finance calculator

Solar Offset Calculator

Estimate monthly kWh a solar array can produce and how much of your bill it could offset.

Results

Estimated system watts: 6000.00
Daily production (kWh): 27.00
Monthly production (kWh): 810.00
Offset vs usage: 90.00%

How to use this calculator

Enter your monthly usage in kWh.
Enter sun hours, system size in kW (or panel count × wattage), and system efficiency.
Review daily/monthly production and your offset percentage.

Inputs explained

Monthly usage (kWh): Your average monthly electricity consumption from your bill.
Sun hours per day: Average peak sun hours at your location (e.g., 4–6+).
System size (kW): Nameplate DC size; leave blank and use panel count if unknown.
Panel count/wattage: Number of panels and wattage per panel to estimate system size if kW is blank.
System efficiency (%): Losses from inverter, wiring, temperature; typical 85–92%.

How it works

We estimate system watts from either a kW entry or panel count × panel wattage.

Daily kWh = watts × sun hours × efficiency ÷ 1000; monthly assumes 30 days.

Offset compares monthly production to your monthly usage.

Formula

System watts = (kW × 1000) or (Panels × Panel W)
Daily kWh = Watts × Sun hours × Eff% ÷ 1000
Monthly kWh = Daily × 30
Offset = Monthly production ÷ Monthly usage

When to use it

Estimating how much of your bill a proposed solar array could offset.
Comparing system sizes or panel wattages for the same roof area.
Checking offset impact of adding a few more panels later.

Tips & cautions

Use realistic sun hours for your location; PVWatts or local installers can provide estimates.
Higher temperatures reduce output; efficiency helps approximate real-world losses.
Round system size to available inverter/array sizes; roof layout can limit panel count.
Does not model azimuth/tilt/shading or seasonal variation; this is a rough estimate.
Assumes constant monthly usage; real bills vary with seasons and rate changes.

Worked examples

6 kW system, 5 sun hours, 90% efficiency, 900 kWh usage

Monthly production ≈ 810 kWh
Offset ≈ 90%

16 panels at 400 W, 5.5 sun hours, 92% efficiency, 1,000 kWh usage

System ≈ 6,400 W
Monthly production ≈ 972 kWh
Offset ≈ 97%

Deep dive

This solar offset calculator estimates how many kWh your array can produce each month and what percentage of your bill it could cover. Enter usage, sun hours, efficiency, and either system kW or panel count to size your system quickly.

Use it early in the solar shopping process to compare offsets, panel wattages, and add-on panels. For a full design, factor in tilt, azimuth, shading, and seasonal production with a detailed tool like PVWatts.

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Rough estimate only. Actual output varies with orientation, shading, temperature, and degradation. Not financial advice.