| Scenario | Outage (h/day) | Days/month | Load (kW) | Coverage | Monthly savings |
|---|---|---|---|---|---|
| Small shop essentials | 3.5 | 24 | 1.5 | 60% | PKR 17,000 |
| Home with inverter upgrade | 4.0 | 25 | 2.0 | 70% | PKR 29,000 |
| Home office reliability | 5.0 | 26 | 2.5 | 80% | PKR 45,000 |
- Monthly Outage Energy (kWh) = outage hours/day × outage days/month × critical load (kW)
- Current Cost = outage energy × generator cost/kWh + generator maintenance/month
- New Cost = (outage energy × (1 − coverage)) × generator cost/kWh + maintenance × (1 − coverage) + O&M/12
- Monthly Savings = current cost − new cost
- Simple Payback (months) = upfront cost ÷ monthly savings
- NPV = −upfront + Σ(annual savings ÷ (1 + discount rate)t)
- Estimate your average daily outage hours and affected days per month.
- Enter your critical load in kW (only what must stay on).
- Use a realistic generator cost per kWh, including wear.
- Set the coverage percentage based on your planned system.
- Review monthly savings, payback, and NPV to guide decisions.
Monthly outage energy and cost
Outage impact begins with energy not served. The calculator estimates monthly outage kWh as critical load (kW) × outage hours per day × outage days per month. Multiply by your generator cost per kWh to approximate today’s variable backup spend, then add maintenance, service, and other fixed charges to reflect real running cost. Example: 4.0 kW load × 5 hours/day × 24 days = 480 kWh; at PKR 120/kWh that’s PKR 57,600 before fixed costs.
Generator economics and fuel sensitivity
Fuel price swings change savings fast. A 10% rise in cost per kWh increases the “current cost” linearly, while the upgraded solution cost grows only for the portion still covered by the generator. Track your true blended cost by dividing monthly fuel plus oil and repairs by delivered kWh during outages. If your set runs at partial load, efficiency drops, so cost per kWh can exceed the fuel-only estimate.
Battery and solar coverage strategy
Coverage represents the fraction of outage energy supplied by solar plus storage. Usable battery kWh is derived from covered daily load adjusted for round‑trip efficiency and depth of discharge. Solar size is estimated from sun hours and derate, then increased for charging losses, giving a starting point for inverter and panel sizing. Use the sizing hints as a minimum; installers may add headroom for surge loads and seasonal sunlight variation.
Payback and NPV interpretation
Monthly savings equals current monthly cost minus new monthly cost. Simple payback is upfront cost divided by annual savings, reported in months. NPV discounts annual savings over your chosen horizon, so a positive NPV suggests the investment beats the discount rate under the same outage pattern.
Practical steps for improving savings
First, lower the critical load by switching off non‑essential circuits during outages. Second, raise coverage gradually: start with batteries for short gaps, then add solar to cut generator hours. Finally, revisit assumptions quarterly because outage schedules and generator performance drift over time.
What does coverage percentage mean?
It is the share of outage energy supplied by solar plus storage. The remaining share is supplied by the generator, so lower coverage reduces upfront cost but also reduces monthly savings.
How do I estimate generator cost per kWh?
Add monthly fuel, oil, routine service, and typical repairs, then divide by kWh produced during outages. If you do not meter kWh, estimate kWh as load (kW) × run hours, but expect some error.
Why is payback shown as N/A sometimes?
If calculated annual savings are zero or negative, the investment does not recover its upfront cost under the entered assumptions, so a simple payback period is not meaningful.
What discount rate should I use for NPV?
Use a rate that reflects your alternative return or borrowing cost. For households, this might align with deposit or loan rates; for businesses, use the required return for similar risk.
Are the battery and solar sizes final design numbers?
No. They are indicative sizing hints based on outage energy, efficiency, depth of discharge, sun hours, and derate. Final design should include surge loads, wiring limits, safety devices, and site shading.
Can I model a hybrid approach with partial generator use?
Yes. Set coverage to the portion you want solar plus storage to cover. The tool will still charge generator cost for the uncovered energy, helping you compare different hybrid mixes.