How to calculate solar module needs: 5 sizing steps

First, tally daily kWh usage (e.g., 3kWh), add 15% loss, divide by local peak sun (4.5h) and module efficiency (20%) to get kWp, then divide by panel wattage (400W) for total count.

List Your Appliances and Usage

A common mistake is underestimating daily energy consumption, leading to an undersized system that dies by dusk or an oversized one that wastes money. For example, a small cabin running just a 40W fridge, five 10W LED lights for 4 hours, and a 60W laptop for 3 hours consumes about 0.59 kWh daily. Underestimating this by even 20% by forgetting a water pump or a fan can lead to power shortages.

The goal is to calculate the total Watt-hours (Wh) or kilowatt-hours (kWh) you consume daily. This number is the single most important figure for your entire solar project. For accuracy, you should distinguish between DC appliances (which run directly from your solar batteries, like 12V LED lights or a car fridge, and are highly efficient) and AC appliances (which require an inverter, like most kitchen gadgets and computers, losing about 5-15% of energy in the conversion process).

A quick tip: For appliances with variable power (like a fridge that cycles on/off), use its running wattage, not its startup surge. A typical 120W fridge might actually only run for about 8 hours total in a 24-hour period.

You can gather power ratings (in Watts) in three ways:

1. Check the manufacturer's label on the back or underside of the appliance. This is the most reliable source.

2. Use a plug-in power meter (a.k.a. a Kill A Watt meter). You plug the device into the meter, and it into the wall; it gives you a precise, real-time measurement of power draw and can even track energy use over 24 hours. This is ideal for appliances without a fixed label or for understanding the real-world consumption of an old refrigerator.

3. Consult the user manual or product specifications online.

Let's look at a practical example for a remote office shed:

The office's core load is 828 Wh per day. If you were to switch to a more efficient laptop (e.g., 45W instead of 80W), you would save (80W - 45W) * 6 hours = 210 Wh per day. Over a year, that 210 Wh reduction would allow for a smaller, less expensive cell (about 20% smaller capacity), potentially saving 150−300 upfront. This granular detail is what prevents you from buying a 2000W solar array when a 400W setup would suffice, or vice versa.

Calculate Your Daily Power Needs

For a typical off-grid cabin, this number often falls between 1.5 kWh and 5 kWh per day. Underestimating by even 10%—for instance, forgetting to account for a well pump that draws 500W for 15 minutes each day (adding 125 Wh)—can mean your system fails on a regular basis. 

The sum of all these individual Watt-hour values is your total daily consumption. However, the critical part that most DIY setups miss is accounting for system inefficiencies. The most significant of these is the inverter loss if you are running AC appliances. A modern pure sine wave inverter is about 90% to 95% efficient. This means if your appliances consume 1000 Wh of AC power, the inverter actually pulls about 1050 Wh to 1110 Wh from your batteries to deliver it. For a system with a mix of AC and DC loads, you should calculate the AC and DC loads separately. Take the total AC Watt-hours and divide by your inverter's efficiency (e.g., 0.90 for 90%) to find the actual load placed on the batteries by those AC devices. A 1000 Wh AC load with a 90% efficient inverter adds roughly 1111 Wh of demand on your cell bank.

A modern television or game console might draw 1 to 3 watts continuously even when switched off, which adds up to 24 to 72 Wh per day. For precision, using a plug-in power meter over a 24-hour period for devices like refrigerators is essential, as their compressor cycles on and off. A 150W fridge might have a compressor that runs for only 8 hours total in a day, leading to a real consumption of 1200 Wh, not the 3600 Wh you'd get by mistakenly assuming it runs 24/7.

You must also apply a safety buffer of 10% to 20% to your final calculated daily need. This buffer accommodates future additions, like a new tool, or days when you use a device more than usual. If your initial math says you need 4000 Wh (4 kWh), you should design your system for a target of 4400 Wh to 4800 Wh. This proactive adjustment is far cheaper than retrofitting your entire system later.

