A load list is a comprehensive inventory of every electrical appliance you plan to power with your off-grid solar system, detailing both their wattage and their estimated daily runtime. By multiplying each appliance's watts by its hours of use per day, you calculate your total daily energy consumption in Watt-hours (Wh), which is the foundational metric required to accurately size your solar panels and battery bank.
A load list is the first and most critical step in solar sizing. Skip this step or guess your numbers, and you risk building a system that either leaves you in the dark after three cloudy days or costs thousands of dollars more than necessary. This guide explains how to properly audit your loads, calculate your daily energy needs, and avoid the common pitfalls that lead to undersized systems.
Step 1: List Every Single Load
Write down absolutely everything that will run from the system. This includes obvious large appliances like refrigerators, well pumps, and air conditioners, but also smaller devices that run continuously or frequently.
Do not forget "phantom" or standby loads. Devices like Wi-Fi routers, smart TVs, microwave clocks, and even the off-grid inverter itself consume power. A router drawing just 15 watts doesn't sound like much, but running 24 hours a day, it consumes 360 Watt-hours—often more than a modern energy-efficient refrigerator!
Step 2: Determine the Watts for Each Appliance
To calculate energy use, you need the power draw (in Watts) for each item on your list.
- Check the Nameplate Label: Most appliances have a sticker on the back or bottom showing their power consumption. If the label only lists Amps (A) and Volts (V), you can calculate Watts using this formula: Watts = Amps × Volts (e.g., 2 A × 120 V = 240 W).
- Use a Plug-in Watt Meter: This is the most accurate method, especially for appliances that cycle on and off (like refrigerators or freezers) or have variable power draw (like laptops or TVs). Plug the appliance into the meter for 24 hours to get a true reading of its daily energy use.
- Note the Surge Watts: While you use running watts to calculate daily energy (Watt-hours), you should also note the surge watts for any appliances with motors. You will need this surge number later for inverter sizing.
Crucial Factors Often Overlooked
When building a load list, many beginners simply multiply the nameplate wattage by 24 hours, leading to wildly inaccurate estimates. Here are the real-world factors you must consider:
Duty Cycle vs. Powered-On Time: A refrigerator is plugged in 24 hours a day, but its compressor (the part that uses significant power) only runs when it needs to cool the interior. This is called its duty cycle. A typical fridge might only have a 30% duty cycle, meaning it actually runs for about 8 hours a day. Using 24 hours in your calculation will result in a massively oversized and expensive system.
Inverter Inefficiency (Tare Loss): If you are powering AC appliances (standard household plugs), you will need an inverter to convert the DC power from your batteries. Inverters are not 100% efficient; they typically lose 10% to 15% of the energy as heat during conversion. You must add this inefficiency to your total AC load. Furthermore, the inverter itself consumes power just by being turned on (standby power), which must be added to your load list as a 24-hour continuous draw.
Seasonal Variations: Your energy use in December will look very different from your energy use in July. In winter, you use lights for more hours, and heating systems (even gas heaters have electrical blower fans) run frequently. In summer, fans or air conditioners dominate the load list. It is highly recommended to create two load lists—one for summer and one for winter—and size your system for the worst-case scenario.
Step 3: Estimate Hours per Day
How many hours does each appliance run on a typical day? Be realistic, and when in doubt, round up slightly to build in a safety margin.
- Lights: Estimate based on the season. (e.g., 4 hours in summer, 8 hours in winter).
- Laptop: 2–4 hours of active charging.
- Well Pump: Often runs for less than 1 hour total per day, but draws massive power when it does.
Step 4: Multiply and Add (The Math)
The formula for daily energy consumption is simple: Watt-hours (Wh) per day = Watts × Hours per day
Calculate this for every appliance, then sum the results.
Illustrative Worked Example: Small Cabin Load List
Note: The following calculation uses illustrative wattages and runtimes to demonstrate the math. Always check your specific appliances.
| Appliance | Running Watts | Hours/Day | Daily Energy (Wh/day) |
|---|---|---|---|
| LED Lights (4 bulbs) | 40 W (total) | 5 hours | 200 Wh |
| Laptop Charger | 60 W | 3 hours | 180 Wh |
| Wi-Fi Router | 15 W | 24 hours | 360 Wh |
| Small Refrigerator | 100 W | 8 hours (duty cycle) | 800 Wh |
| Water Pump | 800 W | 0.5 hours | 400 Wh |
| Subtotal (AC Loads) | 1,940 Wh | ||
| Inverter Inefficiency (10%) | 194 Wh | ||
| Total Daily Energy Need | 2,134 Wh / day |
In this illustrative example, the cabin requires approximately 2.1 kWh of energy per day.
Step 5: Using Your Load List
Once you have your total daily Watt-hours, you have the key to unlocking the rest of your system design. You will plug this number into:
- The peak sun hours formula to determine your solar array size.
- The battery sizing formula (Daily Wh × Days of Autonomy ÷ Depth of Discharge) to determine your battery bank capacity.
Alternatively, you can enter your total daily Wh (or your individual appliances) directly into the WattSizing calculator to instantly receive matched recommendations for panels, batteries, and inverters.
Frequently Asked Questions
Should I use the Energy Guide label on my refrigerator? Yes, the yellow Energy Guide label is highly useful. It provides an estimated annual energy consumption in kWh (e.g., 400 kWh/year). Simply divide that number by 365 to get your daily kWh, then multiply by 1,000 to get your daily Watt-hours. This is often more accurate than guessing the duty cycle.
What is a phantom load, and how do I measure it? A phantom load (or vampire draw) is the power consumed by an appliance when it is turned off but still plugged in. Examples include TVs on standby, microwave clocks, and chargers left in the wall. You can measure these using a plug-in watt meter. To eliminate them, plug these devices into a power strip and turn the strip off when not in use.
Do I need to include DC appliances on my load list? Yes. If you are running 12V or 24V DC appliances directly from your battery bank (like RV lights or a 12V fridge), you must include them in your total daily Watt-hours. However, you do not apply the 10% inverter inefficiency penalty to these specific loads, as they bypass the inverter entirely.
How do I account for appliances I only use once a week? For appliances used infrequently (like a washing machine used once a week), calculate the total Watt-hours used per week, then divide by 7 to get a daily average. Add this daily average to your load list.
Why is my calculated load so much higher than my actual utility bill? If you are moving from a grid-tied home to off-grid, you might be shocked by your load list total. Grid-tied homes often use electric heating, electric water heaters, and electric stoves, which consume massive amounts of power. In an off-grid scenario, it is almost always more economical to switch these heavy heating loads to propane or wood, rather than buying the massive solar array required to power them.
