RV, off-grid and 12V systems

Can One 300 W Panel Refill a 12 V, 100 Ah Battery in a Day?

Published September 23, 2026 · By the SolarPanelX editors · Prices and specs as of that date, from the pages under Sources

Summary

On an average day across the year, yes in Phoenix, no in New York or Seattle. Scaling NLR PVWatts per-kilowatt annual output down to 300 W gives about 1,440 Wh a day in Phoenix, 1,060 Wh in New York, and 890 Wh in Seattle. Victron's Lithium Battery Smart 12.8 V/100 Ah holds 1,280 Wh, so New York's average day covers about 80% of it and Seattle's about 70%.

The charge controller's 12 V ceiling

The panel's 300 W only reaches the battery if the charge controller in between passes it. Victron lists a "Nominal PV power" for each controller at each battery voltage.

Model	Max charge current	At 12 V	At 24 V	Max PV open-circuit voltage
SmartSolar MPPT 75/15	15 A	220 W	440 W	75 V
SmartSolar MPPT 100/20	20 A	290 W	580 W	100 V
SmartSolar MPPT 100/30	30 A	440 W	880 W	100 V

A footnote says the controller limits input power when more PV than that is connected. On a 12 V battery the 100/20 takes 290 W of a 300 W panel. Move the small 75/15 to a 24 V battery and its figure doubles to 440 W. Whether the panel's open-circuit voltage stays under the right-hand column is a separate condition, and which pairings work is set by each manufacturer's specifications; this site does not rule on compatibility.

The daily figures are a grid-tied model (south-facing, 20° tilt, roof-mounted, 14% losses) divided by 365. Winter days run below the average, and battery charging losses are not included.

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Sources

We have not tested these products. Specs, prices, stock and terms are as published on these pages on the date above and can change; check the manufacturer or retailer before you buy.

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