What size solar panel do I need to charge a 12V 100Ah battery?

A 100Ah 12V battery stores 1,200Wh. At 80% DoD, you need to replace 960Wh. With 5 peak sun hours and 15% losses, you need a panel that produces at least 960 / (5 x 0.85) = 226W. A single 250-300W panel handles this comfortably with a small buffer. A 200W panel would work but leave little margin for cloudy days or above-average usage.

Are 400W panels better than 200W panels for home solar?

For rooftop residential installations, 400W panels are almost always the better choice. They produce the same power in less space, require fewer mounting rails and cable connections, and cost less per watt. The only situation where 200W panels make more sense for a home is if you have small, fragmented roof sections where a 400W panel physically will not fit. Modern residential installations overwhelmingly use 380-450W panels.

How do I know if my solar panels are producing the right amount of power?

Compare actual daily production to calculated output. Multiply panel wattage by peak sun hours by (1 - loss percentage). If your 400W system should produce 1,700Wh and your monitoring shows 1,400Wh, you have higher losses than expected — check for shading, dirty panels, or charge controller issues. Most MPPT charge controllers and grid-tied inverters display daily kWh production. If you lack monitoring, a simple DC clamp meter on the panel output cables shows real-time current.

Solar Panel Size Calculator — Free | VoltCalcs

5 min read

Not sure whether you need a 200W panel or a 400W panel, two panels or ten? This calculator matches your energy requirements to specific panel sizes and counts. Enter your daily watt-hour usage, and the tool tells you how many panels of each common wattage you would need.

Solar Panel Wattage Guide by Application

Panel Wattage	Daily Output (5 sun hrs, 15% loss)	Best For	Typical Price (2026)
50-100W	210-425Wh	Trickle charging, gate openers, single device	$50-90
100-200W	425-850Wh	Boat electronics, RV supplement, camping	$80-180
200-300W	850-1,275Wh	Small cabin, van conversion, RV primary	$150-250
300-400W	1,275-1,700Wh	Large RV, small off-grid home, residential	$200-350
400-550W	1,700-2,340Wh	Residential rooftop, medium off-grid	$250-450

Prices are approximate for monocrystalline panels from major brands. Budget panels can cost 20-30% less but may have lower efficiency and shorter warranties.

Bigger Panels vs More Panels

Fewer large panels (400W+) make sense when roof or ground space is limited, you want fewer mounting points, and your charge controller supports higher-voltage strings. A single 400W panel produces as much as two 200W panels but takes up less total area (panels have dead space around the frame) and needs half the wiring connections. Installers prefer large panels because they reduce labour time.

More small panels (100-200W) make sense for irregular mounting surfaces (curved RV roofs, boat decks, odd-shaped ground areas), portable setups where you fold and move panels, and systems where partial shading is unavoidable. Smaller panels with individual MPPT inputs or microinverters handle shade better because one shaded panel does not drag down the others.

Efficiency also favours larger panels. Modern 400W residential panels achieve 20-22% efficiency, while budget 100W panels sit around 17-19%. Higher efficiency means more watts per square foot — which matters when space is your constraint.

Example: Right-Sizing Panels for a Van Conversion

A typical van conversion uses 1,500Wh per day (12V fridge, lights, fan, phone/laptop charging). The van travels the US Southwest with 5.5 average peak sun hours. With 20% system losses: 1,500 / (5.5 x 0.80) = 341W minimum.

Option A: Two 200W panels (400W total). Cost: ~$300-360. Fits on a standard Sprinter roof with room for a vent fan. Provides 17% buffer above minimum. Each panel is light enough for one person to lift onto the roof.

Option B: One 400W panel. Cost: ~$250-350. Uses less roof space but is heavier and needs a larger charge controller input. A single panel failure means zero solar until replaced.

Option C: Four 100W flexible panels. Cost: ~$320-400. Follows the van roof curve without a rack. Lighter total weight but lower efficiency and shorter lifespan (flexible panels degrade faster). Partial shading on one panel does not kill the array if each panel has its own MPPT channel.

For most van builds, Option A wins: two rigid 200W panels balance cost, reliability, and roof space.

Worked Examples

Choosing Panel Size for a Boat Deck Installation

Context

A sailboat owner needs 800Wh/day for navigation, autopilot, lights, and fridge. The boat sails between Florida and the Bahamas with 5.5 PSH average. Deck space is limited and curved, favoring smaller panels. System losses are 22% due to suboptimal panel angles on deck.

Calculation

Minimum array = 800 / (5.5 × 0.78) = 800 / 4.29 = 186.5W

With 25% buffer: 186.5 × 1.25 = 233W

Option: two 120W semi-flexible panels (240W total) or two 200W rigid panels (400W, but need flat mounting area)

Interpretation

Two 120W flexible panels (240W) barely exceeds the buffered minimum. Two 200W rigid panels (400W) provide a 72% margin, which is generous but handles cloudy days and panel degradation with ease. The rigid panels produce more reliably over time since flexible panels lose output faster.

Takeaway

For marine environments with salt spray and limited deck space, reliability per watt matters more than total wattage. After choosing panels, verify your battery bank handles overnight loads when panels produce nothing — the battery runtime calculator gives a clear answer.

Residential Grid-Tied System for Full Offset

Context

A family in Sacramento, CA uses 35kWh per day (central AC, EV charging, all-electric kitchen). Sacramento averages 5.2 PSH annually. System is grid-tied with 15% losses. Goal: offset 100% of annual grid consumption.

Calculation

Minimum array = 35,000 / (5.2 × 0.85) = 35,000 / 4.42 = 7,919W

With 20% buffer: 7,919 × 1.20 = 9,503W

Panel count (450W panels): 9,503 / 450 = 21.1 → 22 panels (9,900W system)

Interpretation

A 9.9kW system with 22 panels of 450W each. In summer with 7+ PSH, this system overproduces significantly and banks net metering credits. In winter with 3.5 PSH, it underproduces, and those credits offset the deficit. Annual production should closely match the 12,775 kWh/year target.

Takeaway

Grid-tied systems with net metering can use annual averages rather than worst-month sizing. To track whether your system hits its target, monitor monthly kWh with the kWh calculator and compare against your utility bill.

Frequently Asked Questions

Glossary

Panel Efficiency

The percentage of incident sunlight converted to electricity. A 22% efficient panel converts 220W of the 1,000W per square meter that hits it under standard test conditions. Higher efficiency means more watts per unit of area.

Flexible Panel

A thin, bendable solar panel that can conform to curved surfaces like RV and boat roofs. They weigh less and need no mounting rack, but typically have lower efficiency (15-18%) and degrade faster than rigid panels.

Net Metering

A billing arrangement where excess solar energy sent to the grid earns credits that offset future grid draws. Net metering makes annual energy balancing possible, since overproduction in summer covers underproduction in winter.

Want to compare output across different panel counts? Our solar panel output calculator lets you adjust wattage and count for instant results. Try it now →

Panel size is a balance between energy needs, physical space, and budget. Do not overbuy panels to compensate for undersized batteries, and do not skimp on panels hoping for sunny-day averages. Match your panel array to your worst-month sun hours and real daily usage, then pick the panel wattage that fits your mounting situation. The cheapest panel per watt that physically fits is usually the right answer.

Last updated: March 20, 2026

Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..

Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.