Solar basics

A 400-watt solar panel on your roof is rated under conditions that rarely exist: 77°F (25°C) ambient air, perfect irradiance of 1,000 W/m², and no wind. Real panels on real roofs in real weather produce 10–30% less. Understanding the gap between rated output and actual production is the difference between a solar system that works and one that leaves you without power on the third cloudy day of an outage.

How photovoltaic works

Photovoltaic (PV) cells are made from semiconductor material — almost always silicon. When photons from sunlight strike the silicon, they knock electrons loose, creating a direct current (DC) flow. A single cell produces less than one volt; manufacturers connect cells in series to build panels rated at 12V, 24V, or 48V nominal systems.

Modern residential and off-grid panels are built from either monocrystalline or polycrystalline silicon cells. Thin-film panels use a different deposition process on glass or flexible substrates. Each type has a distinct efficiency-to-cost tradeoff.

One technical note about temperature: panels are rated at 25°C (77°F), but panels mounted on a rooftop typically run 20–40°C hotter than ambient air. At 60°C (140°F) cell temperature, a typical crystalline panel loses 10–15% of its rated output. This is why hot, sunny days do not produce maximum solar output — the irradiance is high but the cells are hot. Temperature coefficient for standard mono panels runs approximately -0.35% to -0.50% per °C above 25°C (77°F).

Panel type comparison

Three cell technologies dominate the market. The right choice depends on your available space, budget, and installation context.

Type	Efficiency	Annual degradation	Best for	Notes
Monocrystalline	20–23%	~0.3–0.5%/yr	Roof-mounted, space-limited systems	Highest efficiency, highest cost
Polycrystalline	15–17%	~0.5–0.8%/yr	Ground-mounted, budget systems	Lower cost, slightly lower output per sq ft
Thin-film (CdTe, CIGS)	10–13%	~1%/yr	Curved surfaces, flexible installs	Lowest cost per watt, largest footprint

Monocrystalline panels use silicon grown from a single crystal, which allows electrons to move freely with minimal resistance. A 60-cell mono panel measuring roughly 3.25 ft × 5.5 ft (1 m × 1.7 m) can deliver 350–400 W under STC. These dominate serious preparedness and off-grid installations because their higher efficiency per square foot means fewer panels to achieve target output.

Polycrystalline panels are manufactured from multiple silicon fragments melted together. The resulting grain boundaries create slightly more electron resistance, dropping efficiency to 15–17%. The manufacturing process is simpler and cheaper. For ground-mounted arrays where space is not a constraint, polycrystalline is a cost-effective choice.

Thin-film panels deposit semiconductor material directly onto glass or flexible substrates. They are cheap per panel and perform slightly better in diffuse or indirect light — useful in overcast climates. The efficiency penalty (10–13%) means a thin-film array requires roughly twice the area of a mono array for the same output. Thin-film degrades faster than crystalline silicon and typically carries shorter warranties.

Field note

Monocrystalline panels have dropped dramatically in price over the past decade. The price premium over polycrystalline is now small enough that monocrystalline is the default choice for most new installations. Unless you are specifically weight-constrained (flexible thin-film on a van or boat) or have abundant ground space with a tight budget, buy mono.

Reading a panel spec sheet

Every panel ships with a specification sheet listing at minimum six electrical parameters. These are always measured at STC (Standard Test Conditions): 1,000 W/m² irradiance, 25°C (77°F) cell temperature, and 1.5 air mass spectrum.

Parameter	Abbreviation	Meaning	How it's used
Open-circuit voltage	Voc	Voltage with no load connected — the highest voltage the panel will produce	Sets the maximum voltage limit for your charge controller or inverter input
Maximum power voltage	Vmp	Voltage at peak output power	The operating voltage you design around
Short-circuit current	Isc	Current when terminals are shorted — maximum current	Sets fuse sizing and wire current rating
Maximum power current	Imp	Current at peak output power	Imp × Vmp = rated watt output
Power at STC	Pmax	Rated watts under standard conditions	The headline number on the panel
Temperature coefficient	Pmax/°C	Power change per degree above 25°C	Used to calculate real output at operating temperature

A practical example: a 400W monocrystalline panel might list Voc = 49.5V, Vmp = 41.2V, Isc = 10.1A, Imp = 9.7A, and a temperature coefficient of -0.35%/°C. At peak summer cell temperatures of 65°C (149°F) — 40°C above STC — the panel loses 40 × 0.35% = 14% of rated output, dropping effective output to roughly 344W.

STC vs. real-world output: PTC (PVUSA Test Conditions) ratings test at 68°F (20°C) ambient, 1,000 W/m², and 2.2 mph (1 m/s) wind, and are 10–15% lower than STC ratings. For production estimates, PTC is the more honest number. When calculating what a system will actually generate, derate your STC watts by 10–20% before applying peak sun hours.

Don't plan from the STC number

Every solar array in the field produces less than its STC nameplate rating. Account for temperature losses (10–15%), dirt and soiling (2–5%), wiring losses (2–3%), and inverter or controller losses (3–5%). A 1,000W array under real-world conditions typically delivers 750–850W on a clear summer day.

Peak sun hours by region

Peak sun hours (PSH) are not the same as hours of daylight. One peak sun hour equals 1,000 W/m² of solar irradiance received over one hour. On a day with variable clouds, you might have 10 hours of daylight but only 4 peak sun hours. PSH is the design variable; you multiply your array wattage by daily PSH to get daily Wh output.

