Your operator sends you five numbers once a year. Almost nobody knows what to do with them.
Since 2024 the Slovenian grid charge is billed per five time blocks, each with its own agreed power. Lower it and you pay less for power — but you start paying for every quarter-hour you go over. Leave it and you pay nothing for overruns at all. This tool loads your own 15-minute data and works out which of the two actually costs you less.
The rule the whole thing turns on
Article 12, paragraph 8 of the methodology act says it plainly: if you do not change your agreed power, the operator must tell you about your overruns every month — and must not bill you for them. Change it once, and you start paying.
So the real question is not what you pay
It is what it costs you to enter the game at all. Lowering your power is only worth it if the saving on power beats the overruns you take on by lowering it. That comparison is the entire product.
The overrun formula is not what you would guess
Overruns are not added up. Each one is squared, the squares are summed, and the square root is taken — separately for every block and every month. Ten small overruns cost far less than one big one of the same total. No spreadsheet does this by hand.
The agency itself says get advice
On its own information page the Energy Agency writes that lowering your agreed power can end up costing more than leaving it, and recommends working it out properly first. This tool is that calculation.
What it does
Finds the cheapest agreed power
Not block by block — that is the mistake almost everyone makes, because the five powers must increase from block 1 to block 5 and raising one drags the others up with it. The tool searches the whole chain at once and finds the true optimum.
Puts a number on it
Power, overruns, energy, the renewables levy, the efficiency levy and VAT, side by side for both options, scaled to twelve months. And the deadline: submit by the 20th, applies from the 1st, free of charge.
Tells you when not to trust it
It counts missing readings, lists every line it could not read with a reason, and refuses to print a euro figure when your file does not cover the reference window. A number built on a guess should not look like a result.
Handles business connections
All five user groups, from a household on low voltage to industry on 110 kV, with transmission and distribution tariffs kept separate — which is the only way the block 1 discount comes out right. Above 43 kW the minimum power rule changes and the tool knows it.
Spots the seasonal exemption
A business above 43 kW that runs mostly in one season can apply, by 15 January, to be billed on achieved power instead of the five-peak average. The tool checks your figures against each condition — and sends you to your operator rather than declaring you eligible.
Says what solar will not fix
It counts how many of your peaks happened when the sun was down. For those, panels do nothing. It also shows what a battery would need to shave to be worth it — as an upper bound, clearly labelled as one.
How you can tell the numbers are right
Anyone can claim their calculator is correct. Here is what that claim rests on, in full.
Ten published figures as checkpoints
The agency publishes both the ordinary and the discounted block 1 power tariff for all five user groups. The tool reproduces all ten to the fifth decimal. A test that compares code against other code proves nothing; these compare it against a document somebody else signed.
Every guard was broken on purpose
A separate script injects a deliberate error into the source, runs the tests, and checks that the right test fails. Seventeen errors, seventeen failures. A test that has never failed is not a test.
And what we still do not know
Whether the timestamp in the export marks the end of the interval is nowhere stated officially; we infer it from three independent working implementations. Reactive energy is not in the export, so the tool cannot account for it. Both are written down in the product notes, not hidden.
Pricing
One payment, not a subscription. The dividing line is the same one the act itself uses: 43 kW of connection power.
Up to 43 kW
29 €
Households, sole traders, offices, small shops. One payment, no time limit, no account.
BuyBusiness, no limit
79 €
All five user groups and any connection power. Includes the seasonal-activity check and the minimum-power rule for connections above 43 kW.
BuyTrial, free
0 €
Reads your file, shows your consumption, your peaks per block, the month-by-month breakdown and how many readings are missing. It does not compute the decision — that code is not in the trial file at all.
Open the trialQuestions
Where do I get the file?
From mojelektro.si. Sign in, pick your metering point, choose 15-minute data for the last year and export to CSV. The tool reads that file directly.
Does my consumption data go anywhere?
No. The tool is a single HTML file with no server behind it. Your file is read in your own browser. The only thing that ever goes over the network is the licence key, once when you activate it and roughly once a week after that.
Will it always find me a saving?
No, and it says so before you buy. For many people the honest answer is leave it alone. That is still worth knowing — a wrong change is irreversible until the end of the calendar year and starts the overrun billing.
Is this Slovenia only?
Yes. Five time blocks, agreed power per block, the square-root overrun formula and the block 1 transitional discount are Slovenian rules. The tool is in Slovenian for the same reason.
What happens when the tariffs change?
You can edit them yourself, separately for transmission and distribution, and the date of the last update is shown next to them. Only the formulas from the act are fixed — the tariffs, the weighting factor and even the block schedule are data, because the schedule changes on 1 January 2027 and the tool already accounts for it.
I have several metering points.
Run the tool once per point. It does not combine them, and we would rather say so than pretend otherwise.