How to Size a Solar System for an Off-Grid Property in the Pacific Northwest

How to Size a Solar System for an Off-Grid Property in the Pacific Northwest

Wrong sizing is the most expensive mistake in off-grid solar. Here's how to do the math right for eastern WA conditions — before you buy a single panel.

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The Setup

Most people start with panels. They pick a number — 10 panels, 20 panels — based on gut feel or what fits on the roof. Then they wonder why the batteries die in February or why the system can't handle a chest freezer.

Sizing works the other way. You start with your loads, work backward to daily energy needs, account for local sun hours and seasonal variation, and then figure out how many panels and how much battery you actually need.

This isn't complicated, but it does require a spreadsheet.

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Step 1: Calculate Your Daily Load in Watt-Hours

List every electrical load you plan to run. For each one, note the wattage and hours per day. Multiply them.

Example cabin loads:

• LED lighting (8 bulbs × 10W): 80W × 4 hrs = 320 Wh
• Laptop: 65W × 4 hrs = 260 Wh
• Phone charging: 10W × 2 hrs = 20 Wh
• Small chest freezer: 100W × 8 hrs (avg) = 800 Wh
• Water pump: 500W × 0.5 hrs = 250 Wh
• Misc (fans, lights, misc): 200 Wh

Total daily load: ~1,850 Wh (1.85 kWh)

That's a modest but real off-grid cabin. Add electric heat, a large refrigerator, or power tools and you're looking at 5–10 kWh/day easily. Not sure what your appliances pull? Our appliance energy guide has a full table, and the U.S. Department of Energy's appliance energy calculator lets you check specific models.

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Step 2: Account for System Inefficiency

No system runs at 100% efficiency. Battery charging and discharging, inverter losses, and wiring add up to roughly 20–25% overhead.

Divide your daily load by 0.80 to account for losses:

1,850 Wh ÷ 0.80 = 2,313 Wh required from panels per day

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Step 3: Divide by Local Peak Sun Hours

For eastern Washington, use these conservative seasonal figures:

• Summer (May–Sept): 5.5 peak sun hours/day
• Spring/Fall (Mar–Apr, Oct–Nov): 4.0 peak sun hours/day
• Winter (Dec–Feb): 2.5 peak sun hours/day

These hold up surprisingly well east of the Cascades — here's why eastern WA gets more sun than people expect. For numbers dialed in to your exact address, run it through NREL's free PVWatts Calculator.

Size for the worst case if you need year-round reliability. For a winter-critical system:

2,313 Wh ÷ 2.5 hours = 925W of panels required

At 400W per panel, that's 3 panels minimum for a winter day. But you're not building for one good day — you need margin for cloudy stretches.

Rule of thumb: multiply your winter minimum by 1.5–2x for real-world buffer.

925W × 1.5 = ~1,400W → 4 panels at 400W each

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Step 4: Size the Battery Bank

Battery sizing depends on how many days of autonomy you want — how long the system runs without any sun.

Formula: (Daily load Wh × days of autonomy) ÷ usable depth of discharge

For lithium (LiFePO4), usable depth is about 80%. For flooded lead-acid, use 50%.

Example: 2 days of autonomy with lithium:

(1,850 Wh × 2) ÷ 0.80 = 4,625 Wh of battery capacity needed

At 100Ah / 48V = 4,800 Wh per battery, one battery bank covers it. Two gives you real cushion.

For lead-acid (cheaper upfront, heavier, shorter lifespan):

(1,850 Wh × 2) ÷ 0.50 = 7,400 Wh needed → roughly 3–4 100Ah/24V batteries wired in series-parallel.

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Step 5: Pick the Right Inverter

Inverter sizing is straightforward: your inverter must handle your peak simultaneous load.

Add up every load that could run at the same time. A pump (500W) + chest freezer compressor startup (600W) + laptop (65W) + lights (80W) = 1,245W peak. Add 25% headroom: 1,500W inverter minimum.

For larger homesteads or anything with a well pump, start at 2,000–3,000W. Inverters are not where you save money.

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The Bottom Line

Size for loads, not for what fits on the roof. Start with daily watt-hours, divide by your local winter sun hours, multiply by a buffer factor, and then figure out how many panels you need. For eastern Washington off-grid systems, 4–12 panels covers most residential and cabin scenarios when paired with adequate battery storage. Get the math right before you spend a dollar on hardware.

Where panels are concerned, you don't need to pay new-panel prices to do this well — used commercial panels can cut your payback in half. When you've run your numbers, our used 300W panels start at $90 each. Stop by the depot in Oroville and we'll sanity-check your design with you.