How To Calculate Your Solar Energy Needs in kWh

Most people get this wrong on the first try. They buy too few panels and run out of power, or way too many and waste thousands.

The fix is a bit of math. Not hard.

What A kWh Is

A kilowatt-hour is what you'd use running a 100-watt bulb for 10 hours. (100W × 10h = 1,000Wh = 1 kWh.) Power × time = energy. The rest is bookkeeping.

Step 1: List What You Use

Appliance	Power (W)	Hours/day	Daily energy (Wh)
LED light	10W	5h	50Wh
Refrigerator	150W	24h	3,600Wh
Laptop	50W	4h	200Wh
Water pump	500W	1h	500Wh
Total			4,350Wh (4.35 kWh)

Add up every device. Don't forget chargers, the modem, lights you leave on by accident.

Step 2: Add A Buffer

Real batteries and inverters aren't 100% efficient. Some energy is lost as heat. Plan on about 20% extra:

4.35 kWh × 1.2 = 5.22 kWh per day

Step 3: Divide By Sun Hours

Your area gets a certain number of "peak sun hours" per day. Not the number of hours the sun is up. The equivalent number of hours of full-strength noon sun. Most US locations get 4 to 6.

→ Check your sun hours

5.22 kWh ÷ 5 hours = 1.04 kW of panels needed

Divide by your panel size. If you're using 300W panels:

1,040W ÷ 300W = 3.5 panels → round up to 4 panels

Step 4: Size The Battery

Panels make power during the day. Your house uses power day and night. The battery covers the gap.

For one day of backup at 5.22 kWh on a 12V system: 435 Ah. On a 24V system: 217 Ah. Higher voltage means thinner wires and cheaper installs, so most setups use 24V or 48V.

For two days of backup, double it. For three days, triple. This is where battery costs balloon.

→ Pick the right battery type

→ Size your battery with the calculator

Getting It Wrong

Too few panels: you run out of power on grey evenings and lean on the backup generator. Too many: you've spent thousands more than needed, and the extra power has nowhere to go.

A well-sized system fills the battery on a sunny afternoon with a small amount of headroom.

The Cloudy-Week Problem

What if you get a full week of clouds in winter? Three options: add an extra panel or two, add more battery, or keep a backup generator. Most off-gridders do all three.

Cut Usage First

Every kWh you don't use is a kWh you don't have to generate, store, or pay for. LEDs everywhere. Unplug things that draw power when "off". Find the secret hog and decide if it's worth keeping.

A house using 5 kWh a day needs about half the system of one using 10. That can be the difference between $10,000 and $25,000.