Solar Panel Wattage Calculator

To size a solar array, divide your daily energy use in watt-hours by your peak sun hours, then divide again by system efficiency. If your home or rig consumes 3,000 Wh per day, receives 4.5 peak sun hours, and runs at 75% system efficiency, you need about 889 watts of solar — roughly three 400W panels. The calculator above returns the exact wattage, the array size in kilowatts, and the number of standard 400W panels required.

The reason you cannot simply divide 3,000 Wh by 4.5 hours (which would suggest 667 W) is that real arrays never deliver their nameplate rating. Heat, wiring resistance, inverter conversion, dust, and charge-controller losses combine to shave 20–30% off the theoretical output. Building that derate into the math up front prevents the single most common off-grid mistake: an undersized array that never fully recharges the battery bank.

The equation this solar panel output calculator uses is:

Each variable carries real physical meaning:

• Daily Energy (Wh) — the total watt-hours every appliance consumes in 24 hours. Multiply each device's watts by hours of use and add them up, or read kWh from your utility bill and multiply by 1,000.
• Peak Sun Hours (PSH) — not hours of daylight, but the equivalent hours of full 1,000 W/m² sun. A location averaging 5.5 kWh/m²/day has 5.5 peak sun hours. Deserts see 5.5–6.5; the Pacific Northwest and UK see 2.5–3.5 in winter.
• System Efficiency (η) — the fraction of nameplate wattage that survives real conditions. NREL's PVWatts uses ~0.77 as a default derate; off-grid battery systems often use 0.70–0.75 because of extra charge-controller and battery round-trip losses.

Once you know array wattage, you can also estimate current. Divide the array watts by the battery or bus voltage to get amps — the same relationship used in our solar watts to amps calculator. That current figure feeds directly into the solar charge controller size calculator.

These fully worked examples show how the same load changes the array size as sun hours and efficiency shift.

Example 1 — Van build, 3,000 Wh/day, 4.5 sun hours, 75% efficiency: Array = 3,000 ÷ (4.5 × 0.75) = 3,000 ÷ 3.375 = 889 W. Rounding up to standard panels, that is three 400W panels (1,200 W nameplate) to leave headroom for cloudy days and battery charging.

Example 2 — Cabin in cloudy climate, same 3,000 Wh but only 3.0 sun hours: Array = 3,000 ÷ (3.0 × 0.75) = 3,000 ÷ 2.25 = 1,333 W. The identical load now needs 50% more panels purely because of a weaker solar resource — proof that peak sun hours dominate array sizing.

Example 3 — Efficient tiny house, 6,000 Wh, 5.0 sun hours, 80% efficiency: Array = 6,000 ÷ (5.0 × 0.80) = 6,000 ÷ 4.0 = 1,500 W, or roughly four 400W panels. The higher 80% efficiency reflects a modern MPPT controller and short, thick DC runs.

Example 4 — Weekend RV, 1,200 Wh, 5.5 sun hours, 72% efficiency: Array = 1,200 ÷ (5.5 × 0.72) = 1,200 ÷ 3.96 = 303 W — a single 300–400W panel comfortably covers lights, a 12V fridge, and device charging.

The efficiency input is not arbitrary — it is a stack of individual, measurable losses. Understanding them lets you choose an honest number rather than guessing.

A panel's Standard Test Condition (STC) rating is measured at 25°C cell temperature and 1,000 W/m² irradiance. In the field, cells routinely reach 45–65°C, and crystalline silicon loses about 0.3–0.4% of output per degree above 25°C, so a hot roof alone can cost 10–15%. Add wiring voltage drop, connector resistance, panel mismatch, module soiling from dust and pollen, inverter or charge-controller conversion, and battery round-trip inefficiency, and the cumulative derate lands around 20–30%.

For a grid-tied system with a modern string inverter, 0.80–0.82 efficiency is realistic. For an off-grid system that must also push energy into and out of a battery, use 0.70–0.75 because the battery adds another 5–10% round-trip loss. If you route power through an inefficient PWM controller instead of MPPT, drop another 10–20%. This is why our off-grid solar calculator and how many solar panels to run a house calculator both default to conservative efficiency values.

