In What Order Do You Size the Battery and Panels for an Off-Grid Solar System?
Summary
Start with daily watt-hours. The battery is sized by how many sunless days it has to cover, and the array by whether it can put that energy back each day. Storage has two separate ratings, energy in kWh and power in kW, and some energy is lost every time it goes in and out. Size the array against the weakest months, not the annual average.
A cabin that uses 1,500 Wh a day
Take a cabin running lights, a laptop, and a small fridge for 1,500 Wh a day. To get through two cloudy days without shutting anything off, the battery has to hold 3,000 Wh plus whatever conversion eats. DOE puts it plainly: storage "is never 100% efficient," because some energy is always lost converting and retrieving it.
On the supply side, a PVWatts run for Seattle (south-facing, 20° tilt, 14% losses) came to about 1,088 kWh per kW per year, or roughly 3.0 kWh per kW per day. Replacing 1,500 Wh a day would take about 0.5 kW on that average. Refilling the 3,000 Wh drained during two gray days on top of that calls for more panel.
The monthly numbers behind the average
That 3.0 kWh figure blends bright months and dim ones. Alongside the annual total, the PVWatts API returns AC output for each month from January to December. DOE describes longer-term storage as the supply that carries a system over days or weeks when solar production is low, and those low stretches are when off-grid systems run short.
Hours of energy versus watts at once
DOE defines energy capacity as the total that can be stored, usually in kWh, and power capacity as how much can be released at a given time, usually in kW. The cabin's 3,000 Wh is an energy number. Running a kettle and a microwave together is a power question, answered by the battery's output and the inverter's rating. A home with a small daily total can still hit its limit if it runs several large loads at once.
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Sources
- U.S. Department of Energy — Solar Integration: Solar Energy and Storage Basics
- NLR Developer Network — PVWatts API Version 8 (queried by the editors on September 23, 2026: 1 kW, south-facing, 20° tilt, fixed roof mount, standard module, 14% losses)
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