It is the question every motorhome owner asks, usually on a dark pitch somewhere in the Lake District with the fridge humming and the lights on and a nagging feeling that something is about to go flat.

The honest answer is: it depends. But that is not as unhelpful as it sounds, because the things it depends on are easily calculated. Work them out once and you will never be caught short again.

There are three variables. How much battery you have. What you are running. And how much solar is putting back in. Get a handle on all three and the answer becomes a number, however it's still a guess as we can never be certain how much sun we're going to get wherever we are.

How much motorhome leisure battery do you actually have?

Your motorhome leisure battery has a stated capacity in amp-hours. A typical motorhome might have a single 100Ah battery. But that number is not the whole story, because the type of battery determines how much of that capacity you can actually use.

Lead Acid batteries, the most common type in motorhomes, should not be discharged below around 50 per cent without shortening their life. So a 100Ah Lead Acid gives you roughly 50Ah of usable capacity. At 12 volts, that is around 600 watt-hours.

Lithium batteries (usually LiFePO4) can be discharged to around 90 per cent of their rated capacity. The same 100Ah in lithium gives you 1,080 watt-hours; nearly double, from the same rated size, at a fraction of the weight.

This is why two motorhomers with the same battery rating can have entirely different experiences off-grid. The type of battery matters as much as the size.

What are you actually running?

Your daily consumption in watt-hours is the other half of the equation. Most people underestimate it because the fridge runs constantly and invisibly, while everything else is obvious.

A compressor fridge typically draws around 45 watts but only runs intermittently. Over 24 hours, its effective average draw accounts for roughly 450 watt-hours. Add LED lighting for a few hours, phone charging, a water pump, and you are typically somewhere between 200 and 400 watt-hours per day for modest use. Add a laptop, a TV, or a CPAP machine and that figure climbs quickly.

The sum of your daily consumption is the number your battery has to cover. Divide your usable watt-hours by your daily consumption, and you get your answer.

A 100Ah lithium battery with 1,080Wh usable, running a daily consumption of 300Wh, gives you roughly 3.6 days of autonomy without any charging input. The same battery size but Lead Acid gives you about two days.

Use the calculator and find out precisely how long your motorhome leisure battery will last.

Rather than doing that arithmetic in your head, we built a calculator that does it for you. Tick your appliances, enter your battery size and type, and it works out your daily consumption and your days of autonomy. It also shows you how solar changes the picture across three seasonal scenarios: UK winter, UK summer, and Southern Europe.

How does solar affect how long the battery lasts?

Solar extends your off-grid time by putting energy back into the battery during daylight hours. Whether it extends it enough depends on three things: your panel wattage, how many useful sun hours you get in a day, and what the weather is doing.

A 200W panel in UK summer might get four hours of reasonable sun. That generates 800 watt-hours on a good day. If your consumption is 300Wh, your panel generates more than you use and your battery slowly tops up. You could stay off-grid indefinitely in those conditions.

The same panel in UK winter might generate 200 to 300Wh on a decent day. If your consumption is still 300Wh, you are running at roughly break-even and any grey days put you in deficit. This is why people with perfectly adequate summer setups find themselves on hookup from October onwards.

The calculator lets you apply a weather condition; good, mixed, or poor; which adjusts the generation figures realistically. The seasonal comparison table shows you where your setup holds up and where it does not.

Why don't my three-pin sockets work?

The three-pin sockets in a motorhome only work when you are connected to a mains hookup at a campsite, or when you have an inverter fitted and switched on. The habitation area runs on 12V from the leisure battery when you are off hookup. Without an inverter to convert 12V DC to 240V AC, the sockets are dead.

If you are on hookup and the sockets still do not work, check the RCD and circuit breakers in the habitation consumer unit, usually found in a cupboard or under the bed. A tripped RCD is the most common cause, particularly after a previous owner or hire customer has overloaded the circuit.

Can I run a hair dryer or kettle off my battery?

On hookup, yes within the limits of the pitch amperage. Most UK sites offer 10 or 16 amp hookup. At 10 amps you have around 2,400 watts available, which is enough for a hair dryer or a kettle but not both at the same time.

Off hookup it is a different matter. A hair dryer typically draws 1,500 to 2,000 watts. Running one through an inverter from a leisure battery will drain most basic setups in minutes and risks damaging the battery in the process. Most motorhomers off-grid use a 12V travel kettle or the gas hob instead, and have large battery banks with an inverter for the hairdryer.

What is the difference between Lead Acid and lithium?

Lead Acid is the traditional leisure battery. It's Victorian technology but It is reliable, well understood, and costs less upfront. The trade-off is usable capacity: you should only discharge it to around 50 per cent to protect the cells, so half the rated capacity is effectively unavailable.

Lithium (LiFePO4) can be discharged to around 90+ per cent of its rated capacity, charges significantly faster, weighs considerably less, and lasts far longer in cycle terms. The upfront cost is migh higher than Lead-Acid at the same amp-hour rating.

For regular tourers who rely on solar or spend time away from hookup, lithium tends to pay for itself over time through better performance and longer service life. If you are replacing worn-out Lead-Acid batteries, it is worth pricing up the switch rather than simply replacing like for like.

How do I know if my solar setup is big enough?

Use the calculator above. Enter your appliances, your panel wattage, your battery size and type, and your typical weather conditions. The output tells you your daily consumption, your generation across three seasonal scenarios, and how many days of autonomy you have without any sun at all.

If the results show a consistent deficit — your panel is not generating enough to cover what you use — the options are to reduce consumption, add more panel, or upgrade the battery to increase autonomy. Often the most cost-effective first step is understanding exactly where the watt-hours are going, which is what the calculator shows you.

Built a setup you are happy with? Or still trying to work out why the battery keeps going flat? Post in the MotorhomeFun electrical and solar forum and the community will help.