Figuring out how much energy you actually need for a balkonkraftwerk with a battery (a plug-in solar system with storage) is the most critical step to ensure it’s a worthwhile investment. It’s not about guesswork; it’s about matching the system’s output to your specific household’s consumption patterns. Getting this right means maximizing your energy independence and savings, while getting it wrong could leave you with a system that doesn’t meet your expectations. The core of the calculation revolves around three key elements: your daily energy consumption, the solar power generation potential at your location, and the capacity of the battery storage.
Let’s start with your consumption because that’s the foundation. You need to know what you’re powering. A balkonkraftwerk with storage is ideal for offsetting the constant, low-to-medium power draws in your home—what’s known as the baseload. This includes appliances that run continuously or frequently throughout the day.
Typical Household Baseload Appliances and Their Power Consumption:
- Refrigerator: 100-200 Watts when running (cycles on and off)
- Internet Router & Modem: 10-30 Watts (running 24/7)
- Desktop Computer (on): 150-300 Watts
- LED Lighting: 5-15 Watts per bulb
- Television (LED/LCD): 50-150 Watts
- Standby Power for various devices: 1-5 Watts each (this adds up!)
To get a precise figure, you have two main options. The easiest way is to check your electricity bill; it shows your total monthly or annual consumption in kilowatt-hours (kWh). Divide your monthly usage by 30 to get a rough daily average. For a more granular view, use an energy monitor or a plug-in power meter. Measure the consumption of key appliances over a 24-hour period. For example, you might find your fridge uses 1.2 kWh per day, your router uses 0.5 kWh, and your lighting in the evening uses 0.8 kWh. Adding these up gives you a targeted daily baseload.
Next, you must consider solar generation. This is where location and setup are everything. A system in Hamburg will produce significantly less than an identical system in Munich. The key metric is peak sun hours—this isn’t just daylight hours, but the number of hours per day when sunlight intensity is equivalent to 1000 Watts per square meter. This number varies by season.
| City (Germany) | Average Daily Peak Sun Hours (Summer) | Average Daily Peak Sun Hours (Winter) |
|---|---|---|
| Munich | 4.5 – 5.5 hours | 1.0 – 1.5 hours |
| Berlin | 4.0 – 5.0 hours | 0.8 – 1.2 hours |
| Hamburg | 3.5 – 4.5 hours | 0.7 – 1.0 hours |
Now, let’s talk about the system itself. A typical balkonkraftwerk speicher might consist of two 400-watt solar panels (800W total) and a 2.4 kWh battery. To calculate its potential daily energy production, you use the formula: Panel Wattage × Peak Sun Hours × System Efficiency. System efficiency (typically 75-85%) accounts for losses from heat, cables, and the inverter.
Summer Example (Munich, 5 hours of sun, 80% efficiency):
800W × 5 hours × 0.80 = 3,200 Wh or 3.2 kWh of generated energy.
Winter Example (Munich, 1.2 hours of sun, 80% efficiency):
800W × 1.2 hours × 0.80 = 768 Wh or 0.77 kWh of generated energy.
This stark seasonal difference highlights the vital role of the battery. The battery’s job is to store the excess energy produced during the sunny afternoon for use in the evening, overnight, and on cloudy days. The size of the battery, measured in kilowatt-hours (kWh), determines how much of that self-generated power you can actually use. A 2.4 kWh battery can theoretically power a 100-watt load for 24 hours, but in reality, you need to consider the battery’s Depth of Discharge (DoD). Most lithium batteries have a recommended DoD of 80-90%, meaning you should only use 80-90% of their total capacity to preserve their lifespan. For a 2.4 kWh battery with 90% DoD, your usable energy is 2.16 kWh.
So, how do you put it all together? Let’s create a realistic scenario for a one-person household with a daily baseload consumption of 2.5 kWh. They have an 800W balkonkraftwerk and a 2.4 kWh battery in Munich.
A Summer Day:
The system generates 3.2 kWh. The household consumes 2.5 kWh during the day and night. The solar panels directly power the home during the day, and any excess (around 1-1.5 kWh) charges the battery. The battery then powers the home through the evening and night. In this ideal case, the household could be nearly 100% self-powered from solar, with energy to spare.
A Winter Day:
The system only generates 0.77 kWh. This is likely only enough to cover a portion of the daytime consumption. The battery will be called upon heavily to supply power in the evening, but it will be depleted quickly and will need to recharge from the grid or the next day’s limited sun. The system might only cover 30-40% of the daily needs, with the rest coming from the grid.
Therefore, when calculating your needs, you should plan for the winter months if your goal is year-round resilience. This might mean considering a larger battery capacity or slightly more panel wattage than your summer calculations suggest. It’s a balance between budget and desired level of independence. The inverter, which converts the DC power from the panels and battery to AC for your home, also has a power rating (e.g., 600W, 800W). You need to ensure this rating is high enough to handle the combined wattage of the appliances you want to run simultaneously from the battery. You can’t run a 1500W kettle from a 600W inverter, even if the battery is full.
Finally, don’t forget the regulatory framework in Germany. The system must be registered with the Bundesnetzagentur and your grid operator. The inverter must be certified and have a grid protection unit. The plug used for the connection should be a Wieland plug or a Schuko plug according to the new VDE-AR-E 2100-712 standard to ensure safety. Understanding these rules is as crucial as the energy calculation itself for a smooth and compliant installation.