Smaller than you think, until you add a cool box. Phones, a lantern, a speaker, and head torches come to about 110Wh a day, which needs 39W at British summer sun hours, so a 60W folding panel covers it. A 12V cool box on its own wants 250 to 400Wh a day, and that takes the answer to about 200W and a battery to run it overnight.
| Item | Watt-hours per day |
|---|---|
| Two phones | 30 |
| Camera batteries | 30 |
| Rechargeable lantern | 20 |
| Bluetooth speaker | 20 |
| Head torches | 10 |
| 12V cool box (continuous) | 250 to 400 |
The first five come to about 110Wh between them. The cool box is two to four times everything else put together, which is why it is the one decision that changes the size of the kit.
Add up the watt-hours you use in a day, divide by the peak sun hours where you are camping, then divide by 0.7 to cover wiring and charging losses. Peak sun hours run about 5 in the southern US in summer, 3 to 4 in the shoulder seasons, and as low as 2 in a British winter.
So 110Wh ÷ 4 sun hours ÷ 0.7 = 39W for a British August weekend, and 510Wh ÷ 4 ÷ 0.7 = 182W once a cool box joins the trip. Round up to the panel you can actually buy, which is 60W and 200W.
This is the same method the campervan sizing guide uses. The only thing that changes is the list of loads, which is why a van ends up at 300W and a tent at 60W.
Not for phones and lights. A USB power bank charged during the day does that job, weighs little, and costs less than a battery with an inverter in it. You need a proper battery as soon as something runs overnight, because a cool box draws through the dark hours when the panel is making nothing at all.
Face it true south and prop it up rather than leaving it flat on the grass. The angle matters more than the last few watts of panel rating: at British latitudes propping a panel to about 43° collects 12% to 21% more midday sun in summer than lying it flat, and far more than that outside the summer months. Move it once at lunchtime if you are around, and keep it out of the shadow the tent throws as the afternoon goes on.