An off grid solar system for home use has to do everything the utility does: make power, store it and deliver it on the worst winter week. That makes it far bigger and more expensive per kWh than a grid-tied system of the same size. Sizing starts with your loads, not your roof.
Electrical work can be dangerous: follow the manual, fuse and size every cable correctly, and use a licensed electrician for anything connected to household wiring.
Sizing an Off Grid Solar System for Home Use
Step 1: Daily energy. List every load with watts and hours. Example cabin-style home: 10 kWh/day (efficient fridge, lights, well pump, laptop, washer, mini-split part-time). A typical grid home uses closer to 30 kWh/day, so cutting loads is the biggest saving.
Step 2: Battery bank.
Usable kWh = daily kWh × days of autonomy
| Days of autonomy | Usable kWh (10 kWh/day) | Nominal LiFePO4 at 80% DoD |
|---|---|---|
| 1 | 10 | ~12.5 kWh |
| 2 | 20 | ~25 kWh |
| 3 | 30 | ~37.5 kWh |
Step 3: Array, sized for winter. Use PVWatts’ December output for your location. If your array makes 2 kWh per kW per day in December, then 10 kWh ÷ 2 ÷ ~0.85 system efficiency ≈ 5.9 kW. Summer production will be far higher.
Step 4: Inverter. Size for the largest combined load plus motor surge. A 120/240V split-phase inverter is needed for 240V well pumps.
Step 5: Generator. Most full-time off-grid homes keep a generator for long cloudy spells. An inverter-charger can start and charge from it automatically.
Example Price Breakdown (Illustrative)
| Component | Example size | Rough cost range |
|---|---|---|
| Panels | 6 kW | Varies; check current listings |
| Racking (ground mount) | 6 kW | Varies by type |
| LiFePO4 battery, 48V | 25 kWh | Varies by brand |
| Inverter-charger, 120/240V | 8-12 kW | Varies by brand |
| MPPT controller(s) | 6 kW array | Varies |
| Wiring, fuses, disconnects, BOS | — | Often 10-20% of hardware |
| Generator | 7-10 kW | Varies |
Equipment prices change often, so we don’t quote fixed numbers here. Get current quotes for each line. Professional installation adds substantially.
Code and Permits
Off-grid systems still fall under NEC Articles 690 (PV) and 706 (energy storage), plus local building codes. Many lenders and insurers require permitted systems.
Worked Example: A Full Load List
Here is an illustrative load list for the 10 kWh/day home. Your appliance labels and a plug-in watt meter give real numbers.
| Load | Watts | Hours/ | Wh/ |
|---|---|---|---|
| Efficient refrigerator | 150 avg | 24 | 1,500 (about 1.5 kWh) |
| LED lighting | 100 | 5 | 500 |
| Well pump, 240V | 1,000 | 1.5 | 1,500 |
| Mini-split heat pump | 700 | 6 | 4,200 |
| Washer | 500 | 1 | 500 |
| Laptop, router, phones | 100 | 8 | 800 |
| Microwave | 1,200 | 0.25 | 300 |
| Standby and misc. | 30 | 24 | 720 |
| Total | about 10,020 Wh (10 kWh) |
The mini-split is the swing load. In a cold snap it could run twice as long, adding about 4 kWh. That’s why winter sizing and a generator matter. For load math, see watts, volts and amps explained.
Worked Example: Battery Bank in Amp-Hours
Off-grid batteries are often sold in amp-hours at a set voltage. kWh = Ah × V ÷ 1,000.
| Bank | Voltage | Ah | Nominal kWh | Usable at 80% DoD |
|---|---|---|---|---|
| 4 × 100 Ah | 48 V (series strings) | 100 | 4.8 | 3.8 |
| 25 kWh target | 48 V | 520 | 25 | 20 |
| 37.5 kWh target | 48 V | 780 | 37.5 | 30 |
For a 25 kWh bank at 48 V: 25,000 ÷ 48 ≈ 520 Ah. Higher voltage means lower current, which keeps cable sizes manageable. See 12V vs 24V vs 48V systems and depth of discharge and cycle life.
Worked Example: Winter vs Summer Array Output
Using the example rule of 2 kWh per kW per day in December and, say, 5.5 in June, a 5.9 kW array makes:
| Month | kWh per kW per day | Daily output (5.9 kW × 0.85) | vs 10 kWh need |
|---|---|---|---|
| December | 2.0 | about 10.0 kWh | Just covers it |
| March | 4.0 | about 20.1 kWh | Surplus |
| June | 5.5 | about 27.6 kWh | Large surplus |
Off-grid owners often use summer surplus for “opportunity loads” such as water heating or pool pumps. Get real monthly figures for your site from PVWatts.
Choosing a Generator Size
A generator mostly needs to recharge the battery and run loads at the same time. If the inverter-charger accepts 60 A at 240 V of AC charging, that’s about 14 kW at full rate, but most systems limit charge current. A 7–10 kW generator, as in the table, is common for this size of home. Running it at a moderate, steady load is usually more efficient than short runs.
Step-by-Step: Planning Your Off-Grid Build
- Measure or estimate every load in watts and hours.
- Cut loads where you can, starting with heating, cooling and pumping.
- Pick a battery voltage (48 V for whole-home systems).
- Size the bank for 2–3 days of autonomy.
- Size the array from December PVWatts data.
- Pick an inverter for peak load plus motor surge.
- Add a generator and auto-start.
- Get permits under NEC 690 and 706.
Also see how many batteries to power a house.
FAQ
How much does an off grid solar system for a home cost?
It depends mostly on daily kWh and battery size. Get itemized quotes.
How many batteries do I need to go off grid?
Enough usable kWh for your daily use times days of autonomy.
Can I go off grid without a generator?
It’s possible with a large battery and array, but most full-time homes keep one.
What is days of autonomy?
It’s how many days the battery can run your home without sun. Two to three days is a common off-grid target, with a generator for longer stretches.
Is off-grid cheaper than grid-tied solar?
Usually not per kWh, because you must buy batteries, oversize the array for winter and often buy a generator. Off-grid makes most sense where a grid connection is expensive or unavailable.
What inverter size do I need off-grid?
Size it for your largest combined running load plus motor surge. See what size inverter do I need.
Sources: NREL PVWatts; NREL Solar program; NFPA 70 (NEC) 2023 Articles 690 and 706; NREL PVWatts; NEC Articles 690 and 706. Accessed Oct 2026.
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