How the battery estimate works
Battery capacity in amp-hours needs a voltage to become an energy budget. This calculator converts the bank to watt-hours, applies your usable-capacity setting, and divides by the amount of energy you expect to lose each day.
Net use (Wh/day) = daily use − daily solar
Estimated days = usable energy ÷ net use
A worked example
A 200 Ah bank at 12.8 V contains a nominal 2,560 Wh. If you choose to use 80% of it, that gives you 2,048 Wh for planning. At 800 Wh per day and no charging, the estimate is 2.56 days. This is the calculator’s starting example, not a measured result for your RV.
If solar delivers 400 Wh per day under your actual conditions, the modeled daily deficit falls to 400 Wh. The estimate becomes 5.12 days. A cloudy day, battery aging, low temperatures, or higher demand can reduce that time.
Enter the bank, not one battery
Use the capacity and nominal voltage of the assembled battery bank. In a series connection, voltage adds while amp-hour capacity stays the same; in a parallel connection of matching batteries, amp-hours add while voltage stays the same. Confirm your installed configuration rather than simply adding every number on the labels.
The 80% usable setting is an example planning allowance. It is not a universal recommendation for lead-acid or lithium batteries. Choose the discharge limit specified for your model, its condition, and your desired reserve. The calculator assumes you start fully charged.
Measure demand at the battery if possible
Daily watt-hours should include everything drawing from the bank: refrigerator, lights, fans, pumps, control boards, chargers, and any inverter overhead. AC appliance watt-hours must account for inverter losses. For example, 450 Wh of AC demand through an inverter operating at 90% efficiency requires about 500 Wh from the battery, before additional idle consumption.
Appliance power multiplied by hours of actual operation gives energy use. A refrigerator does not necessarily draw its running wattage for 24 continuous hours. A suitable energy meter or battery monitor can help you replace estimates with measured figures.
When solar covers your daily use
A zero or negative modeled daily deficit does not mean unlimited camping. You still need enough stored energy for overnight loads and cloudy weather, and your equipment must handle peak demand. Water, waste tanks, weather, and site restrictions remain separate limits.
This tool uses a simplified average daily balance. It does not model charging curves, load timing, voltage drop, temperature, lead-acid discharge-rate effects, or inverter surge capability. Do not use it to size wiring, protective devices, or a life-critical power system.
Next, estimate a suitable array with the RV solar calculator and check your water and tank capacity.
Sources & further reading
Sources checked September 24, 2026. Confirm local rules and your equipment specifications before your trip.