Are home batteries worth it? The math from a real 5 kWh system

Hi, I'm Joulie, and I count every kilowatt-hour in this house. This summer, this battery filled up and emptied out on roughly nine out of ten days. It worked harder than almost anything in the house. So here's the question everyone asks: did it actually pay off? Let's run the numbers.

Home batteries are sold with big promises: independence, savings, a payback in five years. Batteries are real, useful technology. But "worth it" is a money question, and money questions have an answer you can calculate. So instead of a brochure, we'll use four months of measured data from one real home in central Portugal.

Here's the setup. Ten solar panels of 505 watts each, about five kilowatt-peak. A hybrid inverter. One 5 kWh battery module, of which about four and a half is usable, because the system keeps a ten percent reserve. And me, the meter, between the house and the grid. During the day, the panels produce more than the house uses. The surplus goes into the battery first, and once the battery is full, it goes to the grid. In the evening the sun is gone, so the battery takes over until it hits its reserve. After that, I buy from the grid. And this is what that looked like. Almost every single day: from ten percent to a hundred, and back down again. On a typical August day, it was full around noon, and empty again around one in the morning.

From July to October, the panels produced 2,880 kWh. The house used about 2,200, including the electric car. The battery delivered 540 kWh to the house. That's about 4.4 a day. Now, what is one battery kilowatt-hour worth? Without the battery, that energy would have been exported at three cents, and later I'd buy it back at seventeen cents. So every kilowatt-hour that goes through the battery saves the difference: fourteen cents. 540 times fourteen cents is €75.60 in four months. About sixty cents a day. Winter will be lower, because there's less sun to store. If we assume 2.5 kWh a day in the darker months, a full year comes to roughly 1,370 kWh. Or about €190.

What does the battery cost? A 5 kWh set from this manufacturer costs roughly €2,000 to €3,200 online, before installation. Let's say €3,000, installed. €3,000 divided by €190 a year is almost sixteen years. If you got a great deal at €2,000, it's about ten and a half. The product warranty is ten years. Here's the whole thing in one line. The battery cost, divided by the kilowatt-hours you shift each year, times the gap between what you pay and what you get paid. Look at that last part. That gap matters more than anything else. At seventeen cents: sixteen years. At twenty-five cents, which is normal in much of Europe, the same battery saves about €300 a year, and pays back in ten. At thirty cents: about eight. Same battery. Same sun. Same house. Only the price gap changed.

So, is a home battery worth it? The numbers say it depends on three things. One: the price gap. Low import prices and a decent export price make a battery slow to pay back. High import prices, or near-zero export, make it much faster. Two: whether you can fill it every day. This one fills almost every day, which is the best case. A battery that sits half-empty pays back twice as slowly. Three: what else it does for you. Some people value backup power during a blackout. But be careful: many batteries, this one included, don't power the house during a grid outage unless you add extra backup hardware. Check that before you count it as a benefit. And one more thing. Even with the battery, this house still exported almost 1,300 kWh in four months, at three cents each. That's the next question: is a second battery module, an electric car, or a heat pump the better place for that energy? Spoiler: we're measuring it.

In one sentence: this battery works every day, earns about €190 a year at these prices, and the price gap decides whether that's a good deal. Want to run your own numbers? The calculator is linked below. I'm Joulie. Real data, real costs, your call.