Solar

Solar Panel Sizing: Watt-Hours, Sun Hours, and Battery Math That Works

Every bad solar purchase starts with skipping the arithmetic. The good news: the whole discipline is three multiplications. Know your daily load in watt-hours, your location's average sun hours, and how much of your battery you may actually use, and you can size any system on a napkin and spot a dishonest kit listing in seconds.

Work through the example below with your own numbers. It uses deliberately conservative figures, because a system sized on optimism fails in December, which is when it tends to be needed.

Quick facts

Load unit: Watt-hours per day (Wh/day) = watts x hours of use, summed over every load
Sun hours: US average roughly 3.5-6 peak sun hours/day depending on state and season; winter is the design case
Panel reality factor: Plan on about 75-80% of rated watts after heat, wiring, and controller losses
Usable battery capacity: LiFePO4: about 80-90% of label; lead-acid: about 50% of label
Autonomy: 1-3 sunless days of battery is the usual off-grid design target

Step 1: your daily load

List every load with its watts and honest hours. Example cabin day: LED lighting 60W x 5h = 300 Wh; a small fridge averaging 60W x 24h = 1,440 Wh; laptop 65W x 4h = 260 Wh; phone and radio charging 100 Wh; a water pump 250W x 0.5h = 125 Wh. Total: about 2,225 Wh per day. Fridges and anything with a motor dominate real lists; lights, the thing people picture, barely register.

Step 2: panels from sun hours

Divide daily load by (sun hours x 0.78 system efficiency). For 2,225 Wh in a 4-sun-hour winter location: 2,225 / (4 x 0.78) = about 713W, so 800W of panels (say, four 200W panels) with a little headroom. The same load in Arizona summer sun needs barely half that array, which is why national kit marketing sized on Phoenix sunshine disappoints in Michigan winters.

Find your location's real numbers with NREL's PVWatts tool, linked in the sources, and always size on your worst month, not your average month.

Step 3: batteries from autonomy

Decide how many sunless days to ride out. Two days of autonomy for the example load is 4,450 usable Wh. In LiFePO4 at 85% usable, that is about 5,235 Wh of label capacity, roughly a 48V 100Ah bank (4,800 Wh) or a 12V bank of about 440Ah. In lead-acid at 50% usable, the same job needs about 8,900 label Wh, which is why the cheaper battery usually is not.

Voltage is the last choice: 12V suits small systems, 24V mid-size, 48V anything with a serious inverter, because higher voltage means thinner, cheaper, safer cabling for the same power.

Step-by-step: put it into practice

Tick each step off as you complete it. Your progress is saved on this device.

List loads in watts and hours; sum to watt-hours per day
Look up your location's worst-month peak sun hours (PVWatts)
Panels needed = Wh/day divided by (sun hours x 0.78); round up
Battery usable Wh = Wh/day x days of autonomy
Convert to label capacity: divide by 0.85 (LiFePO4) or 0.5 (lead-acid)
Pick system voltage by inverter size: 12V small, 24V mid, 48V large
Verify the charge controller's voltage and current limits against your panel string

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Sources

Facts checked against the sources above. Last verified: November 11, 2026. If a figure here ever conflicts with an official source, the official source wins, and we want to hear about it via the contact page.