Short answer: a parked RV draws 2–7 amp-hours a day from its house battery even with everything "off". A 10W solar trickle charger puts back 2–3Ah a day in a sunny-winter region and about 1Ah in the Pacific Northwest; a 20W kit roughly doubles that. If your rig's parasitic draw is under 0.1A and you store it somewhere sunny, 10W holds the battery. Anywhere north of Interstate 80, or with a detector, stereo memory and control boards all live, you want 20W with a controller — or you will come back in April to a battery that spent the winter at 11.8 volts.
Below is the math we use when customers ask which Eco kit to buy. The numbers are modeled, not marketing: solar yields come from NREL's published average peak-sun-hour data for each region, derated to 70% of panel nameplate for a flat-mounted panel (dust, angle, temperature, wiring), and parasitic draws are the typical values RV service techs measure with a clamp meter. Your rig will differ a little; the conclusions won't.
Why a stored battery goes flat
Two things drain a battery that nobody is using. The first is self-discharge: flooded lead-acid loses 3–5% of its charge a month sitting on a shelf, AGM 1–3%, lithium (LiFePO4) about 2%. On its own that is slow. The second is the real killer: parasitic loads that stay connected when the coach is "off".
| Typical parasitic load | Draw | Per day |
|---|---|---|
| LP/CO detector | 0.06 A | 1.4 Ah |
| Stereo and TV memory | 0.03 A | 0.7 Ah |
| Refrigerator control board (propane fridge, off) | 0.05 A | 1.2 Ah |
| Furnace and water-heater control boards | 0.04 A | 1.0 Ah |
| Tank-monitor / control panel | 0.02 A | 0.5 Ah |
| Battery disconnect left ON, inverter in standby | 0.15 A | 3.6 Ah |
A simple travel trailer with the disconnect switch pulled might draw nothing. A modern coach with the disconnect left on, an LP/CO detector, two control boards and a stereo that remembers its presets draws 0.15–0.3A around the clock — 3.6 to 7Ah a day, 100–200Ah a month. That empties a 100Ah lead-acid battery in three to four weeks, and a lead-acid battery left flat sulfates and loses capacity permanently. Lithium batteries don't sulfate, but their battery management system will disconnect at low voltage and some will not accept a charge again until they are warmed and woken up.
What 10W and 20W of solar put back, by region and season
Peak sun hours is the number of hours per day of full-strength (1,000 W/m²) sun a location receives on average. Winter is what matters for storage. Daily harvest below = panel watts × peak sun hours × 0.7 ÷ 12.8V.
| Where the RV is stored | Peak sun hours, winter / summer | 10W kit, Ah per day | 20W kit, Ah per day |
|---|---|---|---|
| Pacific Northwest (Bend, Seattle, Portland) | 1.8 / 5.5 | 1.0 / 3.0 | 2.0 / 6.0 |
| Northern Rockies & Plains (Denver, Billings, Minneapolis) | 2.4 / 5.8 | 1.3 / 3.2 | 2.6 / 6.3 |
| Great Lakes & Northeast (Chicago, Detroit, Boston) | 1.9 / 5.2 | 1.0 / 2.8 | 2.1 / 5.7 |
| Southeast (Atlanta, Orlando, Charlotte) | 3.2 / 5.4 | 1.7 / 3.0 | 3.5 / 5.9 |
| Texas & Gulf (Dallas, Houston, San Antonio) | 3.3 / 5.6 | 1.8 / 3.1 | 3.6 / 6.1 |
| Southwest (Phoenix, Las Vegas, Albuquerque) | 4.3 / 7.0 | 2.4 / 3.8 | 4.7 / 7.7 |
| California coast & valley (LA, Sacramento) | 3.3 / 6.6 | 1.8 / 3.6 | 3.6 / 7.2 |
Read the winter column against your parasitic draw. A rig drawing 0.1A (2.4Ah a day) is held by a 10W panel in the Southwest, Texas or the Southeast, and just barely in the Rockies. In the Pacific Northwest and Great Lakes a 10W panel in December covers about 1Ah — it slows the decline but does not stop it. A 20W panel covers a 0.1A draw everywhere and a 0.2A draw in most of the country.
