Generator or solar? — cost comparison
Compares the levelised cost of both. Running costs dominate for the generator, up-front costs for solar — so the comparison hinges almost entirely on the fuel price and the discount rate. Where the uncertainty bands overlap, the calculator gives no recommendation and says so.
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.
Data as of: 2026-08-06
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Levelised cost, solar | (I + Σ opex_t/(1+r)^t) / Σ (E_t/(1+r)^t) | 0.11451 /kWh | exact |
| Levelised cost, generator | (I + Σ (maint_t + fuel_t)/(1+r)^t) / Σ (E_t/(1+r)^t) | 1.0091 /kWh | assumed |
| Optimal solar share | argmin NPV(f_solar) | 1 | assumed |
- Formula
LCOE = (I + Σ (operating + fuel)/(1+r)^t) / Σ (E/(1+r)^t)- Valid for
- Constant prices, uniform annual consumption, correctly discounted LCOE denominator on both sides. Fuel price prefilled from the ADAC national average of 2026-07-29 (E10 €2.14/l, diesel €2.19/l), consumption from manufacturer datasheets (0.3–0.6 l/kWh depending on engine and load). Both are visible assumptions with a ±40% band, not claims.
- Not covered
- Fuel and power price development, degradation, noise and emission rules, the storage cost of the last autarky percent (the oversize exponent is a named placeholder), and the value of supply security — a generator also delivers in December.
- Data sources
- Bundesnetzagentur SMARD electricity price analysis, StromAuskunft price index (as of 2026-08-06) and EEG remuneration rates per § 48 EEG 2023 · Q3/2026 (EEG-Sätze H2/2026) · retrieved 2026-08-06
Frequently asked questions
Which is cheaper for off-grid power, a generator or solar?
The calculator compares the levelised cost of electricity for both. Running costs from fuel and maintenance dominate the generator while solar is dominated by the up-front investment, so the comparison hinges almost entirely on the fuel price and the discount rate. An adjustable uncertainty band is therefore applied to the fuel price.
Why does the calculator sometimes refuse to recommend either option?
When the generator's uncertainty band overlaps the solar cost, any recommendation would be an assertion rather than a result — so the calculator explicitly reports the comparison as inconclusive instead of manufacturing false confidence.
Why doesn't the hybrid optimisation land at the expected 85 to 95 percent solar share?
The model usually produces a corner solution — all solar or all generator — and the reason is nameable: what makes the last percent of autarky expensive is the battery storage needed for multi-day winter gaps, which this model does not represent. The oversizing exponent is a placeholder for that effect and was deliberately not tuned until the expected answer appeared. The hybrid question needs an hourly storage simulation this module does not perform.
Why is generator power so expensive when the unit costs only €2,000?
Because running costs dominate a generator: at 0.4 l/kWh and €2.15/l, fuel alone costs 86 ct per kilowatt-hour — before purchase and maintenance. In the reference case (3,000 kWh/a, 20 years, 4% discount) the generator lands at about €1.01/kWh, the solar system at 10.9 ct/kWh. The low purchase price is an optical illusion: it is the smallest item in the bill.
So what is a generator still good for?
For what this calculator explicitly does not price: supply security. A generator delivers in a sunless December week, as backup when the battery is empty, independent of weather and season. That is exactly why the hybrid curve reports its corner solution as a model limit — whether a small generator for the winter gap pays off is decided by an hourly and storage calculation, not by this LCOE comparison.
What fuel price does the calculator assume — and why the wide band?
The prefill is €2.15/l, between the ADAC national averages of 2026-07-29 (E10 petrol €2.14, diesel €2.19). The ±40% band covers what recent years actually showed: prices between roughly €1.50 and over €2.30 per litre. Since fuel is the generator's largest cost block, any price assumption shifts the result almost one to one — which is why the chart shows a band, not just a line.
What does generator power cost compared with solar?
In the reference case (3,000 kWh a year, 20 years) the calculator finds 11.5 ct/kWh for solar and 100.9 ct/kWh for the generator — a factor of nine. Of the generator figure, 86 ct is fuel alone, the rest purchase and maintenance. Depending on the fuel price the band runs from 66.5 to 135.3 ct.
Why do solar costs depend on the location?
Because the array is paid for by power but delivers energy. 1,300 € per kWp divided by the specific yield gives the cost per annual kilowatt-hour: in the Berlin region, at 1,026 kWh/kWp, about 1.27 €; in southern Europe at a good 1,500 kWh/kWp only about 0.87 €. The same generator costs the same everywhere — so location shifts the comparison one-sidedly in favour of solar.
When does the generator win after all?
At very small energy volumes and infrequent operation. Generator cost per kilowatt-hour falls as annual energy rises, because the purchase spreads out; solar cost stays nearly constant. So the generator does better above the crossover point, not below — a direction easily confused. In the reference case the crossover lies outside the tested range, so solar wins throughout.
Does a hybrid solution make sense?
Often yes, and the calculator quantifies it: it looks for the solar share with the lowest total cost. Solar covers the base load cheaply, the generator the few winter days for which a purely solar design would demand a doubling of module area. The calculator deliberately reports corner solutions too — if 100 percent solar is optimal it says so, instead of constructing a compromise.
Which fuel price does the calculator assume?
2.15 €/l by default — the ADAC national average of 29 July 2026 (E10 2.139, diesel 2.190 €/l). Consumption is set at 0.4 l/kWh, matching the datasheet formula (diesel 0.3 to 0.36, petrol 0.36 to 0.6). Both are flagged as assumptions with ranges, because the fuel price dominates the generator calculation: it alone accounts for 86 of the 101 ct.