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Solar System Sizing Cheat Sheet

The three core sizing formulas plus peak sun hours by region. Use these to estimate panel count, battery bank and inverter size before consulting a professional installer.

Formula 1: Daily panel output

kWh/day = panels × Wp × PSH × efficiency ÷ 1000

Example: 10 panels × 400 Wp × 4.0 PSH × 0.80 efficiency ÷ 1000 = 12.8 kWh/day
Variables: Wp = panel wattage at STC (from datasheet) · PSH = peak sun hours for your location (see table below) · Efficiency = 0.75–0.85 for a typical system (accounts for inverter loss, wiring, soiling, temperature derating)

Formula 2: Battery bank capacity

Ah = (daily kWh × autonomy days) ÷ (voltage × DoD)

Example: 5 kWh/day × 2 days ÷ (48 V × 0.80 DoD) = 260 Ah at 48 V (= 12.5 kWh nominal)
Variables: Autonomy days = days the battery must power loads without sun · Voltage = system voltage (12, 24 or 48 V) · DoD = depth of discharge (see table below)

Formula 3: Inverter minimum size

VA = peak AC load (W) ÷ power factor

Example: 2,000 W peak load ÷ 0.80 power factor = 2,500 VA minimum inverter rating
Variables: Peak AC load = sum of all appliances that might run simultaneously · Power factor = 0.8 for general loads; 1.0 for resistive loads (heaters, lights) only

Depth of discharge by battery chemistry

Chemistry	Safe DoD	Notes
Flooded lead-acid	50%	Never go below 50% or cycle life drops sharply
AGM / sealed lead-acid	50–60%	Slightly more forgiving than flooded but same caution
LiFePO4	80–90%	Best value for off-grid; long cycle life even at 90% DoD
Lithium-ion (NMC)	80%	Good cycle life at 80%; reduces risk of cell stress

DoD = depth of discharge. LiFePO4 offers the best combination of cycle life, safe DoD and thermal stability for solar storage. Lead-acid is lower upfront cost but heavier and requires ventilation.

System efficiency reference

Loss factor	Typical %
Inverter efficiency	94–97%
Battery round-trip efficiency	90–98%
Wiring and connection losses	1–3%
Temperature derating (hot climate)	5–15%
Soiling and shading (typical)	2–5%
Combined system efficiency	75–85%

Use 0.75 for conservative off-grid sizing; 0.80–0.85 for grid-tied systems with modern inverters. Real-time monitoring tools (SolarEdge, Enphase) will show your actual efficiency over time.

Peak sun hours (PSH) by region

Annual average PSH on a south-facing roof at optimal tilt. For off-grid autonomy sizing, use the winter low. For grid-tied annual production estimates, use the annual average. Source: PVGIS (Europe) and NREL (United States).
Region	Annual avg	Winter low	Summer high
Europe
UK, Ireland, Benelux, North Germany	2.7	0.8	4.8
Central Europe (DE, PL, CZ, AT)	3.1	1.1	5.4
France (central), Northern Italy	3.5	1.5	5.8
Southern France, Northern Spain	4.2	2.0	6.3
Spain (central), Portugal, Italy (central)	4.8	2.5	6.8
Southern Spain, Sicily, Greece	5.2	3.0	7.0
United States
Pacific Northwest (Seattle, Portland)	3.5	1.0	6.0
Northeast (Boston, NYC, Philadelphia)	4.2	2.0	5.8
Midwest (Chicago, Minneapolis)	4.0	1.8	6.0
Southeast (Atlanta, Charlotte)	4.6	2.8	5.9
Texas (Dallas, Houston)	5.0	3.2	6.2
Mountain West (Denver, Salt Lake)	5.5	3.5	7.0
Southwest (Phoenix, Las Vegas)	6.5	4.5	7.5
California (LA, San Diego, Bay Area)	5.5	3.5	7.0

Grid-tied vs off-grid sizing rule. For grid-tied systems with net metering, size to the annual average PSH: excess summer production offsets winter shortfall through the grid. For off-grid or battery-backed systems, size to the winter low: that is the worst-case daily production the battery must bridge. Sizing an off-grid system to the annual average almost guarantees winter blackouts.

CovertItAll.com · Solar sizing formulas. PSH data calibrated to PVGIS (EU) and NREL (US). These are estimates; commission a professional energy audit before purchasing equipment. covertitall.com/printables/solar-sizing.html Solar Hub →