How to Calculate Off-Grid System Sizing (2026 Step-by-Step)
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Knowing how to calculate off-grid system sizing is what separates a system that works in February from one that only works in June. The arithmetic is straightforward. The discipline is in being honest about your loads and pessimistic about your winter sun.
The five steps
1. Add up daily watt-hours. 2. Multiply by days of autonomy. 3. Add 20–30% margin. 4. Size solar to replace daily consumption in your worst month. 5. Check inverter and BMS current limits against your largest simultaneous load.
Off-grid system sizing step 1: your true daily consumption
List every load, its wattage, and the hours per day it actually runs. Multiply and total. This is the foundation, and it is where most people underestimate.
| Load | Watts | Hours/day | Wh/day |
|---|---|---|---|
| 12V fridge (avg) | 45 | 24 | 1,080 |
| LED lights | 20 | 5 | 100 |
| Laptop | 60 | 6 | 360 |
| Water pump | 60 | 0.5 | 30 |
| Fan | 25 | 8 | 200 |
| Phone charging | 15 | 3 | 45 |
| Parasitic / standby | 15 | 24 | 360 |
| Total | 2,175 Wh |
Notice the parasitic row. Inverter idle draw, propane detectors, control boards and standby electronics run around the clock and frequently total more than your lighting. Leaving them out is the single most common sizing error.
Use average draw, not label peak. A fridge label shows compressor draw, but the compressor runs perhaps a third of the time.
Step 2 and 3: Autonomy and margin
Days of autonomy is how long the system carries you with no solar input at all. Two days is a reasonable minimum; three is comfortable; five is for genuinely remote winter use.
2,175 Wh × 2 days = 4,350 Wh. Add 25% margin for cold-weather capacity loss, ageing and the loads you forgot: 5,440 Wh, which is roughly a 425 Ah bank at 12 V, or 212 Ah at 24 V.
That 24 V figure is worth noticing. Once a bank passes about 200 Ah at 12 V, moving to 24 V or 48 V halves or quarters your current and dramatically reduces cable cost — see battery bank wiring.
Step 4: Size solar for your worst month
This is where most off-grid system sizing calculations quietly fail. Sizing solar for summer produces a system that dies in December.
The concept you need is peak sun hours — not daylight hours, but the equivalent hours of full-strength sun your location receives. It varies enormously by season and latitude.
| Condition | Typical peak sun hours | Array for 2,175 Wh/day |
|---|---|---|
| Southwest US, summer | 6–7 | ~475 W |
| Mid-latitude, spring/autumn | 4–5 | ~680 W |
| Northern, winter | 1.5–2.5 | ~1,700 W |
| Pacific Northwest, December | 1–1.5 | ~2,500 W |
Those winter numbers are why many off-grid installations include a generator or accept reduced consumption in the darkest months rather than building an array sized for December. Both are legitimate answers — pretending the problem does not exist is not.
The formula: array watts = daily Wh ÷ peak sun hours ÷ 0.75. That 0.75 accounts for controller losses, heat derating, wiring losses and panel soiling. Location-specific solar resource data is published by the National Renewable Energy Laboratory.
Step 5: Check your current limits
Energy capacity is only half of off-grid system sizing. Power capacity is the other half, and it is a separate constraint.
Add up everything that might run simultaneously at its worst moment — microwave plus fridge compressor starting plus water pump. That figure sets your inverter size. Then convert it to battery-side amps and check your BMS allows it: a 2,000 W inverter on a 12 V bank pulls roughly 175 A, and plenty of 100 Ah batteries cap out at 100 A.
Surge matters too. Compressors and pumps draw two to three times their running wattage for a fraction of a second at startup. Your inverter’s surge rating must cover it.
Frequently asked questions
How many days of autonomy do I actually need?
Two days suits most mobile setups, since you can usually move to better weather. Three to five is sensible for fixed cabins where you cannot. More autonomy means more battery; the alternative is more solar, or a generator for the rare bad stretch.
Should I oversize solar or oversize battery?
Solar, generally. Panels are cheaper per watt than batteries are per watt-hour, they have no cycle life to consume, and excess generation simply goes unused rather than degrading anything. Battery capacity buys autonomy; solar buys sustainability.
Why is my real output lower than my calculation?
Panels rarely reach rated output outside laboratory conditions. Heat reduces production, panels are seldom at the ideal angle, and dust and wiring take their share. The 0.75 derating factor exists for exactly this reason — if you omitted it, that is your gap.
Can I just buy a large power station instead?
For loads up to a few kilowatt-hours a day, often yes, and it saves considerable installation complexity. Custom banks become compelling above that, or where you need 48 V, high continuous output, or capacity beyond what portable units offer.
Last updated: July 20, 2026. Peak sun hour figures are typical regional averages; check location-specific data before finalising a design.