Three storage scenarios
1. Travel trailer in a storage lot, Bend, Oregon, disconnect switch pulled
With the disconnect off the only draw is self-discharge — roughly 3Ah a month on a 100Ah AGM. Even 1Ah a day from a 10W panel in December is thirty times what the battery loses. The 10W Eco kit is plenty, and it needs no controller: at 0.6A peak it cannot overcharge a battery of 75Ah or more. Run the 23 ft fused lead straight to the battery with the SAE connector and forget about it until spring.
2. Class C motorhome under cover in Minneapolis, detector and control boards live
Here the draw is around 0.2A, nearly 5Ah a day, and winter sun is 2.4 peak hours — when the panel is clear of snow. A 10W panel returns about 2.6Ah and loses the race; the battery ends the winter around 40% and, if it is lead-acid, comes out with less capacity than it went in. The 20W Eco kit returns about 5.3Ah on an average winter day, enough to hold the bank, and its 10A controller's three-stage profile keeps a lead-acid battery at float rather than cycling it. Mount the panel where you can brush it off, or lay it inside on the dash: output through windshield glass drops 20–30% but the panel stays clean.
3. Bass boat on a trailer in Phoenix, two batteries
Trolling-motor and cranking batteries with no parasitic load, 4.3 peak sun hours in January. One 10W panel on each battery holds both indefinitely. If the two batteries are wired to one charger you want a controller with two outputs; otherwise a panel per battery is the simplest, cheapest answer.
Do I need a charge controller?
The rule of thumb is that a panel under about 1% of the battery's amp-hour rating can be left connected without a controller — a 10W panel (0.6A) on a 75–100Ah battery is below that line. At 20W (1.2A) on a single battery you are at the edge, and a controller is cheap insurance against boiling a flooded battery in July when nothing is drawing. That is why the 20W Eco kit ships with the GP-SB-PWM-10 and the 10W kit does not.
Lithium batteries in cold storage
LiFePO4 batteries must not be charged below freezing; a good BMS blocks charging when the cells are cold. A trickle charger on a lithium battery in a northern winter therefore charges on sunny afternoons when the cells warm up and sits idle the rest of the time, which is fine because lithium's self-discharge is tiny and its BMS draw is small. If the battery has shut itself down after a deep discharge, a 10–20W panel may not have the current to wake it; Go Power! Advanced Lithium batteries include a self-heating function and Bluetooth monitoring so you can see the state of charge from the driveway. The practical advice: bring lithium batteries to 50–60% before storage, pull the disconnect, and the trickle charger has very little work to do.
Which Eco kit
| Kit | Holds a draw of | Best for | Controller |
|---|---|---|---|
| 10W Eco (GP-ECO-10) | ≈ 0.1A in sunny regions; self-discharge anywhere | Boats, tractors, trailers with the disconnect pulled | Not required (≥ 75Ah battery) |
| 20W Eco (GP-ECO-20) | ≈ 0.2A nationwide, 0.3A in the Sun Belt | Motorhomes and trailers with detectors and control boards live | 10A PWM included, lithium/AGM/flooded profiles |
| 80W Eco (GP-ECO-80) | Any storage draw, plus weekend camping | Owners who also want lights, fans and charging on short trips | 10A PWM included |
All three use the same monocrystalline module construction and mounting hardware RV manufacturers have specified from Go Power! since 1996, and the 20W and 80W kits' controller will accept up to 200W of panels later. Browse the full solar panels and kits collection, or start with the kit that matches your winter.
FAQ
Will a 10W solar panel overcharge my battery?
Not a battery of 75Ah or more. At 0.6A peak the panel delivers under 1% of capacity per hour, which the battery absorbs as float. Below 50Ah, or with a 20W panel, use a controller.
Does the trickle charger work through a windshield?
Yes, at reduced output — expect 70–80% of the rooftop figure. It keeps the panel clean and out of the weather, which in snow country is worth the loss.
Can I leave the RV's battery disconnect on so the fridge board stays alive?
Yes, if the panel out-produces the draw. Add up the loads from the table above and pick the kit whose winter column beats the total for your region.
What about a plug-in battery maintainer instead?
A 120V maintainer is the better tool if you have shore power at the storage spot. Most storage lots and driveways don't, which is where solar earns its keep.