Annual totals do not reveal coincidence
Comparing annual consumption with annual solar PV generation conceals when the energy is actually available. Without a battery, solar electricity may be used directly, exported when generation exceeds demand or supplemented by the grid when generation is insufficient.
With a battery, part of the surplus can be stored and used later. This process involves losses and remains limited by storage capacity, charging and discharging power, and operating conditions.
Compare three consistent states
A clear assessment compares an initial state, solar PV only, then solar PV with a battery, using the same data and analysis horizon.
This separates the effect of solar generation from the effect of storage and avoids attributing to the battery a benefit already delivered by solar PV.
Select a state to follow the mechanisms that change energy flows and the economic balance.
Building supplied by the grid
The grid supplies the building’s electricity demand. The reference bill reflects electricity purchases and the applicable tariff components.
- Electricity purchased from the grid
- Payment for energy and applicable tariffs
- Reference energy bill
Local generation without a battery
Solar electricity used at the same time avoids a grid purchase. Surplus energy is exported and may be remunerated.
- Direct self-consumption
- Avoided grid purchase
- Exported surplus and remuneration
- Solar PV investment and associated costs
Part of the surplus is shifted through time
The battery stores part of the solar energy and returns it later. It may avoid further purchases but adds losses, investment and possible replacements.
- Battery charging and discharging
- Potentially avoided grid purchase
- Export remuneration potentially forgone
- Storage losses
- Battery investment, operation and replacement
Conceptual and non-proportional diagram. The actual outcome depends on the consumption profile, generation, tariffs, investments and operating conditions.
Self-consumption, self-sufficiency and savings are different
The self-consumption rate indicates the share of solar PV generation used locally rather than exported. The self-sufficiency rate indicates the share of building demand covered by local generation.
A battery can affect both indicators, but a high value does not automatically mean a better economic or environmental outcome. Annual savings, investment and long-term profitability must be assessed separately.
What a battery simulation must represent
A physically consistent simulation accounts for usable capacity, charging and discharging power, state of charge, efficiency, losses, operating limits and control strategy.
Control is important. A battery charged too early may already be full when solar generation peaks. An appropriate strategy balances building demand, available surplus and grid constraints.
Where economic value may be created
Solar PV can avoid part of the grid purchases through direct use and generate remuneration for exported surplus. A battery may avoid further purchases by returning later energy that would otherwise have been exported.
This shift can also forgo export remuneration, incur losses and add investment, operating costs and possible replacements. Subsidies should be included only when they genuinely apply to the project.
Useful data before making a decision
A study becomes more reliable with measured consumption over a representative period, measured or simulated solar generation, battery characteristics, import and export tariffs, investments, lifetime assumptions and flexible loads.
SynergyFlex places these data on a common time base and compares scenarios through consistent energy, economic and environmental indicators. The result is not a universal battery size, but a transparent view of the project’s trade-offs.
Frequently asked questions
Is a solar battery always profitable?
No. It can increase the solar energy used on site, but profitability depends on the consumption profile, tariffs, losses, investment, replacements and analysis horizon.
What is the difference between self-consumption and self-sufficiency?
Self-consumption compares locally used solar energy with total solar generation. Self-sufficiency compares locally supplied energy with the building’s total consumption.
What data should be analysed before adding a battery?
Consumption and generation should be compared on a sufficiently fine time base, then combined with storage characteristics, tariffs, costs, control strategy and the analysis horizon.
Should maximum self-sufficiency be the objective?
Not necessarily. The final percentage points may require a large battery that is rarely used. The objective should be assessed alongside costs, losses and environmental impact.
Can a battery be added later?
This is often possible, but compatibility with the inverter, grid connection, controls and protection devices must be checked by a qualified specialist.
How should several battery capacities be compared?
Each capacity should be simulated with the same profiles, tariffs, efficiency assumptions and economic horizon. Energy and financial results can then be compared on an equivalent basis.
Sources and references
SynergyFlex grew from a Master’s project focused on energy and the use of time-based data. Master of Science in Engineering – Energy and Environment
This link documents the academic context. It does not imply a partnership or endorsement.
Are you comparing several battery capacities?
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