| Monthly kWh | Offset % | Sun hours | Derate | Panel W | Estimated kW | Panels |
|---|---|---|---|---|---|---|
| 900 | 100 | 5.0 | 0.80 | 400 | 7.40 | 19 |
| 1200 | 90 | 4.5 | 0.78 | 410 | 10.54 | 26 |
| 650 | 80 | 6.0 | 0.82 | 370 | 3.48 | 10 |
- Annual usage (kWh) = Monthly kWh × 12
- Target offset (kWh) = Annual usage × (Offset % ÷ 100)
- Required system size (kW) = Target offset ÷ (Peak sun hours × 365 × Derate)
- Panels needed = ceil(Required kW × 1000 ÷ Panel W)
- Actual DC size (kW) = Panels × Panel W ÷ 1000
- Estimated area (m²) = Panels × Panel area
- Gross cost ($) = Actual DC size × 1000 × Cost per watt
- Net cost ($) = Gross cost × (1 − Incentive % ÷ 100)
- Year‑1 savings ($) = Target offset × Electricity rate
- Simple payback (years) = Net cost ÷ Year‑1 savings
- Enter your typical monthly kWh from recent utility statements.
- Set an offset goal based on bill reduction targets.
- Add local peak sun hours for your roof exposure.
- Use a derate factor to reflect real‑world losses.
- Choose a panel wattage and panel area from a spec sheet.
- Optionally add roof usable area to check fit.
- Enter cost, incentives, and utility rate for estimates.
- Click Calculate, then export CSV or PDF if needed.
Load profile and annual energy baseline
System sizing starts with measured consumption, not estimates. A 900 kWh monthly average equals 10,800 kWh yearly. If you target a 100% offset, the calculator uses that figure. For a 75% goal, it scales the target to 8,100 kWh. This keeps expectations aligned with budget, roof limits, and utility net‑metering rules.
Peak sun hours translate sunlight into production
Peak sun hours convert your site’s solar resource into daily generation. At 5.0 hours, each installed kilowatt can produce roughly 5 kWh per day before losses. If the same roof averages 4.0 hours, required kW rises by about 25% for identical energy goals. The chart visualizes this sensitivity so you can stress‑test winter performance and shading impacts.
Derate factor captures real‑world losses
Losses from temperature, wiring, inverter efficiency, soiling, and mismatch reduce output versus nameplate power. Typical derate values are 0.75–0.85, depending on equipment and climate. With a 0.80 derate, a 7.5 kW array behaves like 6.0 kW of “ideal” energy delivery. Improving design, ventilation, and components can lower required kW and panel count.
Panels, roof area, and practical layout
Once kW is known, the calculator rounds up to whole panels. Using 400 W modules, a 7.40 kW requirement becomes 19 panels, or 7.60 kW DC. If each panel needs 1.9 m², that is about 36.1 m² of collector area before setbacks and walkways. The optional roof area check flags when a higher‑efficiency module may be needed.
Cost, incentives, and a fast payback view
Financial outputs combine installed $/W, incentives, and your utility rate. Gross cost equals DC kW × 1000 × $/W, then incentives reduce it to net cost. Year‑one savings use the targeted kWh offset multiplied by $/kWh. Simple payback is net cost divided by first‑year savings, while the 25‑year estimate applies annual rate escalation and subtracts O&M for a useful benefit view.
FAQs
1) What peak sun hours should I use?
Use an annual average for your roof tilt and orientation from a reputable solar resource map. If you only have city averages, choose a slightly conservative value to reflect shading and seasonal variation.
2) Why is my required kW higher than my neighbor’s?
Different usage, sun hours, roof shading, and loss assumptions change sizing. A lower derate factor or fewer sun hours will increase required kW even with the same monthly kWh.
3) Is DC size the same as inverter size?
Not always. Many systems pair a larger DC array with a smaller AC inverter rating. This tool reports DC array size based on panels, which installers may adjust based on inverter limits and clipping strategy.
4) How accurate is the roof area estimate?
It is a panel‑only surface estimate. Real layouts need setbacks, access pathways, ridge and hip spacing, and obstructions. Add a buffer and confirm with site measurements or an installer’s design software.
5) Do incentives apply to the full installed cost?
Incentives vary by program and may exclude portions of the project. The calculator treats incentives as a percent reduction of gross cost for quick planning. Confirm eligibility details with your local program rules.
6) Should I size for 100% offset?
It depends on policy limits, budget, and future load changes. If net‑metering credits are limited, sizing closer to daytime usage can be better. Consider planned EVs or heat pumps before choosing your target.