How to Calculate Your Daily Energy Needs for Solar Sizing

Every solar system starts with one number: how many watt-hours you consume per day. Get this number wrong and you will either overspend on a system that produces more than you need, or — worse — build a system that cannot keep up with your demand. This guide walks through the load calculation process step by step, from identifying every electrical load to accounting for real-world efficiency losses.

Step 1: List Every Electrical Load

Grab a notebook and walk through your space. For every device, appliance, and system that draws power, record three things: its wattage (found on the nameplate, in the manual, or measured with a Kill-A-Watt meter), the number of hours per day it runs, and whether it runs continuously or cycles on and off.

Cycling loads like refrigerators are the most common source of calculation errors. A refrigerator nameplate might say 150W, but the compressor only runs 30–40% of the time. Its actual consumption is approximately 150W × 0.35 duty cycle × 24 hours = 1,260 Wh/day, not 150W × 24 hours = 3,600 Wh/day. Using the nameplate rating without the duty cycle will cause you to oversize your system by 2–3x.

Step 2: Add Efficiency Losses

Real-world solar systems lose energy at every stage of the conversion chain. Solar panels lose efficiency to heat, dirt, and wiring resistance. Charge controllers lose 2–5% converting panel voltage to battery voltage. Batteries lose 5–10% to internal resistance during charge and discharge. Inverters lose 5–15% converting DC to AC. Cable runs lose 1–3% to resistance depending on length and gauge.

The cumulative effect of these losses is typically 15–25% of your theoretical production. Apply a 20% efficiency factor to your daily consumption to account for these losses: Adjusted daily consumption = Raw daily consumption ÷ 0.80. Our 4,700 Wh example becomes 4,700 ÷ 0.80 = 5,875 Wh adjusted.

Step 3: Size Your Solar Array

Divide your adjusted daily consumption by the peak sun hours at your location. Peak sun hours represent the number of hours per day that solar irradiance equals 1,000 W/m² — the standard test condition for panel ratings. This varies by location and season: Phoenix gets approximately 6.5 peak sun hours annually, Seattle gets approximately 3.5, and most of the continental U.S. falls between 4 and 5.5.

For our 5,875 Wh example in a location with 5 peak sun hours: 5,875 ÷ 5 = 1,175W of panel capacity needed. Round up to the nearest available panel configuration — four 300W panels (1,200W total) or three 400W panels (1,200W total).

Step 4: Size Your Battery Bank

Battery bank size depends on your autonomy target — how many days of storage you want for cloudy weather or reduced production. For most systems, one to two days of autonomy is standard. Multiply your raw daily consumption by your autonomy days, then divide by the usable depth of discharge (typically 80% for LiFePO4).

Our example with two days of autonomy: 4,700 × 2 ÷ 0.80 = 11,750 Wh = 11.75 kWh. At 48V, that is approximately 245Ah — two EG4 LL-S 48V 100Ah batteries (10.24 kWh) gets close, and three (15.36 kWh) provides comfortable margin.

Common Calculation Mistakes

Ignoring phantom loads: Devices on standby (TVs, game consoles, chargers left plugged in, smart home hubs) draw 5–50W continuously. Over 24 hours, that adds up to 120–1,200 Wh/day of consumption you may not have accounted for. Use a Kill-A-Watt meter to measure actual standby consumption.

Using nameplate ratings for cycling loads: Refrigerators, freezers, air conditioners, and well pumps cycle on and off. Their actual daily consumption is far lower than their nameplate wattage multiplied by 24 hours. Always apply a duty cycle factor.

Forgetting seasonal variation: Solar production varies by 30–60% between summer and winter in most U.S. locations. Size your system for the lowest-production season (winter) to ensure year-round adequacy, or plan to supplement with grid power or a generator during the lowest-production months.

For battery selection, see our best LiFePO4 batteries guide. For panel and component selection, check our solar components overview.

Using Your Electric Bill Instead

If listing every appliance sounds tedious, there is a faster approach: use your electric bill. Find your average monthly kWh consumption (most bills list a 12-month average). Divide by 30 to get your daily consumption in kWh, then multiply by 1,000 to convert to Wh. This gives you a quick baseline that accounts for all loads, including phantom draws and seasonal variations.

For example, if your electric bill shows an average of 30 kWh/day (900 kWh/month), your raw daily consumption is 30,000 Wh. Apply the 20% efficiency factor: 30,000 ÷ 0.80 = 37,500 Wh adjusted. In a 5 peak-sun-hour location, that requires 37,500 ÷ 5 = 7,500W of solar panels. This whole-bill approach is ideal for grid-tied systems where you want to offset your entire utility consumption. For off-grid systems, the appliance-by-appliance method is more accurate because it lets you design around essential loads only.

Seasonal Adjustments

Solar production varies dramatically between summer and winter. In most U.S. locations, winter production is 30–60% lower than summer due to shorter days, lower sun angles, and more cloud cover. If you size your system for summer production, it will underperform in winter. If you size for winter, you will overproduce in summer.

The standard approach: size your solar array for the average of the worst three production months (typically December, January, February). This ensures adequate year-round production. For grid-tied systems with net metering, oversizing for winter means summer overproduction gets credited by the utility — a good trade-off. For off-grid systems, supplemental power from a generator during the lowest-production months is a common and cost-effective strategy.

Frequently Asked Questions