters that are fitted in the consumer unit and give you the complete consumption curve. They need an electrician, but they give you the real picture.

A technical warning that will save you trouble: always check the plug's maximum current. Many cheap models handle only 10 A (2,300 W), and an electric water heater or a tumble dryer can easily exceed that. For those appliances you need 16 A.

What to do with the numbers

After two weeks you will see which of three profiles you fit.

Mostly daytime consumption — remote working, a pool pump, air conditioning during the day. This is the ideal scenario: a system without a battery already does the job, and the payback is quick.

Mostly night-time consumption — the house empty during the day, everything running after 7 pm. Here, panels on their own capture little. Either you add a battery, or you change your habits, or the investment returns far less than the brochure promises.

Split consumption — the most common case. This is where correct sizing makes the biggest difference, and where it is really worth having data rather than estimates.

After installation, the same devices serve another purpose

The plugs you used for measuring don't end up in a drawer. Models with automation let you switch on the water heater or the washing machine when the system is producing more than the house is using.

Moving the dishwasher from 10 pm to 2 pm costs nothing, and it turns energy that would have been sold cheaply into energy you no longer have to buy at a high price. It is the cheapest improvement there is in a solar system.

In short

Measure before you buy. Two weeks of data are worth more than any simulation based on an annual figure.

If you prefer, we can do this with you: we analyse your bills, help you interpret the measurements and size the system around your actual consumption profile — not an average.