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). That consumption price is your full price per kWh, including grid tariffs, levies and the fixed costs per year. The simulator takes the supplier's fixed fee (€69 a year) and data management (€18.92 a year) out of it, because a battery doesn't change them. The capacity tariff is on by default, because every household with a digital meter in Flanders pays it: the simulator then also takes the capacity tariff of your monthly peaks out of the price and calculates it separately, the same way as in the advanced version. If you don't pay a capacity tariff, turn it off. That way, a kWh the battery saves no longer counts at the capacity tariff's share as well, and a battery that lowers your peaks earns that back. The battery always runs on maximum self-consumption, and the efficiency (90%) and discount rate (3%) are fixed. Suitable for a quick estimate.
  • Advanced: A detailed calculation specific to Flanders (Belgium), including:
    • Capacity tariff (€/kW/year)
    • kWh grid tariff
    • Data management
    • Maximum tariff for the grid tariffs
    • Fixed fee for energy
    • Green power and CHP contribution
    • Energy contribution and special excise duty
    • A choice between an installed and a plug-in battery
    • A choice of single, dual or dynamic tariff
    • A choice of three charging strategies: maximum self-consumption, Time of Use and optimized charging
    • Adjustable round-trip efficiency and discount rate
    • The option to upload your own consumption data (CSV from Fluvius)

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 the dynamic tariff, the simulator uses historical EPEX day-ahead spot prices for Belgium (2022-2025), from the same year as your consumption data, or the nearest year if that year is missing. For the reference profiles, that is 2024.

The simulator calculates your consumption and injection per quarter of an hour over a full year, first without and then with the battery. When the battery charges and discharges depends on the charging strategy (see Charging strategy). The annual saving is the difference in your energy bill: what you pay less for consumption, minus what you receive less for injection because the battery stores part of your solar energy instead of injecting it.

Because the battery loses some capacity every year (see Assumptions), that saving drops slightly over the lifetime. The financial calculations take this into account:

  • Total profit: Total savings over the lifetime minus the investment cost
  • Payback period: The number of years until the savings added up exceed 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 simulator also calculates the grid tariffs:

  • Capacity tariff: for every month, the simulator takes your highest quarter-hour peak of consumption, and uses the average of those twelve monthly peaks. A month whose peak stays below 2.5 kW counts as 2.5 kW, as the grid operator bills it. If the battery lowers your peaks, the capacity tariff goes down; if it charges from the grid, it can go up.
  • Maximum tariff: together, the capacity tariff, kWh grid tariff and data management never cost more than the maximum tariff per kWh consumed. If they come out higher, the simulator uses that maximum.

Under every result, we also calculate whether another battery size earns your household more than the battery you entered.

  • Sizes: 1.25 to 10 kW of power, in steps of 1.25 kW. If the largest is the best, we calculate further, up to 20 kW.
  • Capacity: 2 kWh per kW of power, like most modular home batteries. A 5 kW battery stores 10 kWh, so the sizes run from 2.5 to 20 kWh. More power per kWh barely changes the result.
  • Price: € 1,800 fixed plus € 420 per kWh, installed, including the inverter, inspection and 6% VAT: the average of Belgian price lists and price overviews from 2025 and 2026. Budget packages cost about € 250 per kWh, premium brands about € 550.
  • Comparison: the profit in today's euros (NPV) of the best size, with your consumption, tariff, charging strategy and battery ageing, against that of your battery at your price. With optimized charging, where each simulation takes much longer, each size is simulated once, as a new battery, and the savings of an aged battery are read from the smaller sizes.
  • Rounded down: we round the difference down to € 100. Between the sizes we calculate, a size may earn a little more, about € 20 on average and about € 40 at most. That way we never promise more than there is to gain. If the difference is less than € 100, your battery is already close to the best size.
  • No battery: if no size earns more than investing the same money, and neither does your battery, we simply say so.
  • A moment's patience: if the server is already running such a comparison for someone else, we skip it, so your result doesn't have to wait.

At the top of the calculator you choose what to calculate: a battery, new solar panels with a battery, or new solar panels alone.