US regional averages (annual daily average):

Region	PSH range	Example locations
Southwest desert	5.5–6.5 h/day	Phoenix AZ, Las Vegas NV, Albuquerque NM
Southern states	4.5–5.5 h/day	Dallas TX, Atlanta GA, Los Angeles CA
Mid-Atlantic / Midwest	4.0–4.5 h/day	Denver CO, Chicago IL, Washington DC
Pacific Northwest / Northeast	3.0–4.0 h/day	Seattle WA, Portland OR, Buffalo NY
Alaska / Great Lakes	2.5–3.5 h/day	Anchorage AK, Cleveland OH

Critical rule: design from worst-month PSH, not annual average. In the Northeast, December averages roughly 2.0–2.5 PSH/day — half of the annual average. A system sized on annual average will be undersized when you need it most.

For any fixed location, NREL's PVWatts calculator provides monthly PSH estimates at no cost. Input your ZIP code, array tilt, and orientation to get hourly production estimates for every month.

System component overview

A complete solar energy system has four functional layers. Each layer can be sized and upgraded independently.

Panels — the generation layer. DC power flows from panels wired in series-parallel combinations to match the charge controller or inverter input voltage window. Series wiring adds voltage; parallel wiring adds current. A 48V system typically uses strings of panels wired in series to reach 60–80V at Vmp.

Charge controller — the regulation layer. The charge controller sits between panels and batteries and manages the three-stage charge cycle (bulk → absorption → float) to protect battery chemistry. Two types exist:

MPPT (Maximum Power Point Tracking): Continuously adjusts the operating voltage to extract maximum power from panels, then steps down to battery voltage. MPPT controllers extract 20–30% more energy from an array than pulse width modulation (PWM), particularly in cold weather when panel Vmp is high. Required for any system above 200W or where panels are wired in series.
PWM (Pulse Width Modulation): Connects panels directly to battery at battery voltage, wasting any voltage above battery voltage. Inexpensive and adequate only for very small, matched-voltage systems (under 100W, 12V).

For the battery bank, MPPT is the correct default for any system intended to carry critical loads.

Battery bank — the storage layer. Batteries store energy for use after dark and during low-production periods. Capacity is measured in kilowatt-hours (kWh) or amp-hours (Ah) at a given voltage. See batteries for chemistry selection, sizing methodology, and maintenance schedules.

Inverter — the conversion layer. The inverter converts DC battery power to AC (120V or 240V) for household appliances. Pure sine wave inverters are required for sensitive electronics, motors, and variable-speed devices. Modified sine wave inverters work for resistive loads (light bulbs, basic heating elements) but can damage or reduce efficiency in many modern appliances. The inverters page covers sizing and waveform selection in detail.

Balance of system (BOS): Wiring, fuses, disconnects, mounting hardware, and monitoring equipment. Wiring losses directly reduce system output; undersized wires create heat and fire risk. Size all DC wiring for 125% of maximum expected current. Install fuses or breakers within 18 inches (46 cm) of every battery terminal and within 18 inches of the first series connection from panels.

Mounting options

Roof-mounted arrays are the most common for residential systems. Fixed-tilt roof mounts are simple and durable. Optimal tilt angle equals your latitude for maximum annual production; in cold climates, steeper tilts (latitude +15°) improve winter output. South-facing (in the Northern Hemisphere) maximizes daily production; within 30° of south is acceptable with minor output penalty.

Ground-mounted arrays offer flexibility: you can tilt them optimally, orient them precisely, and add tracking if the economics justify it. Ground mounts require concrete anchors and conduit runs to the battery or inverter location. Minimum post depth in most soils is 3 feet (0.9 m); cold climates require posts set below frost depth.

Adjustable tilt ground or pole mounts let you change angle seasonally — steeper in winter to capture lower sun angles, shallower in summer. On a system with marginal winter capacity, seasonal adjustment can increase December output by 10–20%.

Field note

For a grid-down preparedness system, portable ground-mounted panels on simple A-frame stands are more practical than roof-mounted fixed arrays. They survive roof repairs, can be repositioned to avoid shade, and can be temporarily relocated if you evacuate. The efficiency penalty for non-optimal pointing is far less costly than losing access to your array entirely.

Sizing vocabulary

Before diving into a full system design, confirm you understand these terms:

Wh/day: Watt-hours consumed or generated per day — your load audit output and your production estimate are both in Wh/day
Autonomy: The number of days the battery bank can power critical loads with zero solar input
Derate factor: A multiplier (typically 0.75–0.85) applied to nameplate array watts to estimate real-world production
System efficiency: Combined losses from controller, wiring, battery, and inverter — typically 80–88% for a well-designed system
Array-to-load ratio: Ratio of daily solar production to daily load; a ratio of 1.3–1.5 is common to allow battery charging during use

The full sizing workflow — from load audit to panel count — is covered in DIY solar installation. For complete off-grid system architecture decisions, see off-grid solar systems.

Solar basics checklist

Understanding the relationship between panel ratings, peak sun hours, and system losses prepares you to move into actual system sizing. The DIY solar installation page walks through a complete load-to-panel worked example, and batteries covers how to match storage capacity to your production numbers.