Solar Panel Array Size Calculator

Work the math in three steps — the calculator does this automatically, but it helps to understand what’s happening under the hood:

• Daily energy — add up the watt-hours of everything you’ll run in 24 hours (the Daily Consumption Calculator handles this).
• Peak sun hours — the equivalent hours per day your location receives at full 1000 W/m² intensity. Typical ranges: 3.5 in the Pacific Northwest, 5.5 in the Southeast, 6.5+ in the Southwest.
• System efficiency — the fraction that survives wiring loss, inverter loss, battery round-trip, temperature, and soiling. Realistic values land between 0.65 and 0.80.

A household using 6,000 Wh/day with 4.5 sun hours at 75% efficiency needs about 1,780 W of panels. Bump that by 15-25% if you want comfortable winter performance.

Divide the total array wattage by the wattage of each panel, then round up. The trick is choosing panel wattage that matches your goals:

• 100-200 W panels — portable, easy to mount, good for RVs and small cabins. More connections = more failure points.
• 350-450 W residential panels — the sweet spot for most rooftop and ground-mount builds in 2026.
• 500-600 W commercial panels — fewer units to wire but heavy, hard to handle solo, and often incompatible with smaller charge controllers.

Example: a 2,000 W array needs twenty 100 W panels, or six 350 W panels, or four 500 W panels. Same output — very different install experience and cost.

Very different beasts, because off-grid has to survive the worst week of the year while grid-tied can lean on the utility.

• Tiny cabin / weekend use — 400-1,200 W. Lights, phones, small fridge, a laptop.
• Full-time off-grid cabin (efficient) — 2,000-4,500 W. Full kitchen, well pump, electronics, propane for heat/hot water.
• Off-grid home with heat pump / AC — 8,000-14,000 W plus serious battery storage.
• Grid-tied residential — 5,000-11,000 W is the common range, sized to offset the utility bill rather than stand alone.

If you’re off-grid, always size to your worst-month sun hours, not the annual average. A system that’s perfect in July will be starved in December.

Total array watts stay the same — but panel wattage affects nearly everything else:

• Roof/ground space — higher wattage per square meter means less footprint.
• Mounting hardware — fewer panels = fewer rails, clamps, and penetrations.
• Wiring runs — fewer strings to manage, but higher string voltages.
• Charge controller compatibility — high-wattage panels can exceed the Voc ceiling of smaller MPPTs. Always check Voc at the coldest temperature you’ll see.
• Redundancy — if a 500 W panel in a 4-panel array fails, you lose 25% of production. Spreading the same wattage over 12 smaller panels drops that to ~8%.

Undersizing fails gradually — and usually in a season you didn’t plan for. The failure modes in order of how you’ll notice them:

• Batteries don’t fully recharge — they sit at partial state-of-charge day after day, sulfating (lead-acid) or losing usable capacity.
• Generator runtime creeps up — you go from “once a month” to “most evenings” in November.
• Depth of discharge goes too deep — you start pulling past the safe DoD and shortening battery lifespan dramatically.
• Seasonal shutdowns — in short-winter-day climates, an undersized system simply can’t keep up and you either add panels, add a generator, or cut loads.

The fix is almost always adding panels, not batteries. Extra panels help you recover from cloudy stretches; extra batteries just give you more capacity to drain when the sun doesn’t come back.

For off-grid systems, yes — almost always. A 20-30% oversize is the single cheapest insurance policy in solar design. It means:

• You recover battery state-of-charge by mid-afternoon instead of sunset, leaving margin for a cloudy tomorrow.
• Winter performance stays usable without emergency generator runs.
• Panel degradation (0.4-0.8% per year) barely matters — the oversize absorbs it.

For grid-tied systems, oversize economics depend on net-metering rules. In states with 1:1 net metering you can essentially “bank” excess summer production against winter usage. In states with avoided-cost compensation (much lower than retail), aggressive oversizing loses money.

A single shaded cell can knock out most of a panel’s output — and in a series string, it drags down every panel wired with it. Shading is the most underestimated variable in off-grid planning.

Three ways to manage it:

• Avoid it — trim trees, relocate the array, use a ground mount instead of a shaded roof. Always the cheapest option long-term.
• Split strings — wire the sunny panels separately from the occasionally-shaded ones so one doesn’t drag the other down.
• Use optimizers or microinverters — each panel reports independently. Adds cost but recovers 15-25% in partially shaded installs.

If shading is unavoidable for part of the day, bump your array size 20-35% to compensate — then model the shaded hours in the advanced calculator.

Start with your loads — daily watt-hours. Everything else flows from that one number.

• Batteries are sized to carry you through your longest cloudy stretch (usually 2-3 days of autonomy).
• Array is sized to refill those batteries in a single worst-case sunny day — typically 10-14% of bank capacity per hour of peak sun.
• Inverter is sized to the biggest simultaneous load, not to the array.

A useful sanity check: your array wattage (in W) should land roughly between 0.8× and 1.4× your battery bank capacity (in Ah at system voltage). Below that range, you won’t recharge fast enough; above it, you’re wasting panels your batteries can’t absorb.

Panels are rated three different ways and most people only see the first:

• STC (Standard Test Conditions) — 1000 W/m² of sunlight, 25°C panel temperature, dead-clean glass. This is the number on the spec sheet. You will almost never see it in real life.
• PTC (PVUSA Test Conditions) — a more realistic 1000 W/m² at 20°C ambient with 1 m/s wind. Typically 10-15% lower than STC.
• Real-world output — factor in temperature losses (panels lose 0.3-0.5% per °C above 25°C), soiling, wiring loss, and inverter efficiency. Most installs produce 70-85% of STC on a good day.

When you size an array with the calculator’s 75% default efficiency, you’re already accounting for this gap. Don’t also discount the panel rating — that’s double-counting the loss.