Solar Excess Capture Calculator

Formula used

• Baseline self-use (kWh/day) = min(Generation × Self-use%, Consumption)
• Baseline excess (kWh/day) = max(0, Generation − Baseline self-use)
• Captured excess (kWh/day) = Baseline excess × Capture% (limited by usable battery throughput)
• Avoided imports (kWh/day) = Captured excess × Efficiency factor
• Baseline value for excess = Baseline excess × Export credit
• Value with capture = (Remaining export × Export credit) + (Avoided imports × Import price)
• Incremental value = Value with capture − Baseline value for excess
• NPV discounts yearly net benefits, subtracting maintenance and upfront cost.

How to use this calculator

1. Enter your average daily solar generation and household consumption.
2. Set baseline self-use based on your current behavior.
3. Enter your import price and export credit from your bill.
4. Choose a capture method and estimate what fraction of excess you can capture.
5. If using a battery, add capacity, usable percent, efficiency, and cost.
6. Submit to see incremental value, payback, NPV, and IRR.
7. Download results as CSV or PDF for record keeping.

Example data table

Market context

Solar households often export mid‑day surplus while buying higher‑priced evening energy. When export credits are below import prices, capturing excess can materially improve cashflow. This calculator quantifies that gap by valuing each avoided import at your import price and each remaining export at the export credit. Even small changes in rates can shift annual savings significantly.

Key drivers of value

Incremental value rises with three inputs: excess energy, the spread between import and export rates, and the share of excess you can reliably capture. For batteries, round‑trip efficiency reduces usable energy, while usable capacity limits daily throughput. For EV charging and load diversion, efficiency is modeled near 98% because losses are typically lower than storage cycling. For many households, capturing 5 to 10 kWh daily can translate into hundreds per year, especially when import prices exceed export credits by 0.10 per kWh or more consistently.

Interpreting incremental value

The results compare two pathways for your excess: export everything, or capture some portion and use it to offset grid purchases. Daily incremental value equals value with capture minus baseline export value. If you have little excess, a high export credit, or a low import price, the incremental benefit can be small even with strong capture.

Investment metrics and sensitivity

If you enter an equipment cost and maintenance, the calculator estimates simple payback, net present value, and IRR. NPV discounts yearly net benefits using your discount rate, then adjusts benefits for energy inflation and solar degradation. Higher discount rates reduce NPV, while higher inflation increases it. Battery costs dominate payback when export credits are already generous.

Using results for decisions

Use the chart to verify whether avoided import value outweighs lost export income. Then stress‑test assumptions: reduce capture percent to reflect winter output, lower efficiency for real‑world cycling, and cap capacity to match your usable storage. A positive NPV supports the investment; otherwise, consider cheaper options like timers, smart diverters, or flexible EV charging.

FAQs

1) What is “excess capture”?

It is redirecting surplus generation that would be exported, so it can offset later grid purchases through storage, EV charging, or controllable loads.

2) Why does export credit matter so much?

Export credit is your baseline value for excess. A higher credit reduces the advantage of capturing because exporting already pays you closer to what imports cost.

3) How is battery capacity used here?

Captured energy is limited by usable capacity per day, reflecting practical depth‑of‑discharge limits. Larger usable capacity increases the amount of excess you can store.

4) What does round‑trip efficiency represent?

It combines charging and discharging losses. If efficiency is 90%, only 0.9 kWh becomes usable for every 1.0 kWh captured.

5) Does this include time‑of‑use pricing?

Not directly. You can approximate it by using a weighted average import price that reflects when captured energy would offset purchases.

6) What if my incremental value is negative?

That typically means export credits are high, captured energy is small, or losses and costs outweigh benefits. Try alternative capture methods or adjust assumptions.