How to Size an Off-Grid Solar System (Without Wasting Thousands)
One of the biggest mistakes new off-gridders make is guessing at their solar system size. Some buy way too many panels and batteries (throwing away money they’ll never recover), while others go too small and end up firing up a noisy generator every other night. The fix? A simple, repeatable calculation that matches your actual daily energy use to your location’s real solar resource. Do it right once, and your off-grid solar system size will deliver reliable power for decades.
Step 1: Calculate Your Daily Energy Needs
Start by making a complete list of everything that will plug in or run on 12V/120V power. Be honest — include the coffee maker, well pump, Starlink dish, chest freezer, lights, fans, tools, and that electric fence charger you’ll “only use sometimes.”
For each item, note:
- Wattage (check the label, manual, or search the model number)
- Average hours used per day
Then do the math:
Watts × Hours = Watt-hours (Wh)
Example:
| Appliance | Watts | Hours/Day | Daily Wh |
|---|---|---|---|
| LED lights (10 bulbs) | 60 | 5 | 300 |
| Laptop + phone charging | 120 | 4 | 480 |
| Efficient fridge | 100 | 8 (effective) | 800 |
| Well pump (1/2 HP) | 800 | 1 | 800 |
| Microwave (short bursts) | 1,200 | 0.25 | 300 |
| Total | 2,680 Wh → 2.68 kWh |
Add 20–25% for inverter losses, wiring, heat, dust, and aging:
2.68 kWh × 1.25 = 3.35 kWh per day (round up to 3.5 kWh for safety).
Step 2: Factor In Your Location’s Peak Sun Hours
The same load requires drastically different array sizes depending on where you live.
- Phoenix, AZ: 6.5–7 peak sun hours (winter average)
- Seattle, WA: 2–3 peak sun hours (winter average)
Always size for your worst month unless you have a generator backup. Use OffGridIQ or NREL PVWatts and enter your exact ZIP code — look at the lowest monthly value.
Step 3: Solar Array Sizing Formula
Solar Array Watts Needed = (Daily kWh × 1,000) ÷ Worst-Month Peak Sun Hours
Example (8 kWh/day, 5.2 winter sun hours):
8,000 ÷ 5.2 = 1,538 W → round up to 1,600–1,800 W real-world array
Add 10–15% more for 25-year degradation and occasional shading/dust.
Step 4: Battery Bank Sizing Basics
Decide on autonomy days (most choose 2–3).
Example (48V lithium, 8 kWh/day, 3 days, 90% DoD):
8 × 3 = 24 kWh usable → buy ~27–30 kWh total capacity.
Step 5: Inverter Sizing — The Most Overlooked Piece
Your inverter has two critical ratings you must respect:
- Continuous rating — must be larger than your normal running load
- Surge rating — must handle startup spikes (pumps, air-conditioners, tools)
Rule of thumb:
- Add up the watts of everything that could run at the same time (fridge + lights + laptop + fans, etc.) → this is your continuous load.
- Identify the single largest motor load (well pump, washing machine, air-compressor) and note its startup surge — often 3–7× running watts for 0.5–2 seconds.
Example from the load list above:
Normal simultaneous load ≈ 1,200–1,500 W
Well pump surge ≈ 3,500–4,500 W for 1 second
→ Choose a minimum 2,000–2,500 W continuous / 5,000–7,000 W surge inverter/charger.
Most modern off-grid homesteaders install 3,000–5,000 W 48V hybrid inverter-chargers (Victron, Outback, EG4, Growatt, etc.) because they give headroom for future loads and run more efficiently.
Never size the inverter to the solar array size — size it to your actual loads and surge demands.
Step 6: Let SolarGridIQ Do the Heavy Lifting
SolarGridIQ handles every step automatically: appliance database, location-specific winter sun hours, array sizing, battery sizing, and full inverter recommendations with surge ratings. Add your biggest motor loads and it flags whether your selected inverter can handle the startup surge.
Final Word
Proper off-grid solar system sizing isn’t hard — it’s just math.
Get your daily kWh right → use real winter sun hours → size panels → size batteries → size the inverter to your loads and surge, not the panels. Do it once, skip the expensive mistakes, and enjoy silent, reliable power for the next quarter-century.
Head to OffGridHQ’s free SolarGridIQ tool right now, enter your ZIP and your appliance list, and get a complete, location-specific system plan — including the exact inverter model you need — in under five minutes.