Monocrystalline 400W Solar Calculator — 900 kWh/month in California

15 × 400W panels needed for 900 kWh/month in California. 5.9 kW system, net cost $12,390 after IRA 30% ITC. Payback: 3.7 years.

Usage & location

Monthly usage (kWh)

Find on your electricity bill

State

Panel & battery

Panel type

Battery storage (kWh)

Panels needed

5.9 kW

System size

$12,390

Net cost (after ITC)

3.7 yrs

Payback period

100% offset$3,321/yr savings5.8 sun hrs/day9,340 lbs CO₂/yr offset

Monthly usage	900 kWh
Annual usage	10,800 kWh
Peak sun hrs/day	5.8 hrs
System size	5.93 kW-DC
Panels needed	15
Panel area total	323 sq ft
Annual production	10,924 kWh
Offset	100%
CO₂ offset	9,340 lbs/yr
Equivalent trees	195 trees/yr

Monocrystalline 400W — 900 kWh/month in CA

A 5.9 kW solar system using 15 × 400W panels will produce ~10,924 kWh/year in CA, covering 100% of your 10,800 kWh annual usage.

Net cost after the 30% federal ITC: $10,500–$14,700. Estimated payback: 3.2–4.4 years.

System sizing result

5.9 kW system

15 panels (5.9 kW)

100% of your usage offset by solar.

Net cost after IRA 30% tax credit

Net installed cost (mid)

$10,500 – $14,700

Gross: $17,700. ITC savings: –$5,310.

System sizing breakdown

Based on NREL PVWatts v8 peak sun hours and 0.86 system derate.

Parameter	Value	Note
Monthly usage	900 kWh	Input
Annual usage	10,800 kWh	× 12
Peak sun hrs/day	5.8 hrs	NREL PVWatts v8, CA
System derate	0.86	Wiring, inverter, soiling
System size needed	5.93 kW-DC	Before rounding
Panels needed	15	400W each
Panel area	323 sq ft	15 × 21.5 sq ft
Annual production	10,924 kWh	100% offset

Cost estimate (LBNL 2023 benchmarks)

Source: LBNL Tracking the Sun 2023 / NREL Cost Benchmarks 2023. IRA 30% ITC applied.

Category	Low	Mid	High
Gross system cost	$15,000	$17,700	$21,000
ITC (30%)	–$4,500	–$5,310	–$6,300
Net cost	$10,500	$12,390	$14,700

Solar sizing formula

\text{panels} = \text{ceil}((\text{monthlyKwh} × 12) / (\text{sunHrs} × 365 × 0.86) / (\text{panelWatts}/1000))

Step-by-step derivation

Monthly usage: 900 kWh → Annual: 10,800 kWh

California peak sun hours: 5.8 hrs/day × 365 days = 2117 hrs/yr

System derate factor: 0.86 (wiring, inverter, soiling, mismatch)

System size needed: 10,800 ÷ (5.8 × 365 × 0.86) = 5.93 kW-DC

Panels needed: ⌈5.93 kW ÷ 0.400 kW/panel⌉ = 15 panels (Monocrystalline 400W)

Actual system size: 15 × 400W = 6.00 kW-DC

Estimated annual production: 6.00 × 5.8 × 365 × 0.86 = 10,924 kWh (100% offset)

Gross cost (mid): 15 panels × 6.00 kW × $2.95/W = $17,700

IRA federal ITC (30%): –$5,310 → Net cost: $12,390

Annual savings: 10,924 kWh × $0.304/kWh = $3,321/yr

Payback period: $12,390 ÷ $3,321/yr = 3.7 years

Environmental impact

CO₂ factor: EPA eGRID 0.855 lbs/kWh. Tree equivalency: ~48 lbs CO₂/tree/yr.

Metric	Per Year
CO₂ offset	9,340 lbs
Equivalent trees	195 trees
Clean energy	10,924 kWh

Complete system sizing

System sized to 100% offset. Actual production varies with shading and orientation.

