| Scenario | Existing (kW) | Added (kW) | Buy Rate | Export Rate | Net Cost | Year-1 Added kWh | Year-1 Savings |
|---|---|---|---|---|---|---|---|
| Starter upgrade | 4.00 | 1.60 | $0.18 | $0.08 | $6,000 | 1,950 | $269 |
| Balanced upgrade | 5.00 | 2.00 | $0.22 | $0.10 | $7,900 | 2,460 | $414 |
| Large expansion | 7.50 | 3.50 | $0.30 | $0.12 | $12,800 | 4,150 | $1,040 |
- Added capacity (kW): added_panels × panel_watt ÷ 1000 (when panels are provided).
- Year-1 added production (kWh): added_kW × production_per_kW × performance_ratio × (1 − loss%).
- Value per kWh: self% × buy_rate + (1 − self%) × export_rate.
- Year-1 added savings: year1_added_kWh × value_per_kWh.
- Net cost: gross_cost − (incentive% × gross_cost) − incentive_fixed.
- Annual cashflow (year y): savings(y) − O&M(y) − loan_payments(y).
- NPV: initial_outlay + Σ cashflow(y) ÷ (1 + discount_rate)^y.
- Enter your existing system size and the planned expansion (kW or panels).
- Set annual production per kW and performance assumptions for your site.
- Choose self-consumption, buy rate, and export credit to value each kWh.
- Add expansion costs and incentives to estimate net out-of-pocket cost.
- Optional: enable financing to see payment impact on annual cashflow.
- Press Submit to view results above the form.
Upgrade economics and sizing context
Expansion decisions usually start with added capacity, measured in kilowatts, and expected first‑year production. A practical planning range is 1.0–3.5 kW added on top of a 4–8 kW base system, producing roughly 1,600–5,500 kWh per year depending on site yield and losses. When households add appliances or an electric vehicle, extra kWh can offset higher bills without replacing the original array.
Production modeling assumptions that matter
This calculator converts added kW into year‑1 kWh using production per kW, performance ratio, and a loss factor. Using 1,500 kWh/kW/yr, an 0.82 performance ratio, and 4% shading loss, each added kW can deliver about 1,181 kWh in year one. With 0.5% annual degradation, production in year 25 is about 89% of year‑1 output, which affects long payback cases.
Valuing energy with self‑use and exports
Cash savings depend on where each kWh goes. When 60% is self‑consumed at $0.18/kWh and 40% exported at $0.08/kWh, the blended value is $0.14/kWh. With 2,460 added kWh, year‑1 savings are about $344 before O&M and financing. If rates escalate 3% annually, the blended value reaches about $0.24/kWh by year 20, increasing later‑year savings even as output declines.
Cost, incentives, and financing impacts
Gross expansion cost combines hardware, labor, permits, and other items. Incentives reduce out‑of‑pocket cost through a percentage credit and any fixed rebate. For example, an $8,100 scope with a 20% credit yields a $6,480 net cost. If financing is enabled, down payment becomes the initial outlay and loan payments reduce annual net cashflow, shifting payback later but smoothing upfront capital.
Interpreting payback, NPV, and IRR
Simple payback divides initial outlay by year‑1 savings, but cashflow payback tracks cumulative net results across years. NPV discounts future net cashflows at the selected discount rate to express today’s value. IRR estimates the annual return of the expansion cashflows. Use the annual table and chart to see when cumulative turns positive, how loan term changes early cashflow, and which inputs drive the biggest overall differences.
How should I choose “annual production per kW”?
Use a site estimate from installer proposals, past generation data, or a regional benchmark. Typical values range from 1,100 to 1,900 kWh per kW per year, depending on sun hours, tilt, and weather patterns.
What does performance ratio represent?
Performance ratio captures system losses from temperature, wiring, inverter efficiency, soiling, and mismatch. For many rooftop systems, 0.75–0.88 is common. Higher values imply better system design, maintenance, and operating conditions.
Why does self-consumption change savings?
Self-consumed energy offsets the full retail buy rate, while exported energy earns the export credit. If export credit is lower than retail, increasing self-consumption (via load shifting or storage) usually increases total savings for the same production.
Should I include rate escalation?
If you expect electricity prices to rise, escalation helps reflect larger future savings. Use a conservative assumption if uncertain, such as 2–4% per year. Setting it to 0% is fine for a cautious baseline comparison.
How does financing affect payback?
Financing reduces upfront cash but adds loan payments that lower early net cashflow. This can delay payback even if the project is attractive long term. Compare cashflow and discounted payback with and without a loan.
Is the IRR value guaranteed?
No. IRR is an estimate based on your inputs and assumes production, rates, and costs follow your assumptions. Use it as a scenario indicator, then validate incentives, tariffs, and system design details before committing.