How the calculation works

Step 1: solar power per hour

For every hour we calculate the position of the sun above the centre of the Netherlands (De Bilt) and the irradiance under a clear sky. Cloud cover is a random but fixed weather year with sunny and overcast spells, tuned so that each month receives exactly the average irradiation reported by KNMI: about 1,013 kWh per m² a year in total.

We convert that irradiation to a roof with a 35-degree pitch in the chosen direction, split into direct and diffuse light. The shape of the profile is then scaled to your yearly production. The total matches your own installation, while the distribution over the day and the seasons comes from the model.

Step 2: consumption per hour

Your yearly consumption is spread over the hours with a profile that differs between weekdays and weekends, summer and winter. A real household consumes erratically: little in most hours, a lot in a few hours of cooking, washing or drying. That is why we add random peaks per hour. Without them the model would let too much solar power be used directly and give the battery too little to do.

• Being at home during the day shifts consumption towards or away from the hours between 9:00 and 17:00.
• A heat pump uses electricity in proportion to the cold: mostly in the winter months, when there is little sun.
• An electric car charges about four times a week, from 18:00, or in the cheap night hours on a dynamic contract.

If you enter the purchase and feed-in figures from your annual statement, the calculator finds the consumption pattern for which the simulated feed-in equals your actual feed-in. That is the most accurate way to calculate.

Step 3: the battery

On a fixed or variable contract the battery does the following every hour:

surplus   = production − consumption
charge    = smallest of: surplus, power, (capacity − charge level) ÷ η
discharge = smallest of: shortage, power, charge level × η
η         = square root of the round-trip efficiency

Standby consumption is added to your consumption. What does not fit into the battery goes to the grid; what the battery cannot deliver comes from the grid.

On a dynamic contract the calculator finds the cheapest charging schedule for the whole year, including charging from the grid in cheap hours. That optimum assumes prices, sun and consumption are known in advance. A real controller works with forecasts. We therefore count the saving of the simple strategy in full and, by default, 85% of the extra gain. When the feed-in price is negative we assume your inverter does not export, with and without a battery.

Step 4: tariffs

Fixed or variable
cost = purchase × electricity price − feed-in × (compensation − feed-in costs)

Dynamic, per hour
buy price  = market price × 1.21 + supplier fee + energy tax
sell price = market price − supplier fee

Fixed charges such as the standing charge and grid fees are the same with and without a battery and are therefore left out. The hourly prices come from a price year tuned to the Dutch day-ahead market of 2025: €87 per MWh on average, 583 hours with a negative price, low prices when the sun shines and high prices in the evening.

Step 5: from saving to payback period

capacity(year)  = (1 − capacity loss) ^ (year − 0.5)
saving(year)    = saving at that capacity × (1 + price trend) ^ (year − 1)
                  + extra income − yearly costs
payback period  = moment the cumulative saving reaches the purchase price
return          = interest rate at which all savings together are worth the purchase price

We simulate the battery twice: new and at the end of its life. The years in between follow from those two results. The verdict is probably worth it for a payback period of up to 10 years (the usual warranty), borderline if the battery only pays for itself later but within its lifetime, and not worth it if it never does.

Defaults and assumptions

Every value below can be changed in the calculator. Reference date: November 2026.

Validation: is the model right?

We test the model against figures from measurements and public statistics. Automated tests also check that the energy balance closes in every hour and that the results stay within these ranges.

What the model cannot do

• Your household is not average. The hourly consumption is an estimate. Your meter readings improve the result, but it is still not exact.
• One weather year. A sunny or gloomy year gives a few per cent more or less saving.
• Hourly resolution. Peaks of a few minutes, such as a kettle, are averaged out. The random peaks largely compensate for that.
• Future tariffs are unknown. Electricity price, energy tax and feed-in compensation will change. That is why we show a scenario with higher and lower prices.
• Trading on the imbalance market is not included. That income is uncertain and depends on your provider; you can enter it as extra income.
• Failure and replacement of the battery or inverter outside the warranty are not included.
• Grid tariffs that depend on the time of day or your peak power do not yet exist for Dutch households and are not included.

In doubt about a result? Try a different electricity price, purchase price or lifetime in the calculator. That shows how sensitive your situation is.