Size a generator for a hybrid system
Sizes the hybrid-system generator via the load factor — a permanently underloaded generator soots up, and a calculator ignoring that gives harmful advice. Deficit days and the daily gap are counted per day of the typical year at your location instead of guessed: for 3,000 Wh and 1,200 Wp in Berlin it is 187 days, not 120. The killer feature remains the marginal-cost comparison of modules versus generator hours — with real numbers it comes out clearer: break-even 5.3 instead of 10.7 years.
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 |
|---|---|---|---|
| Daily coverage gap | max(0, E_demand - E_solar) | 1,765.1 Wh/d | exact |
| Required generator size | max(P_load,peak ; P_charge,max + P_load,cont) | 2,500 W | exact |
| Effective power | P_gen * f_load | 1,875 W | assumed |
| Run hours per year | ΔE * d / (P_eff * eta_charge) | 195.6 h/a | exact |
| Fuel cost per year | t_lauf * P_effektiv * Verbrauch * Preis | 275.98 | assumed |
| Equivalent module power | Delta / (PSH * eta_Kette) | 2,439.3 Wp | assumed |
| Break-even distance | C_PV / C_fuel,a | 5.3032 a | exact |
- Formula
t_run = Δ · days / (P_gen · load factor · η_charge)- Valid for
- Generator size from peak load or charge power plus continuous load; run hours from the annual gap at effective power (rated × load factor — never 1.0). Deficit days and the daily gap are counted per day of the TMY year (location + array size, the same hourly chain as calculators 27/43); the design sun hours for the module comparison come from the module plane at the site. Reference case: 3,000 Wh/d, 1,200 Wp in Berlin → 187 deficit days, 1,765 Wh mean gap, 196 run hours, break-even of additional modules 5.3 years. Fuel price as a visible assumption (ADAC national mean 2026-07-29).
- Not covered
- Storage carry-over between days (a battery lowers real generator starts — calculator 45 computes that hourly), inrush behaviour and part-load curve of the specific unit, maintenance intervals, emission rules and noise limits at the site, fuel price development.
Frequently asked questions
What generator size do I need for a hybrid solar system?
The required rating is the maximum of the peak load and the sum of maximum battery charge power plus continuous load. Run hours and fuel are then derived from the coverage gap using the effective power — nameplate rating times load factor — rather than the nameplate figure. Notably, the fuel volume barely depends on generator size, while lifetime and wear very much do.
Why shouldn't a generator run below 30 % load all the time?
Below roughly 30 % load a generator soots up — with diesel this is called wet stacking — and burns fuel disproportionately. Recommending a 5 kW generator for a 1 kW charging load is therefore technically harmful advice. The calculator warns when the load factor is too low, and also when the unit would run only a few hours per year, because idle deterioration then costs more than fuel.
Would extra solar modules be cheaper than running the generator?
That is the marginal-cost comparison this calculator performs: the last watt-hours of winter coverage cost either extra modules once or generator hours forever. The break-even is module cost divided by annual fuel cost; if it comes out below five years, the calculator explicitly flags that modules are the cheaper option. The specific unit's part-load curve, service intervals and site noise limits are not modelled.
Why does the calculator use a load factor instead of nameplate power?
Because a generator in charging duty never runs at nameplate: realistic is 70–80% load, and run hours follow from the annual coverage gap divided by this effective power. More importantly: a generator permanently below 30% load soots up (wet stacking on diesel) and consumes disproportionately — a 5 kW unit for a 1 kW charging load is technically harmful advice, and the calculator warns exactly about that.
Add modules or keep running the generator — how do I decide?
Via marginal costs: the same winter coverage costs either a one-off module addition or fuel forever. The calculator converts the coverage gap into equivalent module power and draws both cost lines over the years — the intersection is the break-even. If it lies below the module lifetime, modules are the more economical choice; a residual generator for rare extreme weeks often still makes sense.
Which fuel price is used?
The prefill is €2.15/l between the ADAC national averages of 2026-07-29 (E10 €2.14, diesel €2.19) — as a visible assumption with a range, since the price is the biggest lever of running costs. The specific consumption (l/kWh) comes from your unit’s datasheet; small petrol units run 0.4–0.6 l/kWh, diesel 0.3–0.36.
Where do the deficit days come from?
From your location's hourly year and your array size: for every day of the typical year the calculator checks whether solar yield meets daily demand. Previously 120 days were assumed — for 3,000 Wh demand and 1,200 Wp in Berlin it is actually 187. The direction is no surprise: in the off-grid comparison, 5 assumed days turned into 146 real ones.
Why count per day instead of averaging per month?
Because a month can cover demand on average and still contain deficit days. A dull November day delivers a tenth of the monthly mean; comparing monthly totals misses exactly the days the generator is bought for. So counting happens per TMY day — the same hourly chain as the annual-yield calculator, summed to days.
What changes in the marginal-cost comparison?
It sharpens — in favour of the modules. With the real figures (187 days, 1,765 Wh mean gap, 1.03 design sun hours instead of 0.8) the break-even of additional modules drops from 10.7 to 5.3 years. The reason: the guessed values understated generator hours and overstated the required module power at the same time.
Why is the existing array power an input?
Because the coverage gap depends on it — and because the array calculator hands it over directly: if you sized your system there, the wattage arrives pre-filled here. Doubling the example array to 2,400 Wp cuts deficit days from 187 to 119 and run hours from 196 to 120 per year.
Does the calculation account for a battery bridging deficit days?
No, deliberately not. The daily count compares yield and demand per day without storage carry-over between days — a battery bridging two dull days reduces real generator starts below the counted deficit days. That question is answered by the off-grid calculator with hourly storage dispatch; here the count remains the conservative upper bound.