Which storage system is right for my balcony power plant? The base load formula instead of gut feeling

Michael Liu

Aug 11, 2026

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Welcher Speicher passt zu meinem Balkonkraftwerk? Die Grundlast-Formel statt Bauchgefühl

Which storage solution is right for my balcony power plant? The base load formula instead of gut feeling.

Many households in Germany face the same hurdle when purchasing a balcony power plant: the solar modules deliver peak performance in terms of watt-peak (Wp), but the energy often dissipates unused into the public grid. The desire for a battery storage unit is great – but how do you choose the right capacity in kilowatt-hours (kWh)?

Anyone who blindly buys a battery without a sound calculation will either pay for unused overcapacity or be annoyed by a storage unit that is too small and already discharged late in the evening. In this guide, we present a data-driven methodology – the base load formula – with which you can precisely calculate your ideal storage size and optimize the amortization period.

The Misconception: Why Wp-output is not the same as kWh-storage

When it comes to balcony power plants, buyers primarily focus on module output (e.g., 800 W, 1,600 W, or 2,000 Wp). However, module output only describes the instantaneous power under standard test conditions. A storage unit, on the other hand, secures the work (power × time) for times without sunlight.

The sizing depends significantly on two factors:

The midday surplus: How many kilowatt-hours do your modules generate between 11:00 AM and 3:00 PM beyond your current direct consumption?
Night coverage (base load gap): How much electricity does your household consume between sunset and sunrise?

The 3-Step Decision Path: From Measurement to Capacity

Step 1: Determine base load over 7 days

The base load is the continuous electricity consumption of your household (refrigerator, Wi-Fi router, standby devices, heating control). Determine this value as follows:

Read the meter reading on the digital electricity meter (or via smart plug/power meter) before going to bed.
Note the meter reading immediately after waking up (e.g., after 8 hours).
Divide the consumed kWh by the number of hours to get the average base load in watts.

\text{Base load (W)} = \frac{\text{Consumption in kWh}}{\text{Hours}} \times 1.000

Example: 1.2 kWh consumption in 8 night hours = 150 watts continuous base load.

Step 2: Estimate midday surplus

On a sunny summer day, a solar system with approx. 2,000 Wp module output generates up to 1.8 to 2.2 kW in peak hours. If you deduct the direct consumption in the household, there often remain 6 to 12 kWh of daily surplus that would flow into the grid unremunerated without a battery.

Step 3: Calculate night demand

Multiply your base load by the nocturnal dark phase (summer approx. 8–10 hours, transitional period approx. 12–14 hours):

\text{Night demand (kWh)} = \text{Base load (kW)} \times \text{Night hours (h)}

Example: 0.15 kW × 12 h = 1.8 kWh nocturnal electricity demand.

Decision Matrix: Which storage size suits your household?

Household Type	Daily Consumption	Recommended PV Output	Recommended Storage Size	Suitability & Focus
1–2 People (Single/Couple)	1,500 – 2,200 kWh/year	800 W – 1,200 Wp	1.0 – 2.0 kWh	Pure coverage of night base load (100–150 W). Fastest amortization with small PV area.
3–4 People (Family)	3,000 – 4,500 kWh/year	1,500 W – 2,500 Wp	2.5 – 5.0 kWh	Buffers evening peaks (cooking, TV, laundry) and fully supplies the night base load.
Family + Home Office / E-Car / Heat Pump	> 5,000 kWh/year	2,000 W – 3,500 Wp	5.0 – 10.0+ kWh (scalable)	Maximizing the degree of self-sufficiency. Stores large midday surpluses for extended use.

Economic Analysis: When does a 5 kWh storage unit pay off?

Based on a concrete practical example, we examine the amortization period of a model with approx. 5 kWh storage capacity with a modernly sized solar area (approx. 2,000 Wp module output):

Electricity price: €0.38 / kWh
Usable daily yield for storage: approx. 4.5 kWh per day (on average over 220 sunny days/year)
Annual electricity saving through storage: $4.5 \text{ kWh} \times 220 \text{ days} = \text{approx. } 990 \text{ kWh/year}$
Annual savings in Euros: $$990 \text{ kWh} \times 0.38 \text{ €} = \mathbf{376.20 \text{ € / year}}$ $

A high-quality 5 kWh storage system, with acquisition costs of approx. €1,800 to €2,200, amortizes under these conditions in just 4.8 to 6 years. With a lifespan of modern LiFePO4 batteries of over 15 years (6,000+ charging cycles), the system generates a significant return over its total operating time.

Technical Solution for Higher Demands: The Expandable Storage Concept

Those who aim for maximum independence with the latest technology should pay attention to modular scalability and high inverter output when choosing a system. Systems like the SunEnergyXT 500 Pro Head Storage (2,400 W) precisely meet these requirements flexibly:

Full 2,400 W System Power: Up to 2,400 W in grid-connected operation offers sufficient reserves for higher household loads and simultaneous consumers.
Scalable from 5 to 30 kWh: Entry is with a capacity of 5.024 kWh and can be expanded as needed with B500 storage units in clear 5-kWh steps.
Four MPPT Trackers (up to 2,500 W PV): Four independent inputs get the maximum out of different module orientations (e.g., East-South-West) and partially shaded areas.
Emergency Power in max. 10 ms: The separate emergency power output, with appropriate installation, provides up to 2,400 W for important consumers without interruption.

More information on the system can be found directly at: drbo-greenenergy.de – SunEnergyXT 500 Pro Head Storage 2400W.

Frequently Asked Questions

1. Can a storage unit for a balcony power plant be oversized?

Yes. If the storage capacity is significantly higher than the yield of your solar modules or your night consumption, the battery will rarely be fully charged. This unnecessarily extends the amortization period.

2. How do I most easily calculate my nightly base load?

Measure electricity consumption overnight (approx. 8 hours) at the electricity meter or use smart home metering devices. Divide the consumed kilowatt-hours by the number of hours to get the average wattage.

3. Which battery technology is best suited for balcony power plant storage?

Lithium iron phosphate batteries (LiFePO4) are currently considered the standard. They offer high thermal safety, achieve over 6,000 charging cycles, and retain a high capacity even after many years.

4. How does the orientation of the solar modules affect the choice of storage size?

An east-west orientation distributes electricity generation more evenly throughout the day, which increases direct consumption and means a smaller storage unit is sufficient. A pure south orientation generates high midday peaks, which are best captured with a larger storage unit.

5. What happens to the storage unit in winter?

In the low-yield winter months, solar radiation is often only sufficient to cover daily consumption. Many systems have an intelligent battery management system (BMS) that protects the battery from deep discharge.

6. Can I expand a storage system later?

Modular storage systems allow for subsequent expansion with additional battery units (e.g., in 5 kWh increments), so that the system can be adapted to changing consumption needs.

7. Do I need an electrician for the installation of a balcony storage unit?

Standard plug-and-play balcony storage units are connected via standardized plug connections. However, for the integration of special emergency power circuits or fixed permanent connections, a specialist should be consulted.

8. How long does a LiFePO4 storage unit last on average?

With daily full use (365 cycles/year), 6,000 cycles correspond to a theoretical lifespan of over 15 years before the remaining capacity noticeably drops below 80%.