Buy Your Friends Batteries · Domen Kožar

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Your next group birthday gifts should be home batteries.

Most birthday gifts are forgotten within a year. Batteries save your friends<br>money every day and keep essential devices running when the power goes out.

A 5 kWh battery costs about €1,600. That is expensive for one person and easy<br>for a group:

eight people contribute €200 each;

sixteen people contribute €100 each;

thirty-two people contribute €50 each.

Create a battery birthday club. Buy each friend a battery on their birthday<br>until everyone has one.

The idea is simple: buy electricity when it is cheap, store it, and use it<br>when it is expensive. Automatic price optimization reads tomorrow&rsquo;s prices<br>and handles the schedule for you.

The EcoFlow STREAM 5000<br>is one current example: about €1,600 for 5 kWh of storage and up to 3 kW of<br>output.

Germany and Spain are good places to do this. In the second half of 2025, the<br>average household electricity price was €0.3869/kWh in Germany and €0.2669/kWh<br>in Spain, including taxes. Those averages hide large changes during each day.

I calculated what would have happened on every day of 2025. The battery buys<br>5 kWh during the four cheapest hours before 17:00 and delivers 4.5 kWh during<br>the four most expensive evening hours. That assumes 90% round-trip efficiency<br>and only runs the battery when the cycle is profitable.

Country<br>Average cheap price<br>Average expensive price<br>Average saving per day<br>Saving in 2025<br>€1,600 payback

Germany<br>€0.041/kWh<br>€0.133/kWh<br>€0.40<br>€144<br>11.1 years

Spain<br>€0.026/kWh<br>€0.106/kWh<br>€0.34<br>€126<br>12.7 years

The calculation is:

daily saving = 4.5 × expensive price − 5 × cheap price

The table uses the market-linked part of a dynamic tariff, based on German<br>SMARD and Spanish<br>OMIE<br>day-ahead prices. Your bill also contains supplier charges, network fees, and<br>taxes. Some are fixed and some depend on time and location, so you should run<br>the same calculation against your actual tariff. The national household price<br>comparison comes from<br>Eurostat.

Grid arbitrage alone is therefore useful, but not yet an automatic five-year<br>payback. The economics improve when the same battery also stores your solar<br>power, earns grid-service payments, or replaces electricity bought at the full<br>retail price.

You are also safer during a power outage. The STREAM 5000 provides up to 3 kW<br>of off-grid output. Keep essential devices on its backup output and reserve<br>some battery capacity, and your refrigerator, lights, router, and laptop can<br>keep running when the grid goes down. An ordinary grid connection does not<br>back up the whole house automatically; the backup output must be configured<br>correctly.

And when the neighborhood goes dark but their lights stay on, they will<br>remember who gave them the battery.

A battery also decouples when electricity is produced from when it is used.<br>Spain can save abundant midday solar for the evening. Germany can save wind<br>power produced during low demand for the next peak. More low-carbon energy can<br>be used instead of wasted simply because it arrived at the wrong hour.

Do this

Pick the friends with upcoming birthdays.

Confirm that their home, meter, and electricity contract are compatible.

Collect €50–€200 from each person.

Buy a battery with automatic tariff integration.

Configure price optimization and keep 20% available for outages.

Repeat for the next birthdays.

Ten million 5 kWh batteries would create 50 GWh of distributed storage and<br>30 GW of output. At one cycle per day, they could move about 16 TWh of<br>electricity each year.

EUROMAXXING.