  • Yield: the measured and upscaled production of all solar panels in Flanders, per quarter hour, from Elia Open Data, divided by the installed capacity. That gives the kWh per kWp in every quarter hour, from the same year as your consumption data. Over a year that is 860 to 1,100 kWh per kWp (2022 to 2025).
  • An average over Flanders: a single roof has a calmer sunny day and a choppier cloudy day than that average, and we don't (yet) account for the orientation and tilt of your roof. Over a year the average works out well; on a changeable day it slightly overestimates how much solar power you use yourself.
  • On top of your own consumption: we add the new panels' production to your current profile, quarter hour by quarter hour. What you use in that quarter hour you no longer take from the grid; the rest goes to the grid. That way your own consumption pattern is kept. We assume you don't have solar panels yet; if you upload your own meter data, the new panels come on top of what your meter shows today.
  • Price: we fill in € 1,500 fixed plus € 800 per kWp, installed, including the inverter, inspection and 6% VAT: about € 1,175 per kWp for 4 kWp, in line with Belgian price overviews from 2025 and 2026. If you have a quote, enter its total price.
  • Lifetime and ageing: 25 years, during which the panels produce 0.5% less each year.
  • Solar panels with a battery: we first calculate the panels, then the battery on top of the house with panels. The profit in today's euros (NPV) is the sum of both, each over its own lifetime; the yearly savings and payback time come from the combined cash flows.
  • Another number of panels: under the result we calculate 1 to 12 kWp, at the price above, and show how much more the best size earns than your panels at that same price, rounded down to € 100, without saying which size. With a battery, we also look for the best battery size with your new panels and show the larger of the two gains. With optimized charging, we value the battery in that comparison with maximum self-consumption, because each simulation would otherwise take too long.

Under every result, we show which plug-in battery earns your household the most: a small battery you plug into a socket yourself.

  • Which batteries: all plug-in batteries in our home battery overview, each with its price, power and capacity. If the manufacturer states a usable capacity, we use that.
  • How they charge: a plug-in battery steers itself on your digital meter. So we always calculate it with maximum self-consumption: it charges with solar power you would otherwise send to the grid, whatever charging strategy you chose. Without solar panels a plug-in battery therefore earns nothing, and then we show no model either.
  • Efficiency: the manufacturer's round-trip efficiency if it is measured at the socket, otherwise 90%, like the calculator.
  • Best choice: the battery with the highest profit in today's euros (NPV), with your consumption, tariff and battery ageing. If a cheaper battery earns less than € 25 less, we show the cheaper one.
  • An honest verdict: if even the best plug-in battery doesn't pay off for you, we say so. We do name the one that loses the least, without recommending it.

In the advanced version you can also calculate a plug-in battery of your own: choose Plug-in battery under Type of battery. We then calculate it the same way, with maximum self-consumption, and with its purchase price: you install it yourself, so there are no installation costs.

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, for consumption and for injection:
    • Peak hours: 7:00 - 22:00 on weekdays (higher energy price)
    • Off-peak hours: 22:00 - 7:00, and all weekend (lower energy price)
    The simulator applies the matching price to every quarter of an hour. Public holidays count as ordinary days.
  • Dynamic tariff: The energy price follows the EPEX day-ahead spot price, per hour (per quarter of an hour from October 2025). The price is calculated for every quarter of an hour:
    • Consumption: constant factor (€/MWh) + variable factor × spot price + green power and CHP, plus the kWh grid tariff, the energy contribution and the special excise duty
    • Injection: constant factor (€/MWh, usually negative) + variable factor × spot price
    A fixed fee per year comes on top. Because every cost per kWh consumed is already in the consumption price, the optimized charging strategy takes them into account as well.

With all three tariffs, the advanced version adds the energy contribution (€2.04/MWh) and the special excise duty (€50.33/MWh) to every kWh consumed. These amounts are fixed. All prices you enter include VAT; the simulator does not add VAT on top.

