What is seasonal variance in solar production?

Seasonal variance is the percentage difference between your best production month and worst production month. Phoenix has ~33% variance (consistent year-round). Seattle has ~87% variance (dramatically more production in summer than winter). High variance matters most for net metering planning — you need annual true-up to bank summer surplus for winter.

Why does panel azimuth affect production?

South-facing panels receive peak sunlight throughout midday, maximizing daily production. East/west-facing panels lose about 15% annually but may align better with time-of-use rate schedules. West-facing panels can be more economically valuable in utilities that charge premium afternoon peak rates.

What tilt angle maximizes solar production?

Optimal tilt angle roughly equals your latitude. Los Angeles (34°N) produces most at ~34° tilt, Boston (42°N) at ~42°. Steeper tilts favor winter production; flatter tilts favor summer. Most roof-mounted systems are at fixed roof pitch and differ only 3-5% from optimal annual production.

Can I use this to size my battery storage?

Yes — your worst winter month's daily production determines the minimum daily solar charging available for batteries. If December produces 6 kWh/day in your city, that is the design-day input for off-grid battery sizing. Compare to your daily load to determine how many days of storage you need for winter autonomy.

Solar Production Calendar Planner

Select your city, system size, and panel orientation — get a month-by-month production calendar with daily kWh estimates, seasonal variance, and net metering surplus analysis.

System & location parameters

Location?

System size?

Panel tilt angle?

Panel azimuth (direction)?

Weather pattern?

Solar Production Calendar — Phoenix, AZ

18,566 kWh/year annual production

Peak Month

May (61.7 kWh/day)

Low Month

Dec (38.8 kWh/day)

Seasonal Variance

37%

Summer (Apr-Sep)

55% of annual

Tilt Efficiency

99%

Azimuth Factor

100%

Monthly Production Calendar

Jan

41.2 kWh/day

1,276 kWh total

5.2 PSH avg

Feb

46.7 kWh/day

1,308 kWh total

5.9 PSH avg

Mar

53.8 kWh/day

1,669 kWh total

6.8 PSH avg

Apr

59.4 kWh/day

1,781 kWh total

7.5 PSH avg

MayPEAK

61.7 kWh/day

1,914 kWh total

7.8 PSH avg

Jun

58.6 kWh/day

1,757 kWh total

7.4 PSH avg

Jul

52.2 kWh/day

1,620 kWh total

6.6 PSH avg

Aug

51.5 kWh/day

1,595 kWh total

6.5 PSH avg

Sep

53.8 kWh/day

1,615 kWh total

6.8 PSH avg

Oct

49.9 kWh/day

1,546 kWh total

6.3 PSH avg

Nov

42.7 kWh/day

1,282 kWh total

5.4 PSH avg

DecLOW

38.8 kWh/day

1,202 kWh total

4.9 PSH avg

SEASONAL ANALYSIS & NET METERING PLANNING

Summer Surplus (Apr-Sep)

10,283 kWh

55% of annual production

Winter Deficit (Oct-Mar)

8,284 kWh

45% of annual production

Moderate seasonal variance (37%). Net metering banking works well — summer surplus builds credits to offset winter shortfall.

How to Use This Calculator

Select your location and system parameters

Choose the city closest to your installation — the calculator uses location-specific monthly peak sun hours (PSH) data, which captures not just latitude effects but also the seasonal cloud cover and rain patterns of each region. Enter your system size in kW, panel tilt, and azimuth (compass direction). South-facing panels at your latitude angle maximize annual output; east or west deviations reduce annual production but shift when during the day you produce most power.

Choose your weather pattern

Weather pattern applies monthly adjustments beyond what PSH data alone captures — particularly for maritime climates (Seattle's persistent marine layer), tropical climates (Florida's hurricane season storm clouds June-October), and continental climates (New England's snow cover and winter cloud days). Desert climates receive no reduction as cloud cover is minimal year-round.

Read the monthly production calendar

Each month shows daily average kWh production, total monthly kWh, and average peak sun hours. Color intensity indicates relative production — green for peak months, amber for moderate, and grey for low months. The seasonal analysis section shows your summer surplus vs. winter deficit — critical for optimizing net metering in states with annual true-up policies.

