The right storage size is the biggest lever for your returns.
A battery storage system is one of the most effective investments that businesses with a PV system can make today. However, for it to fully unleash its economic potential, it's not about the maximum possible size, but about the optimally suitable size. Those who dimension cleanly double their self-consumption, cut peak loads, and secure emergency power – all from a single system. And the best part: the right storage size can be precisely determined with three clear key figures and a few minutes of calculation.
This guide will lead you step-by-step through the design – from the PV system on a barn roof to an industrial operation with peak loads beyond 200 kW.
The three key figures that determine dimensioning
Every storage design revolves around exactly three metrics:
Capacity (kWh) – meaning how much energy the storage system can absorb and release. It determines how much solar power shifts from day to night and how many hours of emergency power buffer are available.
Power (kW) – meaning how quickly the storage system delivers or absorbs energy. It decides which peak loads can be cut and whether the storage system takes over critical consumers in emergency power mode.
Cycle life – meaning how often the storage system can be fully charged and discharged before its capacity noticeably decreases. Modern LiFePO₄ systems like the MONA Island series offer 6,000 to 8,000 cycles with a full warranty, which corresponds to a service life of well over 15 years.
Those who correctly balance these three values get a storage system that perfectly matches the operation's load profile and is not a size too big or too small.
Step 1: Accurately record daily consumption and night load
The most important calculation step is recording the consumption profile. To do this, obtain the load profile data for the last 12 months from your grid operator – with RLM metering (from approx. 100,000 kWh/year), this data is available as 15-minute values free of charge.
Two figures are relevant: The average daily consumption (annual consumption divided by 365) and the night load between sunset and sunrise. The latter shows how much power the storage system typically has to deliver during the sunless hours.
For a dairy farm with a milking robot and 150 cows, the night load is often between 150–250 kWh. A medium-sized production company can quickly reach 400–800 kWh per night window. And an industrial hall with shift operation can reach night consumption beyond 1,000 kWh.
Step 2: Determine PV surplus – the actual bonus
Now comes the exciting part: How much solar power is currently flowing unused into the grid? Your PV inverter or energy management system shows the monthly feed-in surplus. Sum this for the sunny months of April to September and divide by the number of days – the result is your average daily surplus.
Precisely this surplus is what a battery storage system can capture with full economic efficiency. It should be dimensioned to be at least large enough to fully absorb this surplus – every kWh that goes into the grid instead loses the value difference between the feed-in tariff and the electricity purchase price. Today, this quickly amounts to a real saving of 20 cents per kWh.
Step 3: Choose the right power – for peak shaving and emergency power
Capacity alone says nothing about power. A 100 kWh storage system with only 20 kW output power cannot cut an 80 kW peak load, no matter how full it is. Therefore, you should align the power with two factors:
Peak load from the load profile: Identify your three highest 15-minute values per year. If you want to cut these from, for example, 180 kW to 100 kW, the storage system needs at least 80 kW of continuous output power.
Emergency power requirement: Which consumers must continue to run in the event of a power outage? Milking robots and milk cooling typically require 15–30 kW, a fully automatic cold storage often 40–80 kW, a production hall with a manufacturing line quickly over 100 kW.
The really good news: you don't have to supply the maximum peak power from the battery alone. Intelligent energy management systems like MONA Connect proactively control consumers and shift non-critical loads so that the storage system only cuts the genuine, time-critical peaks. This often halves the required power demand.
The three typical dimensioning scenarios – with suitable MONA Island
To give you a feel for realistic magnitudes, here are three practical cases that cover 90 percent of all design questions:
Scenario A – Family farm or craft business with rooftop PV system up to 100 kWp
Typical key data: 40,000–80,000 kWh annual consumption, night load around 80–150 kWh, PV surplus in summer 40–80 kWh/day. The ideal answer here is the MONA Island 60 with 61.44 kWh capacity and 50 kW hybrid inverter. It is modularly built from 10 to 60 kWh, plug & play without a foundation, and can be used as a full-fledged emergency power system with black-start function. For smaller dairy farms, direct marketers, and commercial businesses with manageable peak loads, this is the most economical solution.
Scenario B – Medium-sized dairy, production, or commercial business with 100–300 kWp PV and peak loads up to approx. 100 kW
Typical key data: 150,000–350,000 kWh annual consumption, night load 200–400 kWh, PV surplus 150–300 kWh/day. Here, the MONA Island 233 with 233 kWh capacity and 105 kW output power comes into play. It completely covers the night demand of a dairy or production facility, effectively cuts peak loads, and is modularly expandable up to 2,330 kWh if growth occurs later.
Scenario C – Large production facilities, agricultural cooperatives, biogas combined heat and power plants, or charging parks
Typical key data: 400,000+ kWh annual consumption, peak loads over 150 kW, PV surpluses from 400 kWh/day upwards, or high charging infrastructure requirements. The MONA Island 418 offers 418 kWh with selectable 125 kW or 215 kW output power – also at 690–800 volt level for direct connection to industrial grids. Modularly scalable up to 8.36 MWh, ideal for businesses that want to supply multiple trades simultaneously with emergency power or secure larger peak loads.
The most common dimensioning opportunities that are often overlooked
Firstly: Power before capacity. Those who want peak shaving need a powerful inverter. This determines the benefit much more strongly than any additional kWh.
Secondly: Consciously utilize modularity. A storage system that can "grow" as a package is a better choice over 15 years than an oversized system that is 40 percent empty today. All MONA Island models can be modularly expanded.
Thirdly: Emergency power capability as a safety bonus. Especially in agriculture and for cold-sensitive businesses, emergency power is a value anchor that pays off with the first real power outage. The black-start function of the MONA Island 60 and the autonomous emergency power capability of the Island 233 and Island 418 models deliver exactly that.
Fourthly: Consider software. A storage system without intelligent EMS is half a storage system. MONA Connect automatically orchestrates between self-consumption, peak shaving, and emergency power – and thus significantly increases the real yield.
Conclusion: With the right dimensioning, the storage becomes a profit center
A perfectly dimensioned battery storage system is more than just an energy storage solution – it is the central component of a modern, independent energy system. Those who accurately calibrate the three key figures of capacity, power, and application profile achieve amortization periods of five to eight years with a guaranteed service life beyond 15 years.
We would be happy to analyze your load profile and PV yield together with you and recommend the appropriate configuration – from the compact MONA Island 60 to the powerful industrial variant MONA Island 418.