Load Calculator for Solar: How to Size Your System

A solar load calculator works out your total daily energy consumption in kilowatt-hours (kWh) so you can size a solar panel and battery system correctly. You enter the wattage of each appliance and how many hours per day you use it; the calculator returns your total daily load, which then drives decisions about panel count, inverter size, and battery capacity.

Formula

The basic formula for a single appliance is:

Daily energy (kWh) = (Wattage × Hours used per day) ÷ 1000

For multiple appliances, you sum each one:

Total daily load = Σ (Wattage × Hours) ÷ 1000

Variables:

• Wattage (W) – power draw of the appliance, usually printed on a label or in the manual.
• Hours (h) – how many hours per day the appliance runs.
• Days – how many days per week you use it (for weekly/monthly totals).
• kWh – kilowatt-hours, the unit your utility bill uses and the unit batteries are rated in.

Worked Example

Using the defaults from the calculator: a 100 W appliance used 5 hours per day, 7 days per week.

1. Daily energy = (100 W × 5 h) ÷ 1000 = 0.5 kWh
2. Weekly energy = 0.5 kWh × 7 = 3.5 kWh
3. Monthly energy = 0.5 kWh × 30 = 15 kWh

The calculator shows 0.50 kWh daily, 3.50 kWh weekly, and 15.00 kWh monthly. That matches the formula exactly.

Why You Need a Load Calculator

Solar panels produce power intermittently—only when the sun shines—and batteries store a finite amount of energy. If you don’t know your daily load, you can’t choose the right panel wattage or battery capacity. A common mistake is buying panels based on roof space or budget, then discovering the system can’t run your refrigerator overnight.

Load calculation is the first step in any off-grid or backup system design. It also helps you decide whether a small portable solar generator can handle a camping trip or whether you need a whole-home system.

How to Use the Load Calculator Correctly

Follow these steps to get an accurate estimate:

1. List every appliance you plan to power. Include lights, phone chargers, laptops, refrigerators, TVs, pumps, and heating elements.
2. Find the wattage for each. Look for a label that says “W� or “Watts.� If you only see amps (A), multiply by the voltage (usually 120 V in North America, 230 V in Europe) to get watts.
3. Estimate hours per day. Be honest—a refrigerator runs about 8 hours per day (compressor cycles), not 24.
4. Enter each appliance separately, or use the calculator multiple times and add the results.
5. Add a safety margin. Multiply your total by 1.25 to account for inverter losses, battery inefficiency, and days with less sun.

For example, a typical small cabin might have: LED lights (10 W × 4 h = 0.04 kWh), a laptop (60 W × 3 h = 0.18 kWh), a mini-fridge (80 W × 8 h = 0.64 kWh), and a phone charger (5 W × 2 h = 0.01 kWh). Total = 0.87 kWh per day. With a 25% margin, you’d design for about 1.1 kWh per day.

Common Mistakes and How to Avoid Them

The most frequent error is confusing watts and watt-hours. Watts measure power at an instant; watt-hours measure energy over time. A 1000 W microwave used for 5 minutes uses far less energy than a 100 W light bulb left on for 5 hours.

Another mistake is ignoring startup surges. Motors (refrigerators, pumps, air conditioners) draw 2–3 times their running wattage for a few seconds when starting. Your inverter must handle that surge, even if your average load is low.

Also, don’t forget that batteries have a usable capacity. A 100 Ah battery at 12 V holds 1.2 kWh, but you shouldn’t discharge it below 50% for lead-acid batteries, giving you only 0.6 kWh usable. Lithium batteries can go to 80–90% depth of discharge.

From Load to System Size

Once you have your daily load, you can size the rest of the system:

• Solar panel wattage: Divide daily load by peak sun hours in your location. If you need 1.1 kWh/day and get 4 peak sun hours, you need 1.1 ÷ 4 = 0.275 kW = 275 W of panels.
• Battery capacity: Multiply daily load by days of autonomy (how many cloudy days you want to cover). For 2 days of autonomy and 1.1 kWh/day, you need 2.2 kWh of usable storage. For a 12 V lead-acid system at 50% DoD, that’s 2.2 ÷ 0.5 = 4.4 kWh of battery, or 4.4 kWh ÷ 12 V = 367 Ah.
• Inverter size: Add up the wattage of all appliances you might run at once, plus the largest surge. If you run a 1000 W microwave and 200 W of lights simultaneously, you need at least a 1200 W inverter, preferably 1500 W to handle surges.

These are rough estimates. Real systems need to account for panel orientation, temperature, wiring losses, and seasonal sun variation.

Limitations of the Load Calculator

This calculator gives a static estimate based on average usage. It does not account for:

• Seasonal changes in appliance use (e.g., heating in winter, fans in summer).
• Battery temperature effects—cold batteries deliver less capacity.
• Inverter idle consumption (many inverters draw 10–20 W even with no load).
• Solar panel degradation over time (about 0.5% per year).

For a precise design, use a solar design software or consult a professional installer. But for most DIY projects, a simple load calculation gets you in the right ballpark.

Frequently Asked Questions

How many solar panels do I need for 30 kWh per day?

Divide 30 kWh by your location’s peak sun hours. If you get 5 peak sun hours, you need 30 ÷ 5 = 6 kW of panels. With 400 W panels, that’s 15 panels. Add a 25% margin for losses, so about 7.5 kW or 19 panels.

What is peak sun hours?

Peak sun hours are the equivalent number of hours per day when solar irradiance averages 1000 W/m². A location with 5 peak sun hours gets the same total energy as if the sun shone at full strength for 5 hours. This varies by latitude, season, and weather.

Can I use the load calculator for a 12V system?

Yes. The calculator works in watts and kWh regardless of voltage. For a 12 V system, you can convert kWh to amp-hours by dividing by 12 V. For example, 1 kWh ÷ 12 V = 83.3 Ah. Remember to account for depth of discharge.

Why do I need to multiply by 1.25?

The 25% margin covers system losses: inverter efficiency (typically 90–95%), battery charge/discharge losses (lead-acid ~20%, lithium ~5%), and wiring resistance. Without this margin, your system will underperform on cloudy days or when batteries age.

Does a refrigerator use its full wattage all day?

No. A refrigerator’s compressor cycles on and off. The label wattage is the running wattage when the compressor is on. Over a day, the compressor runs about 8–12 hours, depending on ambient temperature and how often the door is opened. Use 8 hours as a conservative estimate for daily energy.

What if my appliance only lists amps?

Multiply amps by the voltage to get watts. In North America, standard outlets are 120 V; large appliances like dryers and ovens may be 240 V. In Europe and most of Asia, it’s 230 V. For example, a 5 A appliance at 120 V uses 600 W.

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in watt | Appliance wattage | W | 100 | 0
in hours | Hours used per day | h | 5 | 0
in days | Days of use per week | - | 7 | 1
out daily | Daily energy use | kWh | watt * hours / 1000 | 2
out weekly | Weekly energy use | kWh | watt * hours * days / 1000 | 2
out monthly | Monthly energy use | kWh | watt * hours * days * 30 / 1000 | 2