The Formula

Daily kWh (month M) = System kW × Monthly PSH(M) × Tilt Factor × Azimuth Factor × Weather Factor(M) Monthly kWh = Daily kWh × Days in Month Annual kWh = ∑ Monthly kWh (all 12 months) Tilt Factor = MAX(0.85, 1 - |Tilt - Optimal Tilt| × 0.003) Optimal Tilt ≈ Latitude × 0.76 + 3.1 degrees Azimuth Factor: South = 1.0, SE/SW = 0.95, E/W = 0.85, NE/NW = 0.72, North = 0.55 Seasonal Variance = (Peak Day - Low Day) / Peak Day × 100%

Monthly PSH data for each city is derived from NREL's National Solar Radiation Database (NSRDB) and represents the long-term average solar irradiance in kilowatt-hours per square meter per day — equivalent to hours of peak (1000 W/m²) sunlight. Tilt and azimuth corrections use established solar geometry relationships. Weather pattern factors apply empirical reductions for cloud cover patterns that PSH data partially but not fully captures.

Example

Seattle vs Phoenix — Dramatic vs Consistent Seasonal Patterns

An 8 kW south-facing system at 40° tilt shows strikingly different seasonal profiles in Seattle (maritime) and Phoenix (desert).

Seattle January (worst)~6 kWh/day

Seattle July (peak)~47 kWh/day

Seattle seasonal variance~87%

Phoenix January~38 kWh/day

Phoenix June (peak)~57 kWh/day

Phoenix seasonal variance~33%

Net Metering Implications

Seattle summer surplus (Apr-Sep)~72% of annual production

Seattle winter deficit~28% — needs grid import Nov-Feb

Phoenix summer surplus~59% — more balanced

Annual true-up recommendationCritical for Seattle; nice-to-have for Phoenix

Seattle homeowners need annual true-up net metering (not monthly netting) to avoid losing summer surplus credits before winter deficit arrives. If your utility only offers monthly netting, summer over-production beyond your usage is often credited at low avoided-cost rates (~$0.03/kWh) instead of retail rates ($0.11/kWh) — a significant revenue loss in high-variance locations.

FAQ

What is the difference between this and a basic solar production calculator? ▼

A basic solar production calculator gives you an annual kWh estimate. This calendar planner gives you a month-by-month breakdown with daily averages — showing exactly how much you produce in your worst month (December in New England) versus your best (June in most states). This granularity is essential for net metering planning, battery sizing decisions (how many days of winter autonomy do you need?), HVAC pre-cooling decisions, and EV charging scheduling. It also shows seasonal variance — the ratio between peak and trough production months — which determines how much you rely on net metering banking vs. physical storage.

Why does panel azimuth matter so much? ▼

South-facing panels receive direct sunlight throughout the midday peak, maximizing daily and annual production. East-facing panels peak in the morning; west-facing panels peak in the late afternoon. While east and west each lose about 15% of annual production compared to south, they may align better with your time-of-use rate schedule — some utilities charge 3-5× more for afternoon peak power (3-8pm), making west-facing panels more economically valuable even with lower annual kWh output. Split east-west arrays are increasingly popular for homes with EV charging and time-of-use rates.

What is annual true-up net metering and do I need it? ▼

Annual true-up (used by PG&E in California and some other utilities) means your net metering credits accumulate all year and are reconciled once annually, typically in April or October. This lets summer surplus credits flow into winter months to offset your heating/lighting demand when solar production is low. Monthly netting means credits reset each month — if you over-produce in July, the excess doesn't help you in January. Annual true-up is far more valuable in high-variance climates (Seattle, Boston, Minneapolis) and less critical in low-variance climates (Phoenix, Honolulu).

What tilt angle should I use for my roof? ▼

Use your actual roof pitch — most installers mount panels flat to the roof rather than on adjustable racking. A standard 4/12 pitch is about 18°, 6/12 is about 26°, and 8/12 is about 34°. The optimal tilt angle roughly equals your latitude (Los Angeles = 34°, Denver = 40°, Boston = 42°). Slightly steeper tilts (5-10° beyond optimal) favor winter production — useful in net metering states where you want to minimize winter grid imports. Flat tilts (5-10°) favor summer production. For most homeowners, the difference between an 18° and 35° tilt in the same direction is less than 5% in annual production.

Should I use this data to size my battery storage? ▼

Yes — your worst winter month's daily production tells you the minimum daily solar input available for battery charging. If Seattle produces 6 kWh/day in January, your battery would only receive ~6 kWh of solar charging on an average January day. If your home uses 30 kWh/day, solar alone won't keep a battery charged through multi-day storms in winter — you'd want a backup generator or grid-tied system. For off-grid sizing, divide your worst winter monthly production by 30 days and compare to your daily load — this gives you your design-day scenario, which determines battery bank capacity and backup generation requirements.

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