| Scenario | Baseline Bill (monthly) | Offset Approach | Estimated Savings (monthly) | Notes |
|---|---|---|---|---|
| Efficiency Upgrades | $210.00 | 35% electric, 10% gas | $58.00 | Lower usage; fixed charges remain. |
| Solar Production | $240.00 | 650 kWh/month production | $92.00 | Output varies by season and shading. |
| Mixed Savings Plan | $185.00 | 50% electric, 15% other utilities | $71.00 | Combine device upgrades and billing credits. |
These examples are illustrative; run your own inputs for accurate projections.
- Baseline monthly bill = (kWh × electric rate + electric fixed) + (therms × gas rate + gas fixed) + other monthly.
- Electric offset:
- Percentage mode: offset kWh = kWh × (electric offset % ÷ 100).
- Production mode: offset kWh = min(kWh, production kWh).
- Post-offset bill uses reduced usage; fixed charges remain:
- post electric = max(0, kWh − offset kWh) × rate + fixed.
- post gas = max(0, therms − offset therms) × rate + fixed.
- post other = other × (1 − other offset % ÷ 100).
- Monthly savings = baseline monthly − post-offset monthly.
- Utility inflation grows rates by (1 + inflation)^(year−1).
- Offset degradation reduces offset output by (1 − degradation)^(year−1).
- Loan payment uses the standard amortization payment formula (PMT).
- Net annual cashflow = (annual savings) − maintenance − loan payments.
- NPV discounts each year: cashflow ÷ (1 + discount rate)^year, including year 0 upfront outlay.
- Break-even year is the first year cumulative net cashflow ≥ 0.
- Enter your monthly utility usage and rates (electric, gas, and other).
- Choose an offset mode: percentage reductions or monthly energy production.
- Add project cost, incentives, and estimated annual maintenance.
- Select cash or loan and fill financing details if applicable.
- Set inflation, discount rate, and analysis years for projections.
- Click Calculate Offset to view results above the form.
- Use Download CSV or Download PDF from the results panel.
Tip: Match your bills’ units (kWh, therms) and confirm fixed charges to avoid overstating savings.
Baseline Utility Spend Snapshot
A household using 900 kWh at $0.16 plus an $18 service fee spends about $162 monthly on electricity. Add 35 therms at $1.30 with a $12 fee and the gas bill is about $58. With $25 in other utilities, the baseline totals near $245 per month, or $2,940 per year, before any improvements. Fixed fees represent 12% of the baseline, so offsets cannot eliminate the bill completely. Seasonal usage swings can shift monthly results.
Offset Mechanics and Degradation
Offsets reduce usage-based charges but fixed charges stay. In percentage mode, a 60% electric offset cuts 540 kWh, leaving 360 kWh billed. In production mode, 600 kWh of monthly generation offsets up to the full 600 kWh, capped by your usage. Add 15% gas offset and 10% other-utility reduction to broaden impact beyond electricity. A 0.5% annual degradation factor reduces offset output to about 93% by year 15.
Financing, Payments, and Cashflow
Net project cost equals project cost minus incentives. For example, $9,000 minus $1,500 yields $7,500. With a $1,000 down payment, the financed balance is $6,500. At 10.5% APR for 7 years, the payment is roughly $110 per month, which reduces early net savings until the loan is retired. Add $120 per year in maintenance to keep assumptions realistic.
Inflation and Discounting Effects
Utility inflation compounds savings over time because avoided kWh and therms rise in cost. At 3.5% annual inflation, a $1,000 first‑year savings becomes about $1,630 by year 15, before maintenance and payments. Discounting reverses that growth for NPV: at 6%, $1,000 received in year 10 is worth about $558 today. If inflation is 0%, the projection becomes a straight-line savings profile.
Interpreting Payback, NPV, and IRR
Break-even is the first year cumulative net cashflow turns positive, including maintenance and loan payments. NPV sums discounted cashflows; a positive NPV indicates returns above the discount rate. IRR estimates the discount rate that makes NPV zero; use it to compare options with different costs, incentives, and lifetimes.
What does “offset” mean in this calculator?
Offset is the portion of utility usage you avoid paying for through efficiency, production, or credits. The tool reduces usage-based charges while keeping fixed service fees, then compares baseline versus post-offset costs.
Why can’t my post-offset bill go to zero?
Most bills include fixed customer, meter, or minimum charges. Offsets typically reduce only variable usage charges. If your provider has minimum-bill rules, the real-world floor can be higher than the model.
When should I use production mode?
Use production mode when you have an estimated monthly kWh output, such as from solar. The calculator caps the offset at your usage, then applies annual degradation to reflect performance decline over time.
How are NPV and IRR calculated here?
NPV discounts the upfront outlay and each year’s net cashflow using your discount rate. IRR is the approximate rate that makes the discounted cashflows sum to zero, based on the projected cashflow series.
How does a loan change the results?
Loan payments reduce net cashflow during the loan term, which can delay break-even. The calculator estimates a standard amortized payment from principal, APR, and term, then subtracts annual payments from savings.
Can I export my results?
Yes. After you calculate, use the Download CSV or Download PDF buttons in the results panel. Exports include key assumptions and a year-by-year projection for easy sharing and recordkeeping.