Account for Sunlight and System Loss

In the real world, that panel almost never produces its full rated power for more than a few brief moments a day. Factors like your geographic location, seasonal weather shifts, the panel's angle, and inherent energy losses within the system itself dramatically reduce output. If your calculated daily need is 5 kWh, simply dividing by 5 peak sun hours to get a 1000W array will leave you with a consistently underperforming system.

The first critical variable is peak sun hours. This is not the number of daylight hours, but the equivalent number of hours per day when sunlight intensity averages 1000 watts per square meter. This number varies drastically by location and season. A city like Seattle might average only 3.0 peak sun hours in December, while Phoenix, Arizona, enjoys about 5.5 hours in the same month. During the summer, these numbers can jump to 6.5 and 8.0 hours respectively. You must design your system for the worst-case scenario, typically the month with the least sun, to ensure year-round reliability. Relying on annual averages, which might be a comfortable 4.5 hours for many regions, guarantees winter power shortages.

Beyond available sunlight, the entire energy pathway from the sun to your appliance is lined with small but significant efficiency losses. Failing to account for these will result in a system that is too small. The combined impact of these losses can easily reduce your system's effective output by 25% to 35%.

l Temperature Loss: Solar panel efficiency decreases as temperature rises. For every 1°C (1.8°F) above the standard test temperature of 25°C (77°F), power output drops by approximately 0.3% to 0.5%. On a hot 35°C (95°F) day, a panel's output can be 4-5% lower than its rated capacity.

l Dust and Dirt Accumulation: A layer of dust, pollen, or bird droppings can block sunlight. Studies show this can cause a 5% to 15% reduction in output. Panels installed at a steep angle in rainy climates may stay cleaner, while flat-mounted panels in arid regions will suffer more.

l DC Wiring Losses: As electricity travels from the panels to the charge controller and batteries, resistance in the wires causes a voltage drop. Properly sizing your wiring for a voltage drop of less than 2% is crucial; poor wiring can lead to losses of 5% or more.

l Charge Controller and Inverter Efficiency: These modules are not 100% efficient. A high-quality Maximum Power Point Tracking (MPPT) charge controller might be 97-99% efficient, while a Pulse-Width Modulation (PWM) type might be only 90-95% efficient. Similarly, a good inverter operates at 90-95% efficiency.

l Cell Round-Trip Efficiency: If you are using a cell bank (essential for most systems), you lose energy during both charging and discharging. Lead-acid batteries have a round-trip efficiency of about 80-85%, meaning you put 10 kWh in to get 8-8.5 kWh out. Lithium-ion phosphate (LiFePO4) batteries are better, at 95-98% efficiency.

A conservative and commonly used factor is 1.5 (representing 150% of your initial needs). So, if your adjusted daily load calculation from the previous step is 5 kWh, you would calculate your required array size as follows: 5 kWh / (Your local peak sun hours) * 1.5. For a location with 3 winter sun hours, that becomes (5 / 3) * 1.5 = 2.5 kW (or 2500W) of solar panels. This 2500W array is what's needed to reliably generate your target 5 kWh under real-world, suboptimal conditions, not the 1666W array that simple math would suggest.

Determine Solar Module Capacity

The most common error here is equating the nameplate wattage of a panel directly with its daily output. A 400-watt panel does not produce 400 watts for 8 hours a day, yielding 3.2 kWh. In reality, its output is a curve dictated by sunlight intensity. Your goal is to select modules whose combined potential, when derated for all the inefficiencies we've discussed, reliably meets your energy budget during the worst-sunlight month of the year.

if your final adjusted need is 6000 Wh (6 kWh) and your design peak sun hours are 3.5, you need a 6000 / 3.5 = ~1715-watt array. You don't have to find a perfect 1715W panel; you'll build this from individual modules. The choice of individual panel wattage significantly impacts cost, physical layout, and future expandability.

For our 1715W system example, you could achieve it with four 430W panels (1720W total) or five 350W panels (1750W total). The four-panel array will be more compact and potentially cheaper overall, but the five-panel setup using slightly lower-wattage panels might offer more layout flexibility on a complex roof.

