Inputs
Example data table
These sample rows show typical inputs and outputs for quick comparison.
| PV (kW) | Battery (kWh) | Critical (kW) | Outages/yr × hrs | Utility rate | Year‑1 benefit | Simple payback |
|---|---|---|---|---|---|---|
| 5.0 | 10.0 | 1.5 | 6 × 3 | $0.18 | $951 | ~7–10 yrs |
| 8.0 | 15.0 | 2.0 | 10 × 2 | $0.22 | $2,253 | ~5–8 yrs |
| 3.5 | 7.0 | 1.0 | 4 × 4 | $0.15 | $430 | ~8–12 yrs |
Numbers are illustrative; your local yield, tariffs, and outage costs drive the results.
Formula used
- Year‑1 solar energy: PV_kW × Yield × 365 × (1 − Loss%).
- Year‑1 solar savings: Solar_kWh × SelfUse% × UtilityRate.
- Required battery (nominal): (Critical_kW × DesiredHours) ÷ (DoD × Efficiency).
- Supported hours: BatteryNominal × DoD × Efficiency ÷ Critical_kW.
- Covered outage energy/year: Critical_kW × min(SupportedHours, AvgOutageHours) × OutagesPerYear.
- Resilience value/year: Covered_kWh × OutageValuePerkWh.
- Year‑1 net benefit: SolarSavings + ResilienceValue − AnnualO&M.
- NPV: −NetCapex + Σ(AnnualBenefit_y ÷ (1 + DiscountRate)^y), with optional battery replacement in its year.
How to use this calculator
- Enter PV size and local yield to estimate annual generation.
- Set self‑use and utility rate to value on‑site savings.
- Define your critical loads and target backup duration.
- Enter battery DoD and efficiency to model usable storage.
- Add outage frequency, outage duration, and outage value.
- Review net benefit, payback, NPV, and backup hours.
- Use CSV or PDF buttons to export your results.
Value components captured
The calculator estimates two benefit streams: bill savings from on‑site solar use and avoided outage losses from battery backup. Solar savings use annual generation, self‑use share, and energy price. Resilience value uses covered outage energy, based on critical load, outage duration, and outage frequency. Covered energy equals critical kW × min(supported hours, outage hours) × outages per year. When outages are rare, savings dominate; when downtime is costly, resilience can dominate.
Sizing assumptions used
Battery sizing follows usable energy needs. Required nominal storage equals (critical kW × desired hours) ÷ (DoD × efficiency). With 90% DoD and 92% efficiency, every 10 kWh nominal yields about 8.28 kWh usable. Add 15% headroom for surges and future circuits. Inverter kW limits also matter. If installed battery is provided, the model reports supported hours.
Energy production drivers
Annual solar energy equals PV kW × yield × 365 × (1 − losses). Many sites fall near 3.5–5.5 kWh per kW per day, while losses often run 5–15% from shading and soiling. Example: 5 kW at 4.5 yield and 8% losses produces roughly 7,550 kWh in year one. Degradation reduces later output. Seasonal swings matter; use conservative yield from local studies.
Financial metrics you receive
Year‑1 net benefit equals solar savings plus resilience value minus annual O&M. Simple payback divides net capex by year‑1 benefit. NPV discounts each year’s benefit using the discount rate and can subtract a battery replacement cost in its year. Escalation grows savings when tariffs rise. Positive NPV signals value above your hurdle rate. Use the analysis period to compare alternatives consistently.
Improving accuracy for projects
Use a realistic self‑use percentage: commercial daytime loads may exceed 80%, while homes can be 30–70% without load shifting. For outage value, include downtime labor, spoilage, safety, and restart costs; many small sites land between 1 and 10 per unserved kWh. Validate critical kW from panel schedules or measurements. If export credits exist, adjust the effective rate. Document assumptions for stakeholder approvals.
FAQs
What does self‑use mean in this calculator?
Self‑use is the share of solar generation consumed on‑site. Only that portion is valued at the utility rate in the savings calculation. Exported energy is not priced unless you adjust self‑use to reflect it.
How should I choose outage value per kWh?
Estimate the cost of lost service: downtime revenue, spoiled inventory, equipment disruption, and labor impacts. Divide the expected loss per outage by the energy you would have used during that outage to get a per‑kWh value.
Why include shading/losses and degradation?
Losses reduce usable production in year one, while degradation reduces production over time. Including both prevents overstating savings and produces a more conservative NPV and payback for long‑life systems.
What discount rate should I use?
Use your hurdle rate or cost of capital. For households, 4–8% is common; for businesses, it may be higher. A higher discount rate reduces NPV because future benefits are valued less.
How does battery replacement affect results?
If you enter a replacement year and cost, the model subtracts that cost in that year before discounting. This can lengthen payback and reduce NPV, especially when replacement happens early.
Do CSV and PDF exports include my latest results?
Yes. After you calculate, the export buttons send the same inputs and regenerate the report on the server. The downloaded CSV or PDF reflects the most recent calculation you ran.