From Novice to Off-Grid Pro
How to Calculate Your Daily Amp-Hours and Solar Needs
This is a rough guide and gives the principles of calulating power consumption and Battery and solar needs to support you van demands.
There is a distinct kind of freedom that comes with parking up on a remote clifftop aire, watching the sunset over the Atlantic, and realising you don't need a campsite hook-up to keep the lights on.
Energy autonomy, often called off-grid touring, is becoming increasingly popular among motorhome and campervan owners. More travellers are choosing to spend nights away from campsites with electric hook-ups, seeking greater freedom and flexibility.
However, escaping the campsite power post means moving beyond guesswork. If you've ever experienced the dreaded sinking feeling of a low-voltage warning appearing on your control panel late at night, it's time to understand your vehicle's energy budget.
The good news is that you don't need an engineering degree. With a basic understanding of watts, watt-hours and amp-hours, you can estimate your daily consumption, work out an appropriate battery size and understand how much solar generation you need.
Here's a beginner-friendly guide to calculating your daily energy use and designing a more reliable off-grid electrical system, even when the weather isn't cooperating.
1. Demystifying the Math: Watts vs. Watt-Hours vs. Amp-Hours
Before sizing your battery or solar array, it's worth understanding three basic electrical terms.
- Watts (W): The rate at which an appliance uses electrical power while operating. A 60W appliance consumes power at a rate of 60 watts when it is running.
Watt-hours (Wh): The amount of energy consumed over time. The basic calculation is:
Watts × hours used = watt-hours
Amp-hours (Ah): A measure of electrical charge commonly used to describe battery capacity. For a nominal 12V motorhome system, you can make a quick estimate using:
Amp-hours ≈ watt-hours ÷ battery voltage
For simple calculations, 12V is often used as the nominal voltage. In reality, a 12V battery does not remain at exactly 12.0V, and real-world charging and conversion losses also need to be considered.
For example, a 60W appliance running for two hours consumes:
60W × 2 hours = 120Wh
Using the simple 12V approximation:
120Wh ÷ 12V ≈ 10Ah
So you can think of that appliance as using roughly 10Ah from a 12V battery system.
2. Step-by-Step: Building Your Daily Energy Audit
The best way to size an off-grid electrical system is to start with your actual energy consumption.
Make a list of every electrical device in your motorhome and estimate how long each one operates during a typical day. Remember to include things that are easy to overlook, such as heating systems, USB chargers, control electronics and inverter standby consumption.
Here's an illustrative example for a couple touring without a campsite electrical hook-up:
| Appliance / Device | Power Draw | Daily Usage | Estimated Daily Energy |
|---|---|---|---|
| 12V Compressor Fridge | ~40W while running | ~12 equivalent hours* | ~480Wh |
| LED Interior Lighting | ~20W total | 4 hours | ~80Wh |
| Water Pump & Heater Controls | ~30W | 0.5 hours | ~15Wh |
| Phone & Tablet Charging | ~15W | 3 hours | ~45Wh |
| TV / Laptop | ~60W | 2 hours | ~120Wh |
| Diesel Heater Fan | ~25W average | 8 hours | ~200Wh |
| Total | ~940Wh/day |
*The fridge figure assumes approximately 12 hours of equivalent compressor operation at 40W over a 24-hour period. A fridge doesn't normally consume its rated running power continuously, so actual consumption will vary with ambient temperature, thermostat setting, ventilation, contents and model.
Using the simple nominal 12V calculation:
940Wh ÷ 12V ≈ 78Ah/day
That's a useful starting point, but your real battery requirement may be somewhat different because battery voltage, wiring losses, inverter losses and appliance efficiency all affect the result.
The most accurate approach is to measure your actual consumption over several days using a suitable battery monitor or energy meter.
3. Sizing Your Battery Bank: Lead-Acid vs. Lithium
Knowing your daily energy consumption is only half the equation. You also need enough stored energy to get through periods when your solar system isn't producing much.
Lead-acid and AGM
Traditional lead-acid and AGM batteries are generally designed with a shallower usable depth of discharge if you want to achieve good cycle life.
A commonly used design assumption is to use around 50% of the rated capacity.
For example, if your motorhome uses approximately 80Ah per day, a 160Ah lead-acid bank would provide roughly 80Ah of nominal usable capacity under that conservative assumption.
This isn't a hard safety limit, battery life depends on the battery type, discharge rate, temperature, maintenance and manufacturer recommendations—but regularly discharging lead-acid batteries very deeply can significantly reduce their service life.
LiFePO₄ lithium batteries
Lithium iron phosphate (LiFePO₄) batteries generally offer considerably more usable capacity than lead-acid batteries for the same nominal capacity. They are also substantially lighter and maintain a more stable voltage during discharge.
Many lithium systems are designed to use around 80–90% of their rated capacity as a practical target, although the permitted depth of discharge varies by manufacturer and battery model.
For an 80Ah daily requirement, a 100Ah LiFePO₄ battery may therefore provide approximately one day's energy under suitable conditions, while a 200Ah battery gives considerably more reserve.
Don't choose a battery solely from its advertised Ah rating, however. Check the manufacturer's specifications for recommended depth of discharge, continuous discharge current, low-temperature charging limits and battery-management-system protection.
4. Matching Your Solar Array to Real-World Weather
Solar panels are an excellent way to replenish your batteries while travelling, but their output is highly dependent on location, season, orientation, shading and weather.
A useful starting point is:
Daily solar energy ≈ panel wattage × equivalent peak-sun-hours × system efficiency
For example, a 400W solar array receiving the equivalent of three hours of peak sunlight would theoretically receive:
400W × 3 hours = 1,200Wh
The amount actually delivered to the battery will be lower because of factors such as controller, cable and battery-charging losses, panel temperature, shading and the way the battery is being charged.
Summer vs. winter
A 200W or 400W array can produce very different amounts of energy depending on where and when you're travelling.
A sunny summer day in southern Europe can produce dramatically more solar energy than a short, cloudy winter day in the UK. Conversely, a clear winter day can produce considerably more than a heavily overcast one.
This means there is no universal percentage reduction that can accurately describe winter solar production.
For UK and northern European touring, particularly during late autumn and winter, solar should therefore be treated as a variable energy source rather than a guaranteed daily supply.
Adding more panel capacity can help make better use of limited winter sunlight, but there is no universal "minimum" of 300W or 400W that will guarantee energy independence. The appropriate array size depends on your daily consumption, location, season, available roof space and how many consecutive low-solar days you need