Beyond the basic wattage math, your choice of modules should be influenced by several critical performance and physical parameters:

l Panel Efficiency: This percentage (e.g., 21.5%) tells you how effectively the panel converts sunlight into electricity. A higher efficiency rating means you can generate the same total wattage in a smaller area. If your roof space is limited to 15 square meters, using 20% efficient panels versus 18% efficient panels could be the difference between a 3000W system and a 2700W system—a 300W gain without increasing the footprint.

l Temperature Coefficient: It indicates how much power the panel loses for every degree above 25°C (77°F). A panel rated at -0.30%/°C will lose 9% of its output on a 55°C (131°F) panel surface, a common real-world condition. Look for a coefficient closer to -0.25%/°C or better.

l Power Tolerance: This is the manufacturing tolerance stating how much a panel can exceed or underperform its rated wattage. A 400W panel with a +5%/0% tolerance can actually produce up to 420W, which is a bonus. Avoid panels with a negative tolerance (e.g., -3%), as they are guaranteed to produce less than their nameplate rating.

l Degradation Rate: Panels slowly lose output over time. A quality panel will have a first-year degradation of about 2% and then approximately 0.5% per year thereafter. This means after 25 years, it should still produce at least 85% of its original power. A cheaper panel with a higher annual degradation of 1% will only output about 75% of its original power after 25 years, a significant difference in long-term energy harvest.

Plan Cell and Charge Controller

A standard 12V 100Ah lead-acid cell provides only about 0.6 kWh of usable energy (50% Depth of Discharge), enough to run a 60W laptop for 10 hours. A similarly sized LiFePO4 (lithium iron phosphate) cell provides nearly 1.2 kWh usable (80% DoD) and will last 3-5 times longer. 

If your load audit determined you use 5 kWh per day, and you want 2 days of autonomy (meaning the cell can run your loads for two full days without any solar input), you need 10 kWh of usable energy storage. You must then adjust this for the cell's chemistry and your desired depth of discharge (DoD). DoD is the percentage of the cell's total capacity that you can safely use. Regularly discharging a cell beyond its recommended DoD drastically shortens its life.

For our 10 kWh usable energy example, the required totalcell bank capacity would be:

l For Lead-Acid (50% DoD): 10 kWh / 0.50 = 20 kWh total capacity needed.

l For LiFePO4 (80% DoD): 10 kWh / 0.80 = 12.5 kWh total capacity needed.

A 20 kWh lead-acid bank could weigh over 500 kg (1100 lbs), while a 12.5 kWh lithium bank might weigh under 140 kg (300 lbs). You must also incorporate the cell's round-trip efficiency. To get 10 kWh of usable energy out of a lead-acid cell (80% efficient), you need to put about 12.5 kWh into it from the solar panels.

Its job is to intelligently regulate the voltage and current from the solar panels to properly charge the batteries. The two main types are PWM (cheaper, less efficient) and MPPT (more expensive, highly efficient). For any system over 200W, an MPPT controller is strongly recommended because it can be 20-30% more efficient, especially in cool weather. Sizing the controller involves two key ratings:

l Voltage Rating: The controller's maximum input voltage must be higher than the open-circuit voltage (Voc)of your solar array at its coldest expected temperature. A panel's Voc increases as temperature drops. A string with a Voc of 100V at 25°C can exceed 115V at -10°C. Exceeding the controller's max voltage will destroy it.

l Current Rating: The controller's current rating (in Amps) must be greater than the total short-circuit current (Isc) of your solar array. You calculate the minimum ampacity needed by dividing the total array power by the cell bank voltage. For a 2000W array on a 48V cell system, the current is about 41.7A (2000W / 48V). You would then select a controller with a continuous current rating of at least 50A to provide a safety margin. Properly matching the charge controller to the array and cell is the final step in ensuring your system generates, regulates, and stores energy reliably for years.

A 20 kWh lead-acid bank could weigh over 500 kg (1100 lbs), while a 12.5 kWh lithium bank might weigh under 140 kg (300 lbs). You must also incorporate the cell's round-trip efficiency. To get 10 kWh of usable energy out of a lead-acid cell (80% efficient), you need to put about 12.5 kWh into it from the solar panels.