Off-Grid Battery Storage Calculator: How Much You Actually Need

Most people buying off-grid battery storage get it wrong by a factor of two — either they’re running out of juice by day three or they’ve spent $8,000 on capacity they’ll never touch. The right answer comes down to three variables: your daily kWh load, your solar or generator input, and how many days of autonomy you need when the sun doesn’t show up.

I’ve been running off-grid setups since 2011 — first at a remote hunting cabin in the Ozarks, now at a full-time off-grid property in northern New Mexico. I’ve tested everything from flooded lead-acid banks cobbled together from golf cart batteries to purpose-built LiFePO4 systems pushing 20 kWh. The chemistry changed. The sizing math didn’t.

Start With Your Daily Load — Not a Guess

Before you look at a single battery spec, you need a real number for your daily energy consumption in kilowatt-hours. Don’t estimate. Pull your last utility bill if you’re transitioning from grid power, or build a load list from scratch if you’re starting fresh.

Here’s a quick reference for common loads:

LED lighting (10 bulbs, 4 hrs/day): ~0.2 kWh/day
Laptop + phone charging: ~0.1–0.2 kWh/day
12V refrigerator (Dometic CFX3 55): ~0.3–0.5 kWh/day
Full-size Energy Star refrigerator: ~1.0–1.5 kWh/day
Well pump (1/2 HP, 1 hr/day): ~0.4 kWh/day
Mini-split (1-ton, 4 hrs/day): ~2.0–3.0 kWh/day
Electric water heater: 3.0–5.0 kWh/day — avoid this off-grid if possible
Washing machine (3 loads/week): ~0.3 kWh/day average

Add those up honestly. A minimal weekend cabin with LED lights, a small fridge, and device charging lands around 1.0–1.5 kWh/day. A full-time family home with a well pump, refrigerator, and occasional climate control runs 5–10 kWh/day. The battery bank that handles one is useless for the other.

The Off-Grid Battery Sizing Formula

Take your daily kWh load, multiply by your desired days of autonomy (how long you can run without solar input or generator), then divide by your usable depth of discharge.

Required capacity = (Daily kWh × Days of autonomy) ÷ Usable DoD

For LiFePO4 batteries, usable DoD is typically 0.8 (80%). For lead-acid, use 0.5 (50%) — pushing deeper kills them fast. A cabin drawing 2 kWh/day that needs 3 days of autonomy needs: (2 × 3) ÷ 0.8 = 7.5 kWh of LiFePO4 capacity, or (2 × 3) ÷ 0.5 = 12 kWh of lead-acid to do the same job.

That gap is why I stopped recommending lead-acid for anything but the tightest budgets years ago.

Sizing by Lifestyle: Three Real Scenarios

Scenario 1: Weekend Cabin or Seasonal Retreat

You’re up Friday night through Sunday. Lights, a small fridge, phone charging, maybe a fan. Daily load: 1.5–2.0 kWh. You want 2–3 days of autonomy in case you extend the stay or hit a cloudy stretch.

Target capacity: 5–8 kWh usable. A single EcoFlow DELTA Pro at 3.6 kWh handles this with a second unit or expansion battery added. For a more permanent install, a 10 kWh LiFePO4 bank from Renogy or Battle Born gets you there with headroom.

At this scale, a 400–600W solar array on the roof keeps you topped off through most of the year without generator backup.

Scenario 2: Full-Time Off-Grid, Minimal Lifestyle

One or two people, efficient appliances, no electric heat or water heating, small well pump. Daily load: 3–5 kWh. You want 3 days of autonomy minimum — winter in the Rockies will hand you four consecutive days under 20% solar production without blinking.

Target capacity: 12–20 kWh usable. This is where the Renogy 200Ah LiFePO4 starts making sense in a 4-battery 48V bank configuration, giving you 9.6 kWh usable from four units. Stack two banks and you’re at 19.2 kWh — right in the sweet spot.

Pair that with 1,500–2,000W of solar and a backup generator for winter, and you’re genuinely comfortable full-time.

Scenario 3: Full-Time Family Home, Modern Comfort

Two adults, kids, full-size appliances, well pump, mini-split for climate control, home office. Daily load: 8–15 kWh depending on season. This is my current setup in New Mexico. Summer AC load nearly doubles my winter baseline — that’s not an estimate, that’s what my Victron Cerbo GX logs every August.

Target capacity: 25–40 kWh usable. You’re looking at a serious LiFePO4 bank — Signature Solar EG4 or Fortress Power eFlex territory — paired with 4,000–6,000W of solar and a propane or diesel generator for backup. Budget $15,000–$30,000 for the battery bank alone at this scale.

LiFePO4 vs. Lead-Acid: The Real Trade-Off

LiFePO4 wins on almost every technical metric. Cycle life runs 3,000–5,000 cycles at 80% DoD versus 300–500 cycles for flooded lead-acid at 50% DoD. LiFePO4 holds voltage flat through most of its discharge curve, which means your inverter and appliances run cleaner. It’s also roughly half the weight for equivalent usable capacity.

