Grid connection or off-grid? Compare costs
Compares a grid connection with an off-grid system as a present value over the service life, with the break-even distance as the key figure. New: the deficit days of the off-grid system are computed from the hourly year of your location, not estimated. For 10 kWp and 20 kWh of storage near Berlin that is 63 days a year on which the load is not fully covered — against roughly a quarter of an hour of outage on the German grid.
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
An estimate based on the stated assumptions. The final design must be checked by a qualified professional against the rules that apply where you are.
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Connection cost | K_grund + K_meter * Entfernung | 15,000 EUR | assumed |
| NPV grid connection | K_Anschluss + Barwert(Grundgebuehr + Verbrauch) | 35,457 EUR | assumed |
| NPV island system | I_System + Barwert(Betrieb + Kraftstoff) + Batterieersatz | 39,251 EUR | assumed |
| Break-even distance | (Barwert_offgrid - Netz_fix) / K_meter | 197.43 m | assumed |
- Formula
Grid: C_base + C_metre·d + NPV(standing charge + consumption) · island: I + NPV(operation + fuel) + battery replacement every N years · break-even: d* where grid(d*) = island- Valid for
- Constant real prices, discounting over the service life, battery replacement at fixed intervals; connection costs as regional guide values with bands (annex B8).
- Not covered
- Binding grid-connection costs (DSO-dependent, often not public — obtain the DSO’s actual quote before deciding), electricity price escalation, subsidies, residual values, permitting costs for trenches across third-party land.
Frequently asked questions
From what distance does off-grid beat a grid connection?
Exactly the break-even distance the calculator outputs: the trench length at which the NPV of the grid connection (base cost + cost per metre + running electricity cost over the service life) reaches the island system’s NPV. In the example (€3,000 base, €80/m, 3,500 kWh/a, €25,000 island) it sits at a few hundred metres — at 150 m of trench the grid still clearly wins. The figure reacts strongly to the per-metre cost: in rocky ground or across third-party land it tips earlier.
Where do the grid-connection costs in the calculator come from?
They are explicitly guide values with bands (annex B8 of the specification): grid-connection costs are DSO-dependent and often not public — base costs typically €2,000–5,000, trench €50–150/m including civil works. Both fields are therefore marked as assumptions and editable. Only your DSO’s quote is binding; the calculator does not replace that inquiry, it tells you whether it is worth making.
Why does the calculator compare NPVs instead of simple sums?
Because the cost profiles are mirror images: the grid connection is dominated by running costs (electricity purchases over 20 years), the island by the one-off investment plus battery replacement after 10–15 years. Without discounting, future electricity bills would weigh as much as today’s investments — with it, a euro in 15 years counts appropriately less. The discount rate (default 3%) is marked as an assumption and shifts the comparison noticeably.
What does the reliability comparison mean?
It is the second currency besides money: the island design has calculated deficit days — days when solar plus storage do not cover demand (5 in the example, 0–15 depending on sizing). The German grid, by contrast, is down only about a quarter of an hour per year on average (BNetzA SAIDI). A connection that costs a few thousand euros more but practically never fails is a different good — the calculator says so instead of only comparing euros.
How do I handle battery replacement?
As a plannable second investment: the calculator applies it at fixed intervals (default every 12 years, adjustable 10–15) and discounts it to today — over a 20-year horizon exactly one replacement falls due. Its cost has been falling in real terms for years (2026 storage prices: €250–450/kWh device); stretching the interval through shallow cycling shifts the NPV noticeably. The battery lifecycle-cost calculator supplies the interval for your chemistry and depth of discharge.
Who is this comparison actually relevant for?
For the cases at the edge of the grid: weekend houses, alpine huts, allotments, remote farm buildings, new builds far from the last connection point. In town with a connection at the door the grid nearly always wins — the interesting zone starts where three-digit trench metres and civil works enter. And often the best answer is a hybrid: grid connection plus PV against the running costs — the payback calculator takes over there.
How reliable is an off-grid system really?
Considerably less reliable than rules of thumb suggest. For a household using 3,500 kWh a year in the Berlin region the calculator finds: 5 kWp with 10 kWh of storage gives 146 deficit days a year, 10 kWp with 20 kWh still 63 days, and even 15 kWp with 40 kWh comes to 32 days. In the worst month it is 23 to 30 days — essentially every day in December. For comparison: the German grid is out about a quarter of an hour a year on average.
Why is even a large off-grid system short in winter?
Because in December central Europe simply does not deliver enough energy to carry a household through the night — and a battery only shifts what is left over during the day. Once the daily yield falls below the daily demand, more storage no longer helps. The calculator shows this in the worst month's deficit days: they barely fall as the array grows, while the annual total drops markedly.
At what distance does off-grid become cheaper?
In the reference case from about 197 metres of trench — below that the grid connection is cheaper over the service life. The figure depends heavily on the base connection cost and the price per metre, both of which vary locally and are flagged as assumptions with ranges. Importantly, the break-even distance compares money only. The deficit days sit alongside it because they do not convert into euros.
Can I cover the deficit days with a generator?
Yes, and that is why the calculator carries generator fuel as its own item. A generator makes the off-grid system dependable but shifts the calculation: it costs purchase, maintenance and fuel, and it runs precisely on the cold dark days when you least want to service it. The 63 deficit days in the reference case are therefore less an outage risk than an operating cost item.
What is not included in the comparison?
The value a existing grid connection adds to the property, the option to export and be paid for it, later load growth such as a heat pump or wallbox, and the permitting situation. Also not modelled: the off-grid system has to be sized for the worst month and is therefore massively oversized in summer — the surplus evaporates because there is no one to take it.