Also in: Disaster Response and Recovery
Generator-Battery Hybrid Fuel Savings
A generator carrying a light load around the clock burns fuel inefficiently and wet-stacks.
Example
You enter
- Average load (kW) 15
- Generator rating in use (kW) 55
- Fuel at 1/4 load (gph) 1.5
- Fuel at 1/2 load (gph) 2.5
- Fuel at 3/4 load (gph) 3.4
- Fuel at full load (gph) 4.6
- Generator output while ON (kW) 41.25
- Battery round-trip efficiency (0 to 1) 0.85
- Charge cycles per day 1
You get
- Generator run hours per day 9.65 h (60% fewer than continuous)
- Hybrid fuel 32.8 gal/day at 3.400 gph (75% load)
- Continuous fuel at the light load 38.2 gal/day at 1.591 gph (27% load)
- Fuel saving 5.3778
- Saving (%) 14.0847
- Usable battery energy per cycle 215.3 kWh
- Run hour reduction (%) 59.799
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeThe standard fix in temporary power is to run it hard for part of the day, charging a battery with its surplus, and let the battery carry the load the rest of the time. The daily energy balance -- surplus stored, less the round-trip loss, equals the energy drawn -- gives the generator hours directly: h = 24 L / (L + (P_g - L) x efficiency). The manufacturer's four-point fuel curve, interpolated as in the Generator Fuel at Part Load and Minimum-Load Check calculator, turns hours into gallons for the hybrid and for continuous light-load running, and the battery is sized per charge cycle. A 15 kW average shelter load on a Cummins C60D6R (55 kW prime; 1.5, 2.5, 3.4, 4.6 gph) run at 3/4 load, 41.25 kW, when ON, with an 85% round-trip battery: the generator runs 9.65 hours a day, burning 32.8 gallons against 38.2 continuous -- 5.4 gallons (14%) saved and 60% fewer run hours, never below three-quarters load. The battery is the cost: 215 kWh for one cycle a day, 54 kWh for four. The saving does not change with the cycle count; only the battery does, and more cycles mean more starts. It assumes a constant load and fixed set point, and does not model load variation, charge acceptance, battery derating (the Off-Grid Battery Bank Sizing calculator), inverter losses, start limits, or controls. The generator, battery, and inverter manufacturers and the AHJ govern.
Formula or tableenergy balance (P_g - L) x eta x h = L x (24 - h), so generator hours h = 24 L / (L + (P_g - L) eta); hybrid fuel = h x gph(P_g); continuous fuel = 24 x gph(L), both from the four-point fuel curve; saving = continuous - hybrid; usable battery per cycle = L x (24 - h) / cycles per day.
Source and editionA first-principles daily energy balance, with fuel read from the manufacturer's four-point fuel curve as in generator-part-load-fuel (worked example from the Cummins C60D6R data sheet D-6567); the generator and battery manufacturers named as governing.
Free accessAn energy balance and interpolation on figures the user enters from the equipment data sheets; nothing proprietary is reproduced.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesload_kw, rating_kw, fuel_quarter_gph, fuel_half_gph, fuel_three_quarter_gph, fuel_full_gph, setpoint_kw, round_trip_efficiency, cycles_per_day, generator_hours_per_day, hybrid_gal_per_day, continuous_gal_per_day, saving_gal_per_day, saving_pct, battery_kwh_per_cycle, run_hour_reduction_pct
- Constant average load the day's load is carried at its average; variation through the day is not modelledproject method
- Round-trip efficiency the battery returns eta of the surplus storedbattery manufacturer data
- Four-point fuel curve gph interpolated at the set point and at the load, flagged below 1/4 loadmanufacturer data sheet