How to Size Your Off-Grid Solar System (Real Math, Not Marketing)

Type "how many solar panels do I need" into a search engine and you'll get two kinds of answers. The oversimplified kind — a national-average number that has nothing to do with your house, your loads, or your winter. And the installer kind, where the math has a sales quota standing behind it.

I don't install systems. I sell gear, I run my own, and I live in Cody, Wyoming, where getting this math wrong means a generator running all February. I have no incentive to sell you more panels than you need — and no incentive to pretend you need fewer than you do. So here's the actual math, worked end to end, with real numbers.

Why most sizing guides get it wrong

1. They use average consumption, not your loads. "The average US home uses 30 kWh/day" is useless. Off-grid sizing starts from a list of your actual appliances, not a census statistic.

2. They ignore winter. Solar production in January is not solar production in July — on a flat surface around here, midwinter sunlight delivers barely a fifth of midsummer's energy (NREL's Cody data: ~7.1 kWh/m²/day in July vs ~1.5 in December). A properly tilted array claws a lot of that back, but winter output still runs at roughly half of summer. An off-grid system sized to the annual average is a system that fails in the exact season you need it most.

3. They don't distinguish off-grid from grid-backup. Backup can be undersized — the grid covers the gap most days. Off-grid means there is no gap-coverage. The sizing rules are different, and most calculators quietly assume you have the grid.

Step 1 — Calculate your real load

List everything that draws power, its wattage, and its real hours per day. Here's a worked example for a 3-bedroom household outside Cody — propane heat and propane water heater (the right call off-grid), everything else electric:

Note what's NOT on this list: electric heat, air conditioning, an electric water heater, an EV charger, a welder. Any one of those can double or triple the number above. That's the point of doing your own table instead of borrowing someone's average — and it's why the honest answer to "how many panels do I need" is always "for which loads?"

Two numbers matter from this exercise: your daily watt-hours (sizes your panels and battery) and your peak simultaneous load (sizes your inverter — more on that in Step 5).

Step 2 — Figure out your real sun hours

A peak sun hour isn't an hour of daylight. It's the equivalent of one hour of full-intensity sun (1,000 W/m²) hitting your panels. A 14-hour Wyoming summer day might deliver 6 peak sun hours; a clear day in January might deliver 3.

For Cody, we use 4.5 peak sun hours as the annual fixed-tilt design number — and roughly 3.0–3.5 for deep winter. You'll find sources quoting 6+; those are typically annual irradiance figures under ideal tracking assumptions, not what a fixed ground-mount array sees in December. Look your own location up on a solar irradiance map (NREL's is free), and pull the winter month, not the annual average.

The rule that matters: if you're truly off-grid, size for your worst month, not your average month.

Step 3 — Size your panel array

The formula is short:

Daily Wh ÷ peak sun hours × 1.25 inefficiency buffer = array watts needed

The 25% buffer covers what the spec sheets don't: wiring and conversion losses, dust, snow, temperature derating, and battery charging inefficiency. It is not optional.

For our 9.9 kWh/day household:

• Annual-average sizing: 9,900 × 1.25 = 12,375 Wh ÷ 4.5 sun hours = ~2,750W of panel → four Canadian Solar 705W panels (2,820W).

• Winter sizing (the one that counts off-grid): 12,375 Wh ÷ 3.0 = ~4,125W → six panels (4,230W).

For reference: one 705W panel at 4.5 peak sun hours produces about 3,172 Wh per day. That single number tells you what every panel you add actually buys you.

Scaling up: an all-electric off-grid household — heat pump support, electric cooking, shop loads, 30–35 kWh/day — runs the same math to a very different place: 35,000 × 1.25 ÷ 3.0 winter sun hours ≈ 14,600W, which is a 21–24 panel array (14.8–16.9 kW; 20 panels at 14.1 kW falls just short of the requirement, and rounding down is how systems end up undersized). That's exactly the class of arrays the EG4 12,000XP inverter is built around, with 24 kW of PV input capacity.

Why the Canadian Solar 705W is our baseline

We carry one panel as the default recommendation for serious ground-mount builds, and it's this one. The reasons are specific:

• It's what our customers actually buy. It's the single best-selling product on our site over the last six months — 68 panels into real builds, not a spec-sheet endorsement. [PRICE]

• N-Type TOPCon cells. Better low-light performance, a -0.29%/°C temperature coefficient (it holds output in heat), slower degradation, and a 30-year linear performance warranty. 22.7% efficiency.

