When a dynamic tariff does NOT pay off
An honest case study of a net-metered prosumer. On 35,000 real measurements we show a house for which switching to a dynamic tariff costs almost PLN 8k a year - and a battery only pays off once net metering expires.
In a previous case study, a dynamic tariff and a home battery cut the bills almost in half. Today we show a house where the very same move would be a costly mistake: switching to a dynamic tariff would cost an extra 7829 zł per year, and a battery on the current tariff pays back only after decades. We calculated it on 35,000 real measurements. The lesson: before you change tariff, know your profile.
We sell systems that cut costs on dynamic tariffs - so we have an obvious interest in saying "switch and buy a battery". This article says otherwise. We think an honest analysis is worth more than a quick sale.
The subject: a large prosumer
The owner of a detached house asked us the same question as everyone else: should I switch to a dynamic tariff and add a battery? To answer honestly, we did not guess - we took a full year of real data and calculated every scenario on the same numbers.
- ~30 kWp of PV (two inverters), producing about 30 MWh a year.
- ~38 MWh of annual consumption, a lot of it at night (electric heating).
- G12W tariff with prosumer settlement under the net-metering (discount) system, coefficient 0.7 - an installation above 10 kWp registered before April 2022.
- ~19 MWh exported to the grid each year - nearly half of consumption.
The key words are "net metering" and "large export". You will soon see why they decide everything.
How we calculated it
The basis was 12 months of real data - about 35,000 measurements in 15-minute windows, from three meters (grid + two inverters). On that data the EnergyPal AI engine reconstructed, hour by hour, the grid settlement in each scenario. Crucially, we validated the model against the real invoices from the energy supplier - the settled energy matched to about 1%. Every amount below is the full bill: energy + variable distribution + standing charges.
The result: the status quo beats every alternative
Each bar is the calculated full annual bill. The two cheapest variants keep the current net-metering tariff; the three most expensive are all based on the dynamic tariff.
| Scenario | Cost / year | Δ vs today | Investment (CAPEX) | Net met. 0.7 |
|---|---|---|---|---|
| G12W + net metering (today) | 10 857 zł | - | - | kept |
| Net metering + 26 kWh storage | 10 218 zł | -639 zł | 27 000 zł | kept |
| Net metering + 44 kWh storage | 9964 zł | -893 zł | 43 000 zł | kept |
| Dynamic, no storage | 18 686 zł | +7829 zł | - | lost |
| Dynamic + 26 kWh storage | 15 714 zł | +4857 zł | 27 000 zł | lost |
| Dynamic + 44 kWh storage | 14 884 zł | +4027 zł | 43 000 zł | lost |
Yes, you read that right. Switching to a dynamic tariff without a battery would raise the bill by nearly 7829 zł a year. Even with a battery costing over 43 000 zł, the house would pay about 4027 zł more than it does today.
Two mechanisms almost nobody talks about
1. Net metering is a very cheap "virtual battery". For every 1 kWh exported to the grid you get 0.7 kWh back whenever you need it - even six months later. The grid acts as a storage with 70% efficiency and unlimited capacity, carrying the summer surplus into winter.
This house exports nearly 7,800 kWh of surplus in summer and pulls over 11,500 kWh from the grid in winter. No physical battery moves energy from July to January. Net metering does.
2. Distribution: the silent cost of leaving net metering. Under net metering you pay variable distribution charges only on the energy left after balancing (here: about 15,400 kWh). Under the dynamic tariff (net billing) you pay them on the entire grid import (over 27,000 kWh). For this profile the distribution difference alone is about 3500 zł net per year - a cost that does not disappear even if wholesale prices are the lowest in Europe. Most "dynamic tariff calculators" never show it.
"So maybe just a battery, staying on net metering?"
We checked that too. A battery adds little, because net metering already does its job: a 26 kWh battery saves about 639 zł a year, a 44 kWh one about 893 zł - a payback of 42-48 years, longer than the device's life. The surplus this house has is seasonal (summer to winter), and no daily battery bridges that. The daily profile shows why:
When the same battery starts to make sense
Net metering is a time-limited privilege: it applies for 15 years from the first export to the grid. When it expires, the house moves to net billing - and that is the world in which the same 44 kWh battery saves about 3802 zł a year and pays back in about 11.3 years. On top of that comes a value that never shows on the bill: backup power - a 44 kWh battery keeps the whole house running for 7-13 hours (season depending), and critical circuits for days, recharging from PV during the day.
For this house we advised - counter-intuitively for a company that sells batteries - stay on the current tariff, and buy a battery only when the settlement rules change (or earlier, if backup matters to you today).
How to tell which group you belong to
Three signals that you should not rush into a dynamic tariff:
- Your invoice shows settlement "at a 0.7" (or 0.8) coefficient - you are in the net-metering system, and losing it is irreversible.
- You export an amount of energy comparable to half your consumption, or more.
- Your surplus is seasonal: you export a lot in summer and import a lot in winter (e.g. electric heating, a heat pump).
If instead you buy almost all your energy from the grid, have no net metering, or your installation is young (net billing from the start), the maths looks completely different - and a dynamic tariff with a battery can be an excellent decision, as in our previous case study.
Conclusions
- There is no single answer for every prosumer. The same investment pays back in 8 years in one house and in 48 in another.
- Net metering can be unbeatable - especially for large exporters with a seasonal surplus. While it lasts, it also shields you from distribution charges on your full import.
- Decide on your own data, not on averages. A year of real measurements and a check against the invoices give an answer no universal calculator can.
- An honest analysis sometimes says "don't buy". And that is fine.
Want to know which side of this line your house is on? We will calculate it on your data.