1. Enter daily energy use in watt-hours. Add every appliance's watts × hours per day, or take daily kWh from your bill and multiply by 1,000. A 900 kWh/month home uses 30,000 Wh/day.
2. Enter peak sun hours. Look up your location's annual average (or winter value if you need year-round autonomy) from NREL, PVWatts, or the regional table above.
3. Set system efficiency. Use 80% for grid-tie, 75% for a good off-grid MPPT system, or 70% for older PWM setups.
4. Read the result. The primary output is total array wattage; the secondary outputs give kilowatts and the number of 400W panels.

To turn that array wattage into a full off-grid design, feed it into the solar battery bank calculator for storage and the battery bank size calculator for days of autonomy.

The array wattage formula assumes your panels actually receive the peak sun hours you entered — and that depends heavily on tilt and orientation. A panel facing the equator (south in the Northern Hemisphere) at a tilt roughly equal to your latitude captures the most energy over a full year. Deviations cost real output that must be made up with extra wattage.

Azimuth (the compass direction the panel faces) matters most. True south is the reference; east or west orientations typically lose 10–20% annually, while a north face can lose 30% or more. Tilt angle shifts the seasonal balance: a steep, latitude-plus-15° tilt favors winter sun when the sun is low, which is ideal for year-round off-grid systems; a shallow, latitude-minus-15° tilt favors summer. Flat mounting (common on RVs and low-slope roofs) simplifies installation but loses winter production and collects more dust and snow.

Seasonal variation is dramatic. A fixed array in the northern US can produce three to four times more energy in June than in December. This is why off-grid designers size to the worst month and grid-tie designers size to the annual average. If your entered peak sun hours already reflect your tilt and season, the calculator's result is accurate; if not, shade the efficiency figure down or increase array wattage to compensate. For a whole-house translation of that wattage into panels, use the panels-to-run-a-house calculator.

The same array wattage behaves differently depending on whether it charges a battery or feeds the grid, and that changes the efficiency figure you should enter. In a grid-tie system, energy flows panels → inverter → loads/grid with a single conversion, so 0.80 efficiency is realistic and surplus is simply exported for credit. Nothing is wasted as long as the inverter can handle the power.

In a battery system, energy flows panels → charge controller → battery → inverter → loads, adding a charge/discharge round-trip that costs 5–15% depending on chemistry. Lithium round-trips at 92–98%; lead-acid at 80–85%. That extra loss is why off-grid and hybrid systems use 0.70–0.75 efficiency and are physically larger than grid-tie arrays serving the same load. There is also a subtler effect: a battery can only accept charge so fast, so a very large array charging a small bank may hit the charge controller's current limit and clip midday production.

Match the array to both the load and the battery's charge acceptance. A rough rule keeps array charging current between 0.1C and 0.5C of the bank's amp-hour rating. Once you know array wattage, size the controller with the charge controller size calculator and the storage with the solar battery bank calculator so all three stay balanced.

• Confusing daylight hours with peak sun hours. Twelve hours of daylight might equal only four peak sun hours. Always use PSH, never sunrise-to-sunset time.
• Ignoring the derate. Sizing to nameplate wattage leaves you 20–30% short in real conditions and undercharges the battery.
• Sizing to the annual average when you need winter power. If you live off-grid year-round, size to the worst month, not the yearly mean.
• Forgetting phantom and standby loads. Inverters, controllers, and always-on devices draw power around the clock and must be in the daily Wh total.
• Assuming panels will ever hit their rating. STC numbers are lab values; a "400W" panel typically produces 300–360 W at midday on a warm roof.

RV and van life. Most builds land between 200 W and 800 W of solar. A single 400W panel handles lights, a 12V fridge, fans, and charging; heavy users with induction cooking or air conditioning push toward 600–1,000 W plus a large lithium bank.

Off-grid cabins. A weekend cabin can run on 400–1,000 W, but a full-time off-grid home in a cloudy climate often needs 3,000–8,000 W to survive short winter days. Size to the worst month and pair with generous storage.

Tiny houses. With efficient DC appliances and a heat-pump mini-split, 2,000–4,000 W frequently covers a tiny home. Use the panels-to-run-a-house calculator to translate wattage into panel count.

Grid-tie homes. Here array sizing targets offsetting your annual utility bill rather than surviving autonomously. Most US homes install 5–12 kW; our solar panel size calculator for home works directly from your monthly kWh.