Every off-grid site eventually faces the same spreadsheet: keep burning diesel, or invest in solar and storage? The answer is rarely all-or-nothing — the winning configuration for most sites is a hybrid that lets PV carry the day, batteries bridge the evening, and the genset shrink into a backup role. Here is the full 10-year model for a representative 100 kW site, with the sensitivities that decide it.
The reference site
- Average load: 60 kW, peaking at 100 kW; 16 h/day operation (06:00–22:00)
- Daily energy: ~960 kWh; solar resource: 4.5 peak sun hours (typical for SEA / Africa / Middle East)
- Diesel price: $1.20/litre delivered (remote-site logistics included)
Option A: diesel only
- 200 kVA prime genset running 16 h/day at ~40% average load
- Fuel: ~250 L/day → $109,500/year
- Maintenance (oil, filters, top-end work at these run-hours): ~$12,000/year
- Engine overhaul/replacement around year 5–6 at 25,000+ hours: ~$45,000
- 10-year TCO ≈ $1.30 million (genset CAPEX $55k + fuel $1.09M + O&M + overhaul)
Option B: solar + BESS hybrid, genset backup
- 100 kWp PV array (TOPCon bifacial, ground mount): ~$75,000 installed
- 200 kWh LFP BESS + 100 kW hybrid PCS: ~$95,000
- Existing/new 200 kVA genset retained for cloudy days and backup: $55,000
- System integration, controls, EPC: ~$35,000 → CAPEX ≈ $260,000
- PV yields ~450 kWh/day; BESS shifts surplus to evening; genset runtime falls ~75% to ~4 h/day equivalent
- Fuel: ~65 L/day → $28,500/year; genset O&M drops to ~$4,000/year; BESS/PV O&M ~$4,500/year
- 10-year TCO ≈ $0.63 million
The comparison
| Metric | Diesel only | Solar + BESS hybrid |
|---|---|---|
| Upfront CAPEX | $55,000 | $260,000 |
| Annual fuel | $109,500 | $28,500 |
| Annual O&M | $12,000 | $8,500 |
| 10-year TCO | ~$1.30M | ~$0.63M |
| Break-even | — | ~4.2 years |
| 10-year saving | — | ~$670,000 |
Sensitivity: what moves the answer
- Diesel price. At $0.80/L break-even stretches to ~6.3 years; at $1.60/L it shortens to ~3.1 years. Remote logistics push real delivered prices toward the high end.
- Solar resource. Each 0.5 sun-hour changes PV yield ~11%; at 5.5 PSH the hybrid's fuel displacement approaches 85%.
- Load profile. Day-heavy loads (processing plants, pumping) favour the hybrid strongly; night-heavy loads need proportionally more storage.
- BESS pricing. LFP system prices continue to fall — every 10% drop pulls break-even in by roughly 2 months. See our chemistry guide for why LFP dominates this duty.
- Financing. At 8% cost of capital the hybrid still wins by >$500k over 10 years; leasing structures can make it cash-positive from year one.
Design notes that protect the model
- Keep the genset — 100% diesel displacement is uneconomic; the last 10–15% of autonomy costs more in batteries than a backup genset.
- Size the PCS for the largest motor start, not just average load (see our sizing method — it applies to hybrid PCS too).
- Specify hybrid controllers that enforce genset minimum-load (≥30%) when it does run, avoiding wet stacking.
- Use daily-cycling-rated LFP with 6,000+ cycle warranty — this duty uses one full cycle per day.
Econo Solar supplies every component of this architecture — tier-1 modules, Sungrow hybrid inverters, LFP storage and diesel gensets — and models the TCO against your actual load data. Send your daily load profile for a site-specific version of this analysis.