Loan Solar Cost Estimator
Most solar tools size by roof or by sales script. This one sizes by your actual usage, prices the system at a printed $/W, applies YOUR incentive percentage, and shows the payback arithmetic in the open — roof area needed included.
The incentive field starts at 0 so the default payback is the honest no-credit figure — enter 30 to apply the US federal credit, or your state/local percentage. System losses (inverter, wiring, soiling) are planned at 14%. Savings price production at $0.16/kWh.
How it works
Size follows production: daily kWh ÷ (peak sun hours × (1 − 14% losses)) gives the kW needed; panels are 400W each, rounded up, so the fitted system is slightly larger than the bare need. At 900 kWh/mo on 4.5 sun hours: 30 kWh/day ÷ 3.87 ≈ 7.75 kW needed → 20 panels → 8.0 kW fitted, producing about 11,300 kWh/yr. Cost is kW × $2.85/W (the current US residential mid-band). Payback is net cost ÷ yearly savings — no rate-escalation fairy dust, which is why this payback often looks longer than a sales deck's. Roof area plans at about 17.5 sq ft per panel.
Method & worked example
Method: Size needed (kW) = (monthly kWh ÷ 30) ÷ (peak sun hours × (1 − 14% system losses)) × offset %. Panels are 400W, rounded up. Production = fitted kW × sun hours × 365 × 0.86. Cost = fitted kW × 1,000 × $2.85/W; savings at $0.16/kWh; payback = net cost ÷ yearly savings; roof at 17.5 sq ft per panel.
Worked example: 900 kWh/mo at 4.5 sun hours: 7.75 kW needed, 20 panels fitted (8.0 kW), about 11,300 kWh/yr, $22,800 gross, $1,808/yr saved, 12.6-year simple payback before incentives.
When to use this
- You have a year of electric bills and want a size and price before calling installers.
- Installer quotes disagree wildly and you need the $/W yardstick.
- You are testing how incentives change the payback.
How to use it, step by step
- Enter your average monthly kWh from real bills.
- Pick your region's peak sun hours.
- Set the offset you want — 100% is the default, 75% the conservative case.
- Leave incentive at 0 for the honest gross payback, then try 30% for the federal credit.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Monthly kWh | Average monthly consumption | 600–1,200 kWh typical homes |
| Sun hours | Peak sun hours per day in your region | 3.5 Northeast; 4.5 average; 5.5 Southwest |
| Offset % | Share of usage the system should cover | 50–100% |
| Incentive % | Tax credits and rebates applied to gross | 0% raw; 30% US federal credit |
Another worked example
A smaller desert home: 600 kWh/mo at 5.5 sun hours. Need = 20 kWh/day ÷ (5.5 × 0.86) = 4.23 kW → 11 panels fitted (4.4 kW), producing about 7,596 kWh/yr. Gross $12,540 at $2.85/W; saving $1,215/yr; payback 10.3 years before incentives — sun hours quietly did what a bigger system does elsewhere.
Frequently asked questions
How many solar panels do I need for 900 kWh a month?
About 20 modern 400W panels — an 8.0-kW fitted system producing roughly 11,300 kWh a year on 4.5 peak sun hours. In the Southwest (5.5 hrs) a 600-kWh home needs only 11 panels; sun hours are the silent variable in every solar quote.
What does an 8-kW system cost?
Around $22,800 installed at $2.85/W before incentives. Enter 30% for the federal credit and the net is $15,960. Quotes vary ±30% by region and installer — get three, and compare on $/W so the comparison is apples-to-apples.
How long is the payback?
On the default home with no incentive entered: $22,800 ÷ $1,808/yr ≈ 12.6 years. With the 30% federal credit: about 8.8 years. Rising electric rates shorten real paybacks; poor net-metering rules lengthen them.
How much roof do I need?
About 17.5 sq ft per panel including access spacing — 350 sq ft for the 20-panel default system, ideally on a south, east or west slope without chimney and vent collisions.
Should I offset 100% of my usage?
Usually near it, but not blindly: where exported power is credited far below retail, the last panels earn least, and some utilities cap system size relative to usage. Set the offset to 75% and compare paybacks — the honest way to decide.
🔌 Adding an EV to that roof?
Solar pairs with an EV charging load beautifully. The EV Charging Cost Estimator works out what home charging adds to the same electric bill.
What if my roof faces east-west?
East-west arrays produce roughly 10–15% less than the same array facing south. Size up by a panel or two, or accept a lower offset — enter the reduced expectation by lowering the sun-hours choice one step.
Do I need a battery for the payback to work?
No — the math here is grid-tied without storage. Batteries buy outage insurance and time-of-use arbitrage, not core payback, in most rate territories today.