Calculator Methodology
Calculator Methodology
Plain English documentation of every formula, data source, and assumption behind the ChooseMyPower calculators. Every number you see in a result has a paper trail here. If something looks off, you can verify it against the original source. Last reviewed: 2026-05-08.
This is not tax, legal, or financial advice. Tax credits, financing terms, and incentive programs change frequently. Always confirm with a qualified professional before making decisions based on calculator estimates.
1. Effective Rate Decoder
Calculates what a Texas electricity plan actually costs per kilowatt-hour at any given monthly usage, exposing the bill-credit gotcha hidden in many Electricity Facts Labels.
energy = usage × (energyRate / 100)
delivery = usage × (deliveryRate / 100)
credit = (usage ≥ creditThreshold) ? creditAmount : 0
total = energy + delivery + baseCharge − credit
effectiveRate = (total / usage) × 100
delivery = usage × (deliveryRate / 100)
credit = (usage ≥ creditThreshold) ? creditAmount : 0
total = energy + delivery + baseCharge − credit
effectiveRate = (total / usage) × 100
Limitations
- Does not yet support tiered energy rates (some plans price differently above and below 1,000 kWh).
- Does not yet capture the TDU fixed monthly charge as a separate field.
- Excludes minimum-usage fees that some plans apply below 800 kWh.
- Does not include sales tax, gross receipts reimbursements, or municipal franchise fees.
2. Plan Comparison
Compares two or three plans at four usage tiers (500, 1,000, 2,000 kWh and the user’s actual usage). Uses the same per-plan math as the Effective Rate Decoder.
Limitations
- Same EFL limitations as the Effective Rate Decoder above.
- The “winner” badge is based on lowest dollar bill at the user’s usage. It does not factor in renewable content, contract length, cancellation fees, or plan type (fixed / variable / indexed).
3. Electricity Usage Estimator
Estimates monthly and annual kilowatt-hour usage for households without a recent electricity bill, based on home size, climate region, household size, and high-draw appliances.
annual = sqft × regionFactor
+ (centralAC ? sqft × coolingFactor : 0)
+ max(0, occupants − 2) × 400
+ (electricWaterHeater ? 3500 : 0)
+ (heatPump ? climateAdder : 0)
+ (pool ? 2500 : 0)
+ evCount × 3000
+ windowAcCount × 500
+ (centralAC ? sqft × coolingFactor : 0)
+ max(0, occupants − 2) × 400
+ (electricWaterHeater ? 3500 : 0)
+ (heatPump ? climateAdder : 0)
+ (pool ? 2500 : 0)
+ evCount × 3000
+ windowAcCount × 500
Region factors (kWh per square foot per year, base case)
- North Texas: 3.0 base / 1.8 cooling adder
- Central Texas: 3.5 base / 2.2 cooling adder
- South Texas: 4.0 base / 2.8 cooling adder
- Gulf Coast: 4.5 base / 3.2 cooling adder
- West Texas: 3.5 base / 2.0 cooling adder
- Other deregulated state: 3.0 base / 1.5 cooling adder
Sources
Calibrated against the U.S. Energy Information Administration’s Residential Energy Consumption Survey (RECS) by climate zone. EIA RECS
4. Solar Savings Calculator
Estimates rough system size, monthly savings, and payback for residential rooftop solar across all 50 states plus the District of Columbia. Uses 2026 economics, after the federal residential tax credit ended.
annualUsage = monthlyBill × 12 / (stateRate / 100)
systemSizeKw = annualUsage / (sunHours × 365 × 0.78)
cashCost = systemSizeKw × $2,800
year1Savings = annualUsage × stateRate × netMeteringFactor
paybackYears = cashCost / year1Savings
systemSizeKw = annualUsage / (sunHours × 365 × 0.78)
cashCost = systemSizeKw × $2,800
year1Savings = annualUsage × stateRate × netMeteringFactor
paybackYears = cashCost / year1Savings
Constants and assumptions
- Installed cost: $2,800 per kilowatt installed (2026 post-tax-credit national midpoint).
- System derate factor: 0.78 (matches NREL PVWatts default with 14% loss factor).
- Sun-hour values: per-state averages from NREL PVWatts v8 representative coordinates, refreshed daily.
- Electricity rates: trailing 12-month average of EIA Table 5.6.A residential rates, refreshed daily.
- Net-metering factor: 1.0 for full retail net metering, 0.7 for reduced (NEM 3.0 etc.), 0.85 for Texas-style REP-dependent buyback.
Sources
EIA Table 5.6.A for state residential rates.
NREL PVWatts v8 for solar irradiance.