The charging strategy decides when the battery charges and discharges. In the simple version, it is always maximum self-consumption; in the advanced version, you can choose between five strategies. Which strategy is available depends on your tariff:

  • Maximum self-consumption (default, with every tariff): The battery only charges with solar energy you would otherwise inject, and only discharges to cover your own consumption. It never charges from the grid and never injects. Without solar panels, this strategy therefore saves nothing.
  • Time of Use (TOU, dual tariff only): During off-peak hours (22:00 - 7:00, and all weekend), the battery charges from the grid until it is full, at full power. During peak hours (7:00 - 22:00 on weekdays), it discharges to cover your own consumption. It does not store solar energy: your surplus goes to the grid as before. The saving comes from the price difference between day and night, minus the losses when charging and discharging. Because the battery starts charging at full power at 22:00 after a weekday, your monthly peak, and so your capacity tariff, can go up. Time of Use + capacity tariff avoids this.
  • Time of Use + capacity tariff (dual tariff only): Like Time of Use, but the battery only charges from the grid as long as your consumption in that quarter-hour stays under your limit. That limit is your highest quarter-hour peak so far that month, and at least 2.5 kW: below that, you pay the minimum anyway. Every month, the limit starts again at 2.5 kW. So charging does not raise your monthly peak; your own consumption still can. Several home batteries and energy management systems can limit charging like this, but you or your installer usually has to set it up.
  • Optimized charging (dynamic tariff only): For every day, the simulator plans quarter-hour by quarter-hour when the battery charges and discharges, so that the energy cost of that day is as low as possible. The battery charges with solar energy or with cheap electricity from the grid, and discharges to cover your own consumption or to the grid when the injection price is high.
    • The day is known in advance: the plan knows the prices and your consumption and solar production for the whole day. A real battery controller has to forecast consumption and production, so in practice this strategy earns less than the simulation shows.
    • Day by day: each day is planned separately; the charge left at the end of the day carries over to the next day.
    • Without the capacity tariff: the plan does not take your monthly peak into account. Charging from the grid at full power can raise that peak. The higher capacity tariff is included in the result, and can make the savings smaller than with maximum self-consumption on the same dynamic tariff. Optimized charging + capacity tariff avoids this.
    • More charge cycles: the battery charges and discharges more often, so it ages faster. The calculation takes this into account.
  • Optimized charging + capacity tariff (dynamic tariff only): Like Optimized charging, with the same limit on charging from the grid as Time of Use + capacity tariff. Each day's plan uses the limit as it stands at the start of that day. Here too, charging does not raise your monthly peak, but your own consumption still can.

The round-trip efficiency is the share of the electricity you charge into the battery that you get back out. The rest is lost as heat, in the battery cells and in the inverter that turns direct current into alternating current. At 90%, of every 10 kWh you store, 9 kWh comes back to your home.

  • Default 90%: a common value for a modern home battery, measured at the socket (on the alternating-current side), including the inverter. In the advanced version you can change it.
  • How we use it: we split the loss evenly between charging and discharging.
  • Check the datasheet: look for the round-trip efficiency of the whole system (AC). A value for the battery cells only (DC) is higher than what you really get back.
  • Why it matters: every lost kWh is a kWh you don't use yourself. With Time of Use or optimized charging the battery charges and discharges more often, so a low efficiency weighs more there.
  • Battery efficiency: 90% (round-trip, adjustable in the advanced version). The loss is split evenly between charging and discharging.
  • Battery power: set by the user (default 5 kW), for both charging and discharging
  • The capacity you enter is the usable capacity: the battery may go completely full and completely empty. It starts the simulation empty.
  • Every year of the lifetime has the same consumption and solar production as the simulated year.
  • Constant energy prices over the lifetime, without indexation. With the dynamic tariff, the historical price year repeats every year.
  • 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.

The calculator is free and stays free. To keep improving it, the site may make money, for example with affiliate links: if you buy something from a webshop through such a link, that webshop pays us a commission. You pay nothing extra. Making money is fine, but nobody can buy the outcome. That's why we keep to these rules:

  • The calculation comes first: the simulator works out what is best for you, without looking at what we earn from it.
  • Also without a commission: if the best or cheapest choice is a battery we earn nothing from, we show it anyway.
  • Always said: a link we can earn from says so.
  • An honest verdict: if a battery is not profitable for you, we say so, even when that means we earn nothing.

Manufacturers, webshops and installers cannot buy a place in a ranking and have no influence on the calculation. If you see something that doesn't match these rules, let us know with the feedback button at the top.