Calculator Inputs
Example Data Table
| Item | Example value | Notes |
|---|---|---|
| Solar size | 50 kW | Containerized or rooftop array sizing. |
| Annual yield | 1500 kWh/kW-yr | Adjust for site solar resource and shading. |
| Storage capacity | 100 kWh | Supports shifting and peak reduction. |
| Retail rate | $0.18/kWh | Higher rates improve payback. |
| Self-consumption (no / with) | 55% / 80% | Battery increases on-site usage. |
| Replacement | Year 12 at 55% | Reflects midlife battery refresh cost. |
Formula Used
- Annual PV energy (Year 1): PV_kWh = Solar_kW × Yield
- Usable battery energy per cycle: E_usable = Storage_kWh × DoD
- Annual battery throughput: Throughput = E_usable × Cycles/day × 365 × Efficiency
- Self-consumed energy: Self_kWh = PV_kWh × SelfConsumption%
- Exported energy: Export_kWh = PV_kWh − Self_kWh
- Energy value: Value = Self_kWh×RetailRate + Export_kWh×ExportRate
- Degradation: PV_kWh(y) = PV_kWh(1) × (1 − Deg)^(y−1)
- Escalation: Rates(y) = Rates(1) × (1 + Esc)^(y−1)
- Yearly net cashflow: Net(y) = Value(y) + Demand(y) + Backup(y) − O&M(y) − Replacement(y)
- Upfront net cost: Upfront = SolarCapex + StorageCapex + SoftCosts − Incentives
- NPV: NPV = −Upfront + Σ Net(y)/(1+Discount)^y
- Simple ROI: ROI = (Σ Net(y) − Upfront) / Upfront
How to Use This Calculator
- Enter solar size, expected yield, and battery capacity.
- Fill in installed costs and any incentives or rebates.
- Set electricity and export rates from your tariff.
- Choose realistic self-consumption values before and after storage.
- Add demand savings or backup value if you can monetize them.
- Review project life, degradation, escalation, and discount rate.
- Click Calculate ROI to view results above the form.
- Use CSV or PDF to share assumptions with stakeholders.
Cost Drivers for Solar and Storage
Installed cost is usually driven by solar equipment ($/kW), storage hardware ($/kWh), and site soft costs such as transport, foundations, permits, and commissioning. Enter incentives as a direct reduction to net capex to mirror rebates or tax credits captured by the project. For short-duration jobsites, set a conservative project life so you do not spread benefits across years the system will not operate.
Energy Value and Self-Consumption
Annual value is built from two streams: self-consumed energy valued at the retail rate and exported energy valued at the export rate. The self-consumption percentages represent how well site loads match solar output before and after shifting with storage. If daytime loads are high, 50–65% is common without storage; with storage, 70–90% can be achievable when evening lighting and equipment charging are scheduled.
Battery Throughput Limits
Storage benefits are capped by battery throughput. The calculator estimates annual throughput from usable capacity (kWh × depth of discharge), cycles per day, and round-trip efficiency. When the added self-consumed energy exceeds this throughput, savings are limited even if you enter a higher target self-consumption percentage. Use realistic cycling: 0.5–1.0 cycles/day fits many sites, while >1.5 requires deliberate operational control.
Lifecycle Effects and Replacement
Performance changes over time matter. Degradation reduces yearly production, while electricity escalation increases the value of avoided grid energy and may also raise operating expenses. The replacement year and replacement percent represent a midlife battery refresh, creating a visible dip in annual cashflow. Align this entry with warranty terms and expected cycle intensity, and include procurement lead time for critical components.
Decision Metrics for Stakeholders
Use multiple metrics to decide. Simple payback shows how quickly annual net savings recover the upfront net cost. Simple ROI compares lifetime net savings to upfront investment, supporting quick screening. NPV discounts each year’s net cashflow by your discount rate to match capital budgeting rules. IRR estimates the annualized return where NPV equals zero, helping compare against alternative equipment purchases. Review the yearly cashflow table to confirm replacement costs do not coincide with peak expenditure months onsite.
FAQs
What does self-consumption mean here?
It is the share of solar energy used on-site instead of exported. Storage raises self-consumption by shifting excess daytime energy to evening or peak periods.
How should I choose the annual yield value?
Use a site-specific estimate from production tools or past projects. Typical construction deployments range from 1,200 to 1,900 kWh per kW per year depending on location and shading.
Why can savings be capped even with a high storage percentage?
Because the battery can only move a limited amount of energy each year. Throughput depends on usable capacity, cycles per day, and round-trip efficiency, so unrealistic cycling will overstate savings.
Where do demand savings fit in?
If your tariff includes demand charges, enter the expected monthly reduction from peak shaving. For flat energy-only tariffs, leave demand savings at zero.
What should I enter for battery replacement?
Set the year when a major refresh is expected and the replacement cost as a percent of storage capex. Use warranty duration, cycle intensity, and vendor pricing trends to guide the assumption.
How can I model generator fuel offsets or outage benefits?
Add the avoided fuel and maintenance value into backup value per year, or reflect it in the electricity rate for displaced kWh. Keep assumptions documented so stakeholders can validate them.