Join Date: Nov 2016
Location: Little Point Sable, MI
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We assume your proposed lithium battery is 100 amp-hours (implying it will deliver 100 amps for an hour). This is pretty common for drop-in lithium batteries.
100 amp-hours is equivalent to approximately 1,300 watt-hours (implying you can put a 100 watt appliance on it for 13 hours or, at the other extreme, a 1300 watt appliance for 1 hour - - or any combination of watts x time (hours) that equals 1300 watt-hours).
If you use a 100 watt solar panel, the most that it will 'output' is 100 watts. But it will only do this under the best of conditions - - mainly, that the sun is directly overhead. The sun doesn't stand still. It moves across the sky and, even at noon, will not be directly overhead unless you happen to be parked on the Tropic of Cancer on June 21st.
So here are some of our experiences (we have 800 watts of solar, but will work our numbers out assuming we have just 100 watts).
The most we've ever 'collected' is 560 watt-hours of energy in a single day for a 100 watt solar panel. If you have a completely discharged a 100 amp-hour lithium battery (1300 watt-hours), it will take you over TWO PERFECT DAYS to recharge your battery. Thus, easily, you could attach 200 watts of solar (or more, since, again, you rarely get 560 watt-hours/day from a 100 watt solar panel). In fact, you could use 300 watts of solar.
Most people on this forum suggest working from another angle. They suggest deciding/determining what your power requirements will be . . . what appliances do you intend on running . . . then spec solar panels, if possible, to meet that requirement. LED lighting draws little power, a compressor refrigerator is good, but can draw more power - - we probably use 750 watt-hours/day on our refrigerator. Thus, a single 100 amp-hour (1300 watt-hour) lithium battery can supply such a frig and any LED lighting you may have. But, if a 100 watt solar panel can only, in perfect conditions, provide 560 watt-hours/day . . . you'll need another source of charging (alternator or shore-power) to meet the difference. And we'd caution to assume something more like 250 watt-hours/day (rather than the perfect 560) for your 100 watt solar panel expectations. And even that won't be met if it's overcast or you choose to park under a tree.
Bottom line, go for 200-300 watts of solar . . . and don't rule-out alternator and shore-power alternative chargers.