What 200W of solar panels actually does in winter sun
Panel nameplate ratings are tested at 77°F and noon-summer sun. Real winter input is closer to half. Here is the math, and what it changes.
A 200-watt solar panel does not deliver 200 watts. It delivers 200 watts at 77°F under perpendicular noon sun on a clear summer day, which is roughly the conditions of the lab bench it was rated on. In a real backyard in February at 41° latitude, it delivers closer to 70 watts on a good day. On a cloudy day, 15 to 25 watts.
This guide is the math you need to size a real winter system. The numbers are not optimistic. They are the numbers people who have actually run these systems through a winter report.
The two numbers that matter
Solar panel ratings are written for marketing. The real input is the product of two adjustments to the rating:
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Peak sun hours per day. The number of hours the panel sees the equivalent of full-rated sun. In Long Island in December, that number is about 2.5. In Phoenix in December, it is 5. In Seattle in December, it is 1.5. This number is the most misunderstood part of the math.
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System efficiency. The fraction of the panel's rated output that actually reaches the battery. Charge controller losses, cable losses, panel angle, panel temperature, and dust each take a slice. A reasonable end-to-end efficiency for a portable winter system is 70 percent.
Daily energy in watt-hours equals: panel rating × peak sun hours × system efficiency.
For 200W in coastal NY in December: 200 × 2.5 × 0.70 = 350 watt-hours per day.
A LFP power station with a 1,500 Wh battery, depleted, takes about four full sun days in December to refill from solar alone. In summer, the same 200W of panels delivers about 1,000 watt-hours per day, refilling the same battery in roughly a day and a half.
The ratio is not 1:1 between summer and winter. It is closer to 3:1.
Where peak sun hours come from
The National Renewable Energy Lab publishes monthly average peak sun hours for every US zip code. The numbers most people quote ("five sun hours") are annual averages, which hide the December reality.
Rough seasonal averages for a few representative latitudes:
| Latitude | Region | June peak hours | December peak hours |
|---|---|---|---|
| 26° | South Florida | 6.3 | 4.2 |
| 33° | Phoenix, AZ | 7.5 | 5.0 |
| 41° | NY metro / Chicago | 6.0 | 2.5 |
| 47° | Seattle | 5.5 | 1.0 |
These are clear-day averages. Cloudy days knock the number down to 30-50 percent of clear-day output.
Use the December number, not the annual number, when you are sizing for winter resilience. If you cannot keep batteries charged through December, the system fails in the season that matters most.
What the inverter and charge controller take
A panel produces DC at a panel voltage that is rarely the battery voltage. A charge controller bridges them. Two types matter:
PWM (pulse-width modulation) charge controllers are the cheap option. They run at 75-80 percent efficiency. They drop panel voltage to battery voltage by chopping it, wasting the difference. Fine for small systems where the panel voltage and battery voltage are close.
MPPT (maximum power point tracking) charge controllers run at 92-97 percent efficiency. They actively track the panel's optimal power point as light changes through the day. They are worth the extra cost for any panel above 100W or any system with significant cable-run.
Most modern portable power stations have an MPPT controller built in. Most cheap solar generators have a PWM controller. Check the spec sheet. If it does not say MPPT, it is PWM.
The 70 percent end-to-end system efficiency in the math above assumes MPPT plus reasonable cabling. Drop that to 60 percent for a PWM system or for a system with a long undersized cable run.
Panel angle
A panel laid flat receives less light than a panel tilted toward the winter sun. The optimal winter angle is your latitude plus 15°. For 41° latitude, that is 56° from horizontal. Fairly steep.
In practice, most portable panels have folding kickstands set to a single angle, usually around 45°. That is a reasonable compromise. A panel laid flat in winter loses 25 to 35 percent of its possible output to angle alone.
If you are running panels stationary on a balcony or roof, the angle adjustment is one of the highest-leverage things you can do. If you are deploying portable panels each day, set them up at the steepest angle the kickstand allows in winter, and flatter in summer.
