How to Size an Off-Grid Solar System (Battery + Inverter Math)

Step 1: Add Up Your Daily Load in kWh

Everything downstream depends on one number: how many kilowatt-hours you use in a day. Don’t guess. List every load, estimate its watts and hours of runtime, and multiply. Watts × hours ÷ 1,000 = kWh. Here’s a realistic worksheet for a year-round off-grid cabin — the example we’ll carry through the whole article:

Notice what’s not on the list: electric water heating, electric ranges, and electric resistance space heat. Off-grid designs that work put those big thermal loads on propane or wood. Every kWh you move off the battery bank saves you real money in Step 2.

Step 2: Size the Battery Bank (Days of Autonomy × DoD)

The battery bank has to carry you through cloudy stretches when the array produces next to nothing. The standard design target is 2–3 days of autonomy — two days for sunnier climates or generator-backed systems, three for cloudy regions or no generator. Then divide by usable depth of discharge. LFP (lithium iron phosphate) chemistry tolerates 80–100% DoD; designing around 80% leaves headroom for cold weather and aging.

Cabin math: 8 kWh/day × 2 days = 16 kWh usable. 16 ÷ 0.8 = 20 kWh of nominal LFP capacity. Three real ways to build that bank from our battery storage collection:

• Four Fortress Power eFlex 5.4 kWh modules ($3,788.12 each) — 21.6 kWh nominal. Modular 48V wall- or rack-mount units rated for 6,000 cycles; start with four and stack more later.
• Two HomeGrid Stack’d 9.6 kWh modules plus one 4.8 kWh ($2,614 per 9.6 kWh module) — 24 kWh in a single floor-standing tower on HomeGrid’s base unit.
• Two EG4 280Ah indoor LFP batteries ($3,440.70 each) — 280 Ah at 51.2 V nominal is roughly 14.3 kWh per unit, so a pair gives you ~28.6 kWh and a third day of autonomy.

If you’re comparing against lead-acid quotes: lead-acid banks are typically designed around 50% DoD, so replacing 20 kWh of LFP takes roughly 32 kWh of nominal lead-acid — one of several reasons almost every off-grid system we quote today is LFP.

Step 3: Size the Inverter — Continuous vs Surge

The inverter needs two ratings checked, not one. Continuous output must cover your largest realistic combination of simultaneous loads — for the cabin, the mini-split, well pump, fridge, and microwave running at once is roughly 4–5 kW. Surge capacity must cover motor startup. Induction motors like submersible well pumps briefly draw 3–5× their running current at startup, so a 1 hp pump running near 1 kW can momentarily demand 3–5 kW on top of whatever else is on. That surge — not the continuous number — is what trips undersized inverters.

For year-round off-grid living we quote 48V hybrid inverters in the 8–18 kW class. The EG4 FlexBOSS21 12K ($3,641.15) is our most common pick: 12 kW continuous output, three MPPT channels, up to 24 kW of PV input, and full off-grid capability with EG4’s LFP battery line — with an 18K version for bigger homes. The Fortress Power AVALON 11.4 kW ($3,612.50) is the natural pairing if you go with Fortress batteries. And if you’re weighing the premium option, our Sol-Ark vs EG4 comparison breaks down where Sol-Ark’s extra cost buys you something and where it doesn’t. Whatever you choose, verify the surge rating on the datasheet against your largest motor load.

One code note: off-grid doesn’t mean inspection-free. Battery systems, disconnects, and wiring still fall under the NEC in most jurisdictions — under the 2020 NEC, Article 706 covers energy storage systems — so verify requirements with your AHJ before you build.

Step 4: Size the Array for December, Not July

Grid-tied systems get sized for annual average production because the grid absorbs the seasonal swings. Off-grid, there is no grid to absorb anything — your array has to cover the daily load in your worst month. The formula: daily kWh ÷ worst-month peak sun hours ÷ ~0.7 system efficiency (charge losses, battery round-trip, temperature, wiring, the occasional dusting of snow). Here’s what that does to the cabin in a northern-US location:

Prices are pulled live from the Pyra Solar catalog and refreshed hourly. Click any part to see current stock.

Sizing for July would leave this cabin producing less than half its winter need. Sizing for December — call it eleven 440W panels, about 4.8 kW — means summer overproduction, and that’s fine: the charge controller simply tapers off once the batteries fill. If your panel count surprises you, our walkthrough of how many panels a house really needs shows the same math for grid-tied homes, and our winter production article explains where those December numbers come from.

The Cabin, Fully Specced

Pulling the four steps together for the 8 kWh/day cabin: a FlexBOSS21 12K hybrid inverter, ~20 kWh of LFP battery (four eFlex 5.4s or a HomeGrid Stack’d tower), and eleven 440W panels wired for the inverter’s MPPT windows — series vs parallel stringing matters here. Budget roughly $3,641.15 for the inverter and $8,098.71–$15,152.48 for the battery bank at current pricing, before panels and racking.

Every load profile is different — a well pump on a 400-foot bore or a shop full of power tools changes the inverter answer. Run your own numbers through our Solar Builder and we’ll turn them into a priced, compatible parts list. And if you’re not yet sure off-grid is even the right architecture, start with grid-tied vs off-grid vs hybrid.