Parameter	Value	Note
Monthly usage	900 kWh	Your input
Annual usage	10,800 kWh	× 12
Peak sun hrs/day	5.8 hrs	NREL PVWatts, CA
System size	5.93 kW-DC	-
Panels needed	15	400W Monocrystalline 400W
Panel area total	323 sq ft	Roof space required
Annual production	10,924 kWh	100% offset

Cost & payback (IRA 30% ITC applied)

LBNL 2023 / NREL benchmarks. State incentives not included (check DSIRE.org).

Category	Low	Mid	High
Gross cost	$15,000	$17,700	$21,000
ITC (30%)	–$4,500	–$5,310	–$6,300
Net cost	$10,500	$12,390	$14,700
Annual savings	$3,321	$3,321	$3,321
Payback	3.2 yrs	3.7 yrs	4.4 yrs
25-yr net savings	$72,520	$70,630	$68,320

CA vs west region — peak sun hours

NREL PVWatts v8 annual average. West region states shown.

Compare all panel types for this system

All options sized to same 900 kWh/month usage in CA.

Panel	Panels	Area (sq ft)	Net cost (mid)	Payback (mid)
Monocrystalline 400W	15	323	$12,390	3.7 yrs
Monocrystalline 450W (High-Efficiency)	14	330	$14,112	4 yrs
Polycrystalline 350W	17	359	$10,829	3.3 yrs
Bifacial Monocrystalline 420W	15	347	$14,994	4.3 yrs
Thin-Film CIGS	40	388	$10,500	3.2 yrs

Environmental impact

EPA eGRID 0.855 lbs CO₂/kWh average.

Metric	Per Year	Over 25 Years
CO₂ offset	9,340 lbs	233,500 lbs
Equivalent trees	195 trees	4,875 trees
Clean energy	10,924 kWh	273,093 kWh

Sources & Standards

Dobos, A. P. "PVWatts Version 5 Manual." National Renewable Energy Laboratory, NREL/TP-6A20-62641, 2014.PV system output estimation methodology (irradiance, system losses, tilt and azimuth)
National Renewable Energy Laboratory. "National Solar Radiation Database (NSRDB)."Solar irradiance data underlying production estimates

Monocrystalline 400W Solar Calculator — 900 kWh/month in California

15 × 400W panels needed for 900 kWh/month in California. 5.9 kW system, net cost $12,390 after IRA 30% ITC. Payback: 3.7 years.

Usage & location

Monthly usage (kWh)

Find on your electricity bill

State

Panel & battery

Panel type

Battery storage (kWh)

Panels needed

5.9 kW

System size

$12,390

Net cost (after ITC)

3.7 yrs

Payback period

100% offset$3,321/yr savings5.8 sun hrs/day9,340 lbs CO₂/yr offset

Monthly usage	900 kWh
Annual usage	10,800 kWh
Peak sun hrs/day	5.8 hrs
System size	5.93 kW-DC
Panels needed	15
Panel area total	323 sq ft
Annual production	10,924 kWh
Offset	100%
CO₂ offset	9,340 lbs/yr
Equivalent trees	195 trees/yr

Monocrystalline 400W — 900 kWh/month in CA

A 5.9 kW solar system using 15 × 400W panels will produce ~10,924 kWh/year in CA, covering 100% of your 10,800 kWh annual usage.

Net cost after the 30% federal ITC: $10,500–$14,700. Estimated payback: 3.2–4.4 years.

System sizing result

5.9 kW system

15 panels (5.9 kW)

100% of your usage offset by solar.

Net cost after IRA 30% tax credit

Net installed cost (mid)

$10,500 – $14,700

Gross: $17,700. ITC savings: –$5,310.

System sizing breakdown

Based on NREL PVWatts v8 peak sun hours and 0.86 system derate.

Parameter	Value	Note
Monthly usage	900 kWh	Input
Annual usage	10,800 kWh	× 12
Peak sun hrs/day	5.8 hrs	NREL PVWatts v8, CA
System derate	0.86	Wiring, inverter, soiling
System size needed	5.93 kW-DC	Before rounding
Panels needed	15	400W each
Panel area	323 sq ft	15 × 21.5 sq ft
Annual production	10,924 kWh	100% offset

Cost estimate (LBNL 2023 benchmarks)

Source: LBNL Tracking the Sun 2023 / NREL Cost Benchmarks 2023. IRA 30% ITC applied.

