In every household, there is usually a distinct dynamic when it comes to investments and upgrades. I have always considered myself the resident tinkerer, especially when it comes to optimizing our small, vintage solar array here in Belgium. We run on a dynamic, hourly-variable energy contract, constantly trying to beat the volatile energy market by timing our consumption.
But recently, my wife dropped a suggestion that made me stop and re-evaluate my entire strategy. And to be fair, since she actually works in the energy sector, I probably should have taken her idea seriously from the very first second. She asked: “Shouldn’t we just buy one of those plug-in home batteries to store our solar power?”
My initial reaction was typical skepticism. When you hear "home battery," you immediately think of massive, heavy boxes bolted to the wall, requiring a certified electrician and a mountain of paperwork. We have naturally looked into installing a "real," traditional home battery system before. However, the bottleneck for us is our electrical grid connection. To install a heavy-duty battery system that can power the whole house and charge quickly, we would need to upgrade our home from a standard 1-phase connection to a 3-phase connection.
If you have ever dealt with grid operators, you know that this is not just flipping a switch. Upgrading to 3-phase power means replacing the main fuse box, paying hefty inspection fees, and sometimes even digging up the driveway for new cables. Add the cost of a high-end battery and a hybrid inverter, and you are easily looking at a €5,000 to €10,000 project. With a major kitchen renovation coming up on our schedule, draining our savings for a grid upgrade and a massive battery is an absolute hard pass right now. The budget simply isn't there for a complete electrical overhaul.
That is exactly why my wife’s suggestion was so intriguing. She wasn't talking about a massive 3-phase installation. She was talking about the new generation of plug-and-play batteries—specifically the HomeWizard Plug-in Battery. Now that we are well into 2026 and the local grid operators (Synergrid) have finally given these specific devices the official green light in Belgium, you literally just plug them into a standard wall socket. No breaking down walls, no electrical panel upgrades, no expensive electricians. Just plug, play, and store.
But the real question remained: Is this actually worth the €1,195 price tag, or is it just an expensive gadget? I decided to dive down the rabbit hole and run the actual numbers.
To understand if this makes financial sense, you have to look at the physical limitations and the specifications. This specific unit holds 2.7 kWh of energy. It has a built-in micro-inverter that can charge and discharge at a maximum rate of 800 Watts. The concept is beautifully simple: it communicates directly with the smart meter via Wi-Fi. When our vintage solar panels are producing more than the house is actively consuming, the battery soaks up that excess energy. When the sun goes down, it slowly trickles that 800W back into the house to power the fridge, the router, and the lights, ensuring we buy as little expensive peak-hour grid electricity as possible.
Because we have a dynamic hourly contract, the battery can even be programmed to buy cheap electricity from the grid during the night or afternoon slumps (when prices sometimes drop to just a few cents) and discharge during the expensive morning and evening peaks.
However, looking at this strictly from a standard household consumption perspective, the Return on Investment (ROI) was looking a bit bleak. With a regular profit margin of roughly €0.07 to €0.10 per kWh between peak and off-peak prices, it would take around 11 to 15 years to pay off a €1,195 battery. Since the lifespan of the LFP cells inside is rated for about 15 years, it felt like breaking even at best. I was ready to tell my wife that her idea, while innovative and free of installation hassles, was a financial dud.
But then I remembered our electric vehicle (EV), and the entire mathematical equation flipped on its head.
We currently get a flat reimbursement of €0.25 per kWh for all the electricity used to charge the EV at home. This specific detail is where the plug-in battery transforms from a slow-burning savings account into a highly efficient, ROI-crushing machine.
Initially, I had a slight misconception about how this interaction would work. I assumed that if I plugged the EV into the charger, the car would instantly suck the tiny 2.7 kWh home battery dry in about 30 minutes. Physics, however, gently tapped me on the shoulder. Because the plug-in battery is strictly limited to discharging a maximum of 800 Watts to protect the regular household wiring from melting, it cannot dump all its stored energy into the car at once.
Instead, when the EV starts pulling a massive load from the grid to charge, the home battery simply chimes in with its maximum 800W contribution. It essentially acts as a steady drip-feed alongside the grid power. This means it takes about 3.5 hours for the home battery to fully empty its 2.7 kWh reserve into the car's massive battery pack.
But here is the magic part: it doesn’t matter how fast it discharges. Energy is energy.
If we fill the home battery with our own free solar power during the day, or with dirt-cheap grid energy at 2:00 PM, we acquire those 2.7 kWh for practically nothing. When we plug the car in during the evening, the home battery feeds that stored energy into the vehicle. Because of our reimbursement plan, we are essentially “selling” our own cheap, stored energy to the EV charging plan at a premium rate of €0.25 per kWh.
Let’s run those numbers:
If we manage to cycle the battery this way for 365 days a year, that is €246 in pure annual profit.
Suddenly, that €1,195 initial purchase price doesn't look like a 15-year burden anymore. The ROI completely collapses from over a decade to under 5 years. Anything after year five is pure, unadulterated profit.
Before I completely buy into the hype, it is important to acknowledge what this system cannot do.
First, it is entirely useless in a blackout. Because of strict regulations, the moment the grid goes down, the battery shuts off instantly. This is to prevent electrocuting line workers who might be fixing the street cables. You cannot use it as an emergency off-grid generator.
Second, the system is strictly designed for self-consumption. It will not allow you to actively trade or inject your stored battery power back into the public grid for profit. It only releases energy when your house explicitly demands it.
After crunching the numbers and looking at our upcoming kitchen renovation budget, I have to admit defeat: my wife was absolutely right. The combination of our hourly variable contract, our vintage solar array, the avoidance of a 3-phase grid upgrade, and the EV reimbursement loophole makes this a surprisingly solid investment.
Because the system is modular, our strategy is going to be incredibly simple. We aren't going to buy a massive system right out of the gate. We are going to buy exactly one plug-in battery. We will plug it into a standard socket, monitor the app, and see if it consistently empties out into the EV every evening. If we notice that it’s hitting 0% too early and we are still leaving money on the table, we can simply buy a second unit, stack it on top, and double our capacity to 5.4 kWh without paying a single cent for installation.
Sometimes, the best financial and energy decisions start with your partner casually pointing out the obvious solution you were too busy overthinking to see.
Cheers,
Peter