• Bifacial gain you'll actually get in Wyoming. The rear face captures reflected light at up to 80% of front-side power. Over snow — the most reflective ground cover there is — real-world rear gain shows up exactly when your sun hours are worst. We treat bifacial gain as winter margin, not as a number to size against. Don't let anyone sell you the "up to 850W" figure as the planning number; 705W nameplate is the planning number.

• The honest caveat: it's a commercial-format panel — roughly 7'10" × 4'3" and about 85 lbs. This is a two-person, ground-mount panel for a permanent build. It is the wrong panel for an RV roof, and we'll tell you that before you buy it.

Step 4 — Size your battery bank

Panels make power; batteries make it useful at 9 PM in January. Three inputs:

1. Days of autonomy — how many sunless days you ride through. With a generator as backstop, 1.5–2 days is a sane target. Without one, 3+.

2. Depth of discharge — LiFePO4 batteries run comfortably to 80% DoD (lead-acid wants 50%, which is one of several reasons we don't recommend it for new builds).

3. Your daily load — from Step 1.

Daily Wh × days of autonomy ÷ 0.8 = battery bank Wh

Our 9.9 kWh/day household, 2 days autonomy: 9,900 × 2 ÷ 0.8 = ~24.8 kWh. Two EG4 WallMount units (14.3 kWh each, 28.6 kWh total) cover it with margin. The all-electric 30 kWh/day household lands at 75 kWh for the same autonomy — and since 75 ÷ 14.3 = 5.2, that means six units (85.8 kWh), not five; rounding battery banks down means buying a shorter outage than you planned for. Many builds start at four units and lean on a generator for the deepest stretch of winter — a legitimate strategy, as long as it's a decision and not a rounding error.

One number worth knowing on the WallMount: it's rated for 8,000+ cycles at 80% DoD. At one full cycle a day, that's over 21 years before the battery is the thing you're replacing. That's LiFePO4 chemistry doing the quiet work.

The complete system stack

What the full build looks like, panel to outlet, at the two sizes we've worked:

Browse the full power lineup.

Step 5 — A word on the inverter (and peak loads)

Your daily Wh sizes panels and batteries; your peak simultaneous load sizes the inverter. Add up what could realistically run at once — well pump starting (surge), microwave, washer, blower — and make sure the inverter's continuous rating clears it with room. The 12,000XP's 12 kW continuous output clears any realistic residential peak in the tables above, which is why we treat it as the buy-once answer rather than stepping through three smaller inverters over five years.

Who should DIY vs. call an installer

Honest guidance, not a soft close:

• DIY territory: ground-mount array, off-grid (no utility interconnection), and you're comfortable with 48V DC wiring, torque specs, and reading the NEC sections your county inspector will quote at you. Thousands of people build exactly this every year, and the EG4 ecosystem is genuinely friendly to it.

• Installer territory: anything touching your main service panel or utility interconnection, whole-home transfer arrangements, roof structural questions, or a jurisdiction with strict permitting. Paying for a day of licensed electrician time on the AC side of an otherwise DIY build is the best money in this entire project.

If you take one thing from this guide: do the Step 1 table for your own house — just your critical loads, twenty minutes. That number turns every solar decision after it from guesswork into arithmetic.

FAQ 

How do I size an off-grid solar system for my home? List every appliance with its wattage and hours of use to get daily watt-hours. Divide by your location's winter peak sun hours (not the annual average), then multiply by 1.25 to cover system losses — that's your solar array size in watts. Size the battery bank at daily watt-hours × days of autonomy ÷ 0.8 for LiFePO4. Size the inverter to your peak simultaneous load, not your daily total.

How many solar panels do I need to run a house off-grid? A household running essentials (refrigeration, well pump, lighting, communications, furnace blower) on ~10 kWh/day needs roughly 4–6 high-output panels like the Canadian Solar 705W, sized for winter sun. An all-electric home at 30–35 kWh/day needs a 21–24 panel array (15–17 kW). The real answer always starts with your own load calculation, not an average.

What is the best solar panel for off-grid use? For permanent ground-mount off-grid builds, high-wattage bifacial panels offer the best value per watt — the Canadian Solar 705W N-Type TOPCon bifacial (22.7% efficiency, 30-year performance warranty, rear-side gain over snow) is our baseline. For RVs and mobile setups, smaller-format panels are the better tool; the 705W's commercial size is a feature for fixed arrays and a flaw on a roof rack.