Category	Low	Mid	High
Gross system cost	$15,000	$17,700	$21,000
ITC (30%)	–$4,500	–$5,310	–$6,300
Net cost	$10,500	$12,390	$14,700

Solar sizing formula

\text{panels} = \text{ceil}((\text{monthlyKwh} × 12) / (\text{sunHrs} × 365 × 0.86) / (\text{panelWatts}/1000))

Step-by-step derivation

Monthly usage: 900 kWh → Annual: 10,800 kWh

California peak sun hours: 5.8 hrs/day × 365 days = 2117 hrs/yr

System derate factor: 0.86 (wiring, inverter, soiling, mismatch)

System size needed: 10,800 ÷ (5.8 × 365 × 0.86) = 5.93 kW-DC

Panels needed: ⌈5.93 kW ÷ 0.400 kW/panel⌉ = 15 panels (Monocrystalline 400W)

Actual system size: 15 × 400W = 6.00 kW-DC

Estimated annual production: 6.00 × 5.8 × 365 × 0.86 = 10,924 kWh (100% offset)

Gross cost (mid): 15 panels × 6.00 kW × $2.95/W = $17,700

IRA federal ITC (30%): –$5,310 → Net cost: $12,390

Annual savings: 10,924 kWh × $0.304/kWh = $3,321/yr

Payback period: $12,390 ÷ $3,321/yr = 3.7 years

Environmental impact

CO₂ factor: EPA eGRID 0.855 lbs/kWh. Tree equivalency: ~48 lbs CO₂/tree/yr.

Metric	Per Year
CO₂ offset	9,340 lbs
Equivalent trees	195 trees
Clean energy	10,924 kWh

Complete system sizing

System sized to 100% offset. Actual production varies with shading and orientation.

Parameter	Value	Note
Monthly usage	900 kWh	Your input
Annual usage	10,800 kWh	× 12
Peak sun hrs/day	5.8 hrs	NREL PVWatts, CA
System size	5.93 kW-DC	-
Panels needed	15	400W Monocrystalline 400W
Panel area total	323 sq ft	Roof space required
Annual production	10,924 kWh	100% offset

Cost & payback (IRA 30% ITC applied)

LBNL 2023 / NREL benchmarks. State incentives not included (check DSIRE.org).

Category	Low	Mid	High
Gross cost	$15,000	$17,700	$21,000
ITC (30%)	–$4,500	–$5,310	–$6,300
Net cost	$10,500	$12,390	$14,700
Annual savings	$3,321	$3,321	$3,321
Payback	3.2 yrs	3.7 yrs	4.4 yrs
25-yr net savings	$72,520	$70,630	$68,320

CA vs west region — peak sun hours

NREL PVWatts v8 annual average. West region states shown.

Compare all panel types for this system

All options sized to same 900 kWh/month usage in CA.

Panel	Panels	Area (sq ft)	Net cost (mid)	Payback (mid)
Monocrystalline 400W	15	323	$12,390	3.7 yrs
Monocrystalline 450W (High-Efficiency)	14	330	$14,112	4 yrs
Polycrystalline 350W	17	359	$10,829	3.3 yrs
Bifacial Monocrystalline 420W	15	347	$14,994	4.3 yrs
Thin-Film CIGS	40	388	$10,500	3.2 yrs

Environmental impact

EPA eGRID 0.855 lbs CO₂/kWh average.

Metric	Per Year	Over 25 Years
CO₂ offset	9,340 lbs	233,500 lbs
Equivalent trees	195 trees	4,875 trees
Clean energy	10,924 kWh	273,093 kWh

Sources & Standards

Dobos, A. P. "PVWatts Version 5 Manual." National Renewable Energy Laboratory, NREL/TP-6A20-62641, 2014.PV system output estimation methodology (irradiance, system losses, tilt and azimuth)
National Renewable Energy Laboratory. "National Solar Radiation Database (NSRDB)."Solar irradiance data underlying production estimates