How does the calculation work?

A transparent methodology for reliable results.

The simulator comes in two versions:

  • Simple: A simplified calculation with one energy price for consumption and one for injection (€/kWh). Suitable for a quick estimate.
  • Advanced: A detailed calculation specific to Flanders (Belgium), including:
    • Capacity tariff (€/kW/year)
    • kWh grid tariff
    • Data management
    • Fixed fee for energy
    • Green power and CHP contribution
    • A choice of single, dual or dynamic tariff
    • The option to upload your own consumption data (CSV from Fluvius)
    • Charging strategy options

The simulator supports two data sources:

  • Manual input (default): Representative consumption profiles derived from 2,400 anonymous smart meters from Fluvius Open Data (measurement year 2024). For every type of household (with or without solar panels, EV, heat pump) there are three reference profiles:
    • Low consumption: typical of smaller households
    • Average consumption: the most common profile
    • High consumption: typical of larger households

    Based on the annual consumption and injection you enter, the simulator automatically selects the best-matching reference profile. That profile is then scaled to your exact values. This is more accurate than simply scaling a single profile up or down, because households with different consumption levels also have different consumption patterns (for example peak power and the day/night split).

  • CSV upload (advanced version only): Upload your own quarter-hourly totals from Fluvius for a more personal analysis. Download them via MijnFluvius → My consumption → Export quarter-hourly totals. If the data covers less than a year, the results are extrapolated to a full year.

For dynamic tariffs, historical EPEX day-ahead spot prices (2022-2025) are used to calculate the energy cost.

The simulator simulates the battery's behaviour per quarter of an hour over a full year. When there is excess production, the battery charges; when production falls short, the battery discharges. The financial calculations include:

  • Total profit: Total savings over the lifetime minus the investment cost
  • Payback period: The number of years it takes to earn back the investment
  • NPV (net present value): The profit in today's euros. A saving ten years from now is worth less than money today, because you could also have invested that money. That is why every future saving is reduced by 3% a year (the discount rate, adjustable in the advanced version). A positive NPV means the battery earns more than such an investment.
  • IRR (internal rate of return): The battery's average annual return, comparable to the interest rate of an investment. If the IRR is higher than the discount rate, the NPV is positive.
  • ROI (return on investment): Total profit divided by the investment cost, over the whole lifetime. This is not a return per year.

Based on this, you get one final verdict: not profitable if the savings don't earn back the investment, marginally profitable if you earn back the investment but the NPV is negative, and profitable if the NPV is positive.

In the advanced version, the monthly peak power is also calculated, so the capacity tariff is applied correctly.

The advanced version supports three tariff structures:

  • Single tariff: One fixed energy price for consumption and one for injection, whatever the time of day.
  • Dual tariff (day/night): Different energy prices for peak and off-peak hours:
    • Peak hours: 7:00 - 22:00 (higher energy price)
    • Off-peak hours: 22:00 - 7:00 (lower energy price)
  • Dynamic tariff: The energy price changes every hour, following the EPEX day-ahead spot price. The price is calculated with:
    • A constant factor (€/MWh)
    • A variable factor that scales the spot price
    • Separate parameters for consumption and injection

With the dual tariff, the simulator determines for every quarter of an hour whether it falls in peak or off-peak hours and applies the matching energy price.

In the advanced version, you can choose between two charging strategies:

  • Maximum self-consumption (default): The battery charges when your solar panels produce more than you use, and discharges when you use more than they produce. This makes the most of your own solar energy.
  • Time of Use (TOU): Only available with the dual tariff. The battery charges during off-peak hours (cheap electricity) and discharges during peak hours (expensive electricity), regardless of solar production. This can bring extra savings through price arbitrage.
  • Battery efficiency: 90% (round-trip, adjustable in the advanced version)
  • Battery power: set by the user (default 5 kW)
  • Constant energy prices over the lifetime (except with the dynamic tariff, where historical prices are used)
  • Discount rate for the NPV: 3% (adjustable in the advanced version). Because the savings are calculated in today's euros (without indexation), this is a real discount rate: slightly above what a savings account or defensive investment earns after inflation, plus a margin for risk.
  • Minimum peak power for the capacity tariff: 2.5 kW

Battery ageing. A home battery loses part of its capacity every year, even if it is hardly used. We count 1% a year for age plus 3% per 1,000 full charge cycles. A typical home battery does 100 to 300 cycles a year, which comes to about 1.3 to 2% a year: after 10 years, some 80 to 87% remains. We simulate the battery twice: new, and with the smaller capacity it still has in its final year. For the years in between, we interpolate the savings linearly between those two results, according to the capacity that remains at that point. The difference is often smaller than you might expect, because a battery rarely goes from completely full to completely empty every day. Field measurements of home batteries show a loss of 2 to 3% a year for the first generation (Figgener et al., Nature Energy 2024); modern LFP cells do better in lab tests. Manufacturers usually guarantee 60 to 80% after 10 years.