Cold is good, snow is not
Counterintuitively, cold weather helps solar output. Panel efficiency is rated at 77°F (25°C). For every degree Celsius below that, output rises by about 0.4 percent. A panel at 30°F produces about 8 percent more rated output than the same panel at 90°F.
What ruins winter output is not cold. It is snow on the panel, ice on the panel, low sun angle, and short days. Of these, only snow is something you can fix in the moment. Sweep the panel after every snowfall. A snow-covered panel produces zero, regardless of cell temperature.
A worked example
You have a 1,500 Wh LFP power station and 200W of portable solar. You live in coastal NY. You need to know how long you can run a basic loadout from solar input alone in December, on partly-cloudy days.
Daily input on a clear December day: 200 × 2.5 × 0.70 = 350 Wh. Daily input on a partly-cloudy December day (50% of clear): 175 Wh. Daily input on a fully overcast December day (25% of clear): 88 Wh.
Average across a typical December week (3 clear, 3 partly cloudy, 1 overcast): about 240 Wh per day average input.
Now the demand side. A reasonable "lights and phone" loadout:
- Two LED lamps at 8W each, 6 hours per night = 96 Wh
- Phone charge for two phones = 30 Wh
- A small radio = 5 Wh
- Wifi router for a few hours = 60 Wh
- Total: 191 Wh per day
You break even on average. You also have no margin for a cold snap, a stretch of overcast days, or a load you forgot about (a fridge, a CPAP, a heating pad). The 1,500 Wh battery is your buffer for those gaps.
If you add a fridge running at 50W average (a realistic small chest freezer or 12V cooler), demand jumps to 1,200 Wh per day. 200W of panels in December does not cover that. Either the panels grow to 600W, or the battery supplements diesel-generator-style for the off-sun days.
This is the calculation everyone skips, and the reason most prepper solar systems fail in the second week of January.
Sizing the panel array, not the battery
The most common mistake in a backup solar build is buying a big battery and a small panel. The battery is finite. The panel is infinite if it gets enough sun. In a multi-day outage, the panel determines how long you can run; the battery only smooths the day-to-day cycles.
Rule of thumb for a winter backup system: panel watts should be roughly equal to the watt-hours of daily demand divided by your December peak sun hours, divided by 0.70.
For 1,200 Wh per day in coastal NY in December: 1,200 / 2.5 / 0.70 = 686 watts of panel.
Most prepper-built systems have 200 watts of panel. The math says they cover lights and phones. They do not cover a fridge.
If a fridge is in scope, plan for 600 watts of panel minimum at 41° latitude. In Phoenix, you can do it with 250 watts because December peak sun is double. The local sun resource is the input variable nobody ports between forums.
What about wind, hand-crank, and bicycle generators?
Quick math on the alternatives:
- Hand-crank radio. Three minutes of cranking buys about a 10-minute weather forecast. Useful for the radio, useless for charging a phone.
- Bike generator. A fit adult on a stationary bike produces about 100 watts continuous for a half hour before fatigue. That is 50 Wh of input in half an hour of work. A phone charge.
- Wind turbine, residential scale. Variable. In coastal areas with steady wind, a 400W rated turbine can complement solar significantly in winter when sun is short. Inland, the math rarely justifies the cost and noise.
None of these replace solar at the scale a household needs. They are supplements at best.
What to do this weekend
Three things, in order:
- Look up your December peak sun hours on the NREL PVWatts calculator using your zip code. Note the number.
- Multiply that number by your panel rating and by 0.70. That is your real daily December input in watt-hours.
- Add up the daily watt-hours of every load you would run in an outage. Compare to step 2.
If demand is greater than input, you have one of two problems: too few panels or too much demand. Most people choose to cut demand because panels are expensive and roof space is limited. If you are sizing a backup power system right now, this is the calculation that determines whether it works in January.
For the broader question of which battery chemistry to put behind the panels, that is its own guide.