Solar Load Calculator
Calculate the total daily energy demand of your home's lighting, appliances, and HVAC to correctly size a solar system. Use this before getting solar quotes to know exactly how many kilowatt-hours per day you need to generate.
About this calculator
Accurate load calculation is the essential first step in solar system design. Total electrical load is the sum of all power consumers: lighting, appliances, and HVAC, each measured in watts. Multiplying this combined wattage by the average daily usage hours gives watt-hours per day. Dividing by 1,000 converts to kilowatt-hours (kWh), which is the standard billing and sizing unit. A system efficiency factor (typically 0.85–0.95) accounts for inverter losses, wiring resistance, and battery charge/discharge inefficiencies. The formula is: dailyLoad (kWh) = ((lighting + appliances + hvac) × usageHours) / 1,000 / efficiency. The result tells you the gross solar generation your system must produce each day. Divide by your location's peak sun hours to find the required system size in kilowatts.
How to use
Assume lighting = 300 W, appliances = 800 W, HVAC = 1,500 W, average daily usage = 8 hours, and system efficiency = 0.90. Total load = (300 + 800 + 1,500) × 8 = 20,800 Wh. Convert to kWh: 20,800 / 1,000 = 20.8 kWh. Apply efficiency: 20.8 / 0.90 = 23.1 kWh/day. With 5 peak sun hours, the required system size = 23.1 / 5 ≈ 4.6 kW. You would specify at least a 5 kW solar array to reliably cover this household's daily load.
Frequently asked questions
How do I measure my home's actual wattage for a solar load calculation?
The most accurate method is to read your utility bills for monthly kWh consumption and divide by 30 to get a daily average. For a bottom-up estimate, find the wattage printed on each device's label or lookup table, then multiply by the hours it runs per day. Smart plugs with energy monitoring (such as the Kill-A-Watt meter) give real measured values for individual appliances. HVAC is the largest and most variable load—check the nameplate on your unit and use seasonal averages to avoid over- or under-sizing.
What system efficiency factor should I use when calculating solar energy needs?
A system efficiency factor of 0.85 (85%) is a conservative but widely used default for inverter-based grid-tied systems. This accounts for roughly 4–6% inverter losses, 1–3% wiring losses, and 2–3% soiling and shading losses. Battery-based off-grid systems are less efficient—use 0.75–0.80 to account for charge and discharge losses in the battery bank. String inverters tend to be less efficient than microinverters or DC power optimizers, especially under partial shading. Using a lower (more conservative) efficiency factor builds a safety margin into your design.
Why is HVAC the most important load to measure accurately for solar sizing?
HVAC systems typically account for 40–60% of a home's total electricity consumption, making them the dominant load in any solar calculation. Unlike lighting or appliances that run fairly consistently, HVAC loads swing dramatically with outdoor temperature and season. A heat pump or central air conditioner can draw 2,000–5,000 W, and run times vary from 2 hours on mild days to 10+ hours during a heat wave. Underestimating HVAC load leads to an undersized solar array that falls short on peak summer days. Use seasonal daily kWh data from your utility bills to derive a realistic annual average.