Lead-acid still makes sense in one specific situation: extreme cold without battery heating. LiFePO4 can’t charge below 32°F without a built-in heater, and most budget units don’t have one. If you’re in Minnesota running a winter cabin and your battery bank lives in an unheated space, either get LiFePO4 with a self-heating BMS or accept the limitations of AGM.

For most people reading this, Battle Born 100Ah LiFePO4 batteries are the benchmark mid-market option — 3,000+ cycle rating, built-in BMS, and a 10-year warranty that Battle Born actually honors. I’ve had a 4-battery bank running since 2019 with zero cell failures.

AGM is the acceptable compromise on a strict budget. It’s sealed, maintenance-free, and handles cold better than LiFePO4 without heating. Plan to replace it in 4–6 years under regular cycling. Flooded lead-acid is cheaper still but requires monthly maintenance and venting — not worth it for most modern installs.

Seasonal Variance: Size for December, Not June

Your battery sizing can’t be based on June solar production if you’re living off-grid year-round. In most of the continental US, December solar production is 40–60% of June production. In the Pacific Northwest, it’s worse.

Size your battery bank for winter. That means more capacity than feels necessary in July. It also means accepting that a generator is part of a serious full-time off-grid system — not a failure, just smart engineering. I run a 6,500W propane generator roughly 15–20 hours per month in December and January. The rest of the year it sits.

If you’re in the Sun Belt — New Mexico, Arizona, Texas, southern California — your winter production drop is less severe, maybe 25–35%. You can size more aggressively toward summer loads. If you’re in Vermont or Oregon, build in a bigger buffer or a more capable generator.

Temperature also affects battery capacity directly. A LiFePO4 bank rated at 100Ah delivers closer to 85Ah at 32°F and around 70Ah at 14°F. Factor a 15–20% capacity derating into your winter calculations if your batteries aren’t climate-controlled.

Quick-Reference Sizing Table

Use this as your starting point, then adjust based on your specific loads and location:

Weekend cabin, minimal loads: 5–8 kWh usable LiFePO4 | 400–600W solar
Weekend cabin, full comfort: 10–15 kWh usable | 800–1,200W solar
Full-time, minimal (1–2 people): 15–20 kWh usable | 1,500–2,500W solar
Full-time, moderate (family, efficient): 20–30 kWh usable | 3,000–4,000W solar
Full-time, modern comfort (family, AC/heat): 30–45 kWh usable | 5,000–8,000W solar + generator

These assume 3 days of autonomy and LiFePO4 at 80% DoD. If you’re using AGM, multiply usable capacity by 1.6 to get nameplate capacity needed.

Battery Banks Worth Buying Right Now

For portable and semi-permanent setups, the EcoFlow DELTA Pro is the most capable all-in-one unit I’ve tested — 3.6 kWh, expandable to 25 kWh with add-on batteries, 3,600W AC output, and a solid app for monitoring. It’s not the cheapest path to capacity, but the integration is clean and it works out of the box without a separate inverter or BMS.

For permanent installs where you’re building a 48V bank, the Renogy 200Ah LiFePO4 gives you 2.4 kWh per unit at a competitive price point. Wire four in series for a 48V/200Ah bank (9.6 kWh usable) and you’ve got a solid foundation for a cabin or small full-time system.

If budget is the primary constraint and you’re willing to do the maintenance, the Mighty Max 100Ah AGM is a workable starting point for weekend use. Don’t expect more than 4–5 years of cycling, and don’t discharge below 50%.

For serious full-time installs, I’d look hard at the Battle Born 100Ah LiFePO4. The per-kWh cost is higher than some Chinese-manufactured alternatives, but the warranty support and cell consistency matter when this is your primary power source and not a backup toy.

The Mistakes That Cost People Money

Undersizing to save money upfront is the most expensive mistake in off-grid power. Run a LiFePO4 bank below 10% state of charge regularly and you’re burning through cycle life fast. Run lead-acid below 50% and you’re cutting its lifespan in half with every deep discharge. Buy the capacity you actually need.

The second mistake is ignoring the inverter-battery relationship. A 3,000W inverter pulling from a 100Ah 12V battery bank is asking for 250 amps — that’s serious stress on connections, wire, and the battery itself. Size your bank voltage appropriately: 12V for small systems under 1,500W, 24V for 1,500–3,000W, 48V for anything larger. Most serious off-grid installs run 48V for efficiency and lower amperage on the DC side.

Third: don’t skip the battery monitor. A shunt-based monitor like the Victron BMV-712 tells you actual state of charge, not a voltage-based guess. It’s a $100 investment that protects a $5,000 battery bank. I’ve watched people kill $3,000 worth of batteries because they trusted a cheap voltage display that read “full” while the bank sat at 40%.

Bottom Line

Size for your real load, not your optimistic load. Build in 3 days of autonomy minimum. Use LiFePO4 unless you have a specific reason not to. Treat a generator as the insurance policy that lets you size your battery bank for 90% of your days instead of the worst 10%.

Do the math once, buy right, and you’re done. Skip it and you’ll be buying again in two years.

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