RV Power & Solar Sizing
Amp-hours a day from a real load list, the battery bank that carries it, the solar that replaces it, and why 50 amp is 3.3 times 30 amp.
What you use in a day
Size the bank first, then the solar
Work out what you actually use, size a battery bank to carry you through the days you expect without sun, and only then work out how much panel it takes to put that back. Doing it in the other order is how people end up with a roof full of panels feeding a battery that was never the constraint.
Panel ratings are laboratory figures. Flat on a hot roof, slightly dusty, at whatever angle the parking spot gave you, an array delivers around 75% of its rating — panels lose output as they heat, which is why summer is less of a win than it sounds. Anything through the inverter costs another 15% on top.
30 amp against 50 amp
| Service | Volts × amps | Watts available |
|---|---|---|
| 15 A household outlet | 120 × 15 | 1,800 W |
| 20 A household outlet | 120 × 20 | 2,400 W |
| 30 A RV service | 120 × 30 | 3,600 W |
| 50 A RV service | 240 × 50 | 12,000 W |
A 50 amp service is not 1.7 times a 30 amp one. It is 3.3 times, because 50 amp gives you two 120-volt legs where 30 amp gives you one. That single fact decides whether you can run the air conditioner and the microwave at the same time.
What AC appliances draw
| Appliance | Watts | Share of a 30 A service |
|---|---|---|
| Air conditioner (13.5k BTU), running | 1,400 | 39% |
| Air conditioner, starting surge | 2,800 | 78% |
| Microwave | 1,200 | 33% |
| Electric water heater element | 1,400 | 39% |
| Coffee maker | 900 | 25% |
| Hair dryer | 1,500 | 42% |
| Toaster | 1,100 | 31% |
| Electric kettle | 1,500 | 42% |
| Converter/charger at full output | 700 | 19% |
| Residential fridge | 150 | 4% |
Things worth knowing
- Size the bank from what you use, then size the solar from the bank. Doing it the other way round is how people end up with four panels feeding a battery that was never the problem.
- A 30 amp service is 3,600 watts and a 50 amp service is 12,000, because 50 amp gives you two legs rather than one. Running an air conditioner and a microwave together is comfortable on 50 and impossible on 30.
- Lithium wins on charge acceptance more than on capacity. Lead-acid slows to a crawl over about 80% full, so on a short winter day it never actually reaches the figure on the label.
- The inverter costs power simply by being on. If yours idles at most of an amp and you leave it on all evening, that is a real load with nothing plugged into it.
- Panels are rated in laboratory conditions. Flat on a hot roof, at the wrong angle, slightly dusty, a 400 watt array delivers closer to 300 — which is what the derate here assumes.
How to use
- Set hours per day for each load — the amp-hours update as you go.
- Pick a battery chemistry and how many days you want without sun or shore power.
- Read the rated bank size, the panel wattage to break even, and the generator alternative.
Frequently asked questions
How many amp-hours does an RV use per day?
A modest boondocking setup with LED lights, a 12 volt fridge and some charging runs 50 to 90 amp-hours a day. Add a furnace blower in real cold or an inverter left switched on and it climbs fast.
Should I size the battery or the solar first?
The battery. Work out what you use, size a bank to carry the days you expect without sun, and only then size the panel to replace it. The other order is how people end up with a roof full of panels feeding a battery that was never the constraint.
How much solar do I need?
Around 340 watts to replace 100 amp-hours a day on a five-sun-hour day. Panel ratings are laboratory figures — flat on a hot roof at whatever angle the parking spot gave you, an array delivers about three quarters of its rating.
What is the difference between 30 amp and 50 amp service?
More than the numbers suggest. A 30 amp service is one 120 volt leg, so 3,600 watts. A 50 amp service is two legs, so 12,000 — three and a third times as much, not one and two thirds. That is what decides whether the air conditioner and microwave can run together.
Is lithium worth it?
Often, but usually for charge acceptance rather than capacity. Lead-acid slows to a crawl above about eighty per cent full, so on a short winter day it never actually reaches the number on the label. Lithium takes charge fast right to the top.
Does the inverter cost anything when nothing is plugged in?
Yes. A large inverter idles at most of an amp simply for being switched on. Left on all evening that is a real load with nothing connected to it, and switching it off between uses is free capacity.
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