DSIRE for state incentives.
IRS Section 25D for federal credit status.
Tax-credit guidance is for education only. The federal residential clean energy credit (Section 25D) ended for systems placed in service after December 31, 2025. State and local incentives change frequently. Verify with a qualified tax professional before relying on these figures.
5. Home Battery Backup Calculator
Sizes a battery system based on which loads the user wants to run and for how long. Outputs capacity in kilowatt-hours, a cost range, and a recommended product category.
totalDailyKwh = sum of selected load daily kWh
rawKwhNeeded = totalDailyKwh × (backupHours / 24)
recommendedCapacity = rawKwhNeeded / 0.85 (depth-of-discharge buffer)
rawKwhNeeded = totalDailyKwh × (backupHours / 24)
recommendedCapacity = rawKwhNeeded / 0.85 (depth-of-discharge buffer)
Daily energy assumptions
- Refrigerator and freezer: 1.5 kWh/day
- LED lights (a few rooms, evening hours): 0.4 kWh/day
- Internet, router, phone chargers: 0.5 kWh/day
- Medical equipment (CPAP, oxygen): 1.5 kWh/day
- Well or sump pump: 1.5 kWh/day
- Central air conditioning: 30 kWh/day
- Window air conditioner (per unit): 8 kWh/day
Cost per kilowatt-hour installed
- Small portable (under 5 kWh): $500/kWh
- Large portable / stackable (5 to 15 kWh): $700/kWh
- Whole-home wall battery (over 15 kWh): $1,100/kWh
Capacity sizing alone does not size a backup system. Real installations also depend on inverter output, surge capacity, 120V vs 240V circuit support, transfer-switch type, permitting, and local electrical code. Medical-device users should not rely solely on a calculator estimate.
6. 2026 Solar Lease vs Buy vs PPA Calculator
Compares cash purchase, solar loan, lease, and PPA across 25 years and selects a winner based on highest 25-year net savings among paths the user is eligible for.
// 25-year savings with 3% annual utility inflation
lifetimeSavings = sum_{y=0..24} (year1Savings × 1.03^y)
// Cash purchase
net = lifetimeSavings − cashCost
// Solar loan (20-year, 6.99% APR, $0 down)
monthlyPmt = PMT(0.0699/12, 240, cashCost)
net = lifetimeSavings − (monthlyPmt × 240)
// Lease (year 1 = $13/kW/month, 2.5% escalator)
net = lifetimeSavings − sum_{y=0..24} (yr1Lease × 1.025^y)
// PPA (year 1 = $0.11/kWh, 2.9% escalator)
net = lifetimeSavings − sum_{y=0..24} (yr1PPA × 1.029^y)
lifetimeSavings = sum_{y=0..24} (year1Savings × 1.03^y)
// Cash purchase
net = lifetimeSavings − cashCost
// Solar loan (20-year, 6.99% APR, $0 down)
monthlyPmt = PMT(0.0699/12, 240, cashCost)
net = lifetimeSavings − (monthlyPmt × 240)
// Lease (year 1 = $13/kW/month, 2.5% escalator)
net = lifetimeSavings − sum_{y=0..24} (yr1Lease × 1.025^y)
// PPA (year 1 = $0.11/kWh, 2.9% escalator)
net = lifetimeSavings − sum_{y=0..24} (yr1PPA × 1.029^y)
Constants and assumptions
- Utility rate inflation: 3% per year (long-term EIA average).
- Loan terms: 20 years at 6.99% APR with $0 down (mainstream 2026 solar loan rates).
- Lease pricing: $13 per kilowatt of system size per month with a 2.5% annual escalator.
- PPA pricing: $0.11 per kilowatt-hour produced with a 2.9% annual escalator.
- Federal Section 25D credit: $0 for cash and loan paths (ended December 2025). Lease and PPA providers can still claim the commercial Section 48 credit and pass savings through to homeowners as lower payments.
This calculator is for education only and is not tax or financial advice. Tax credits, lease pricing, loan APRs, and PPA escalators all vary by provider, state, and credit profile. Real quotes will differ from the model. Consult a qualified tax professional and an independent solar advisor before signing any contract.
Refresh cadence and audit trail
A daily refresh script (06:00 UTC) pulls fresh state rate data from EIA and fresh sun-hour data from NREL PVWatts. The canonical state-energy-data.json file is updated automatically when changes fall under a 5% drift tolerance. Anything larger is held for human review and surfaced via Telegram and Discord notifications.
A daily change report is published at /refresh-report-{date}/. Past reports are retained for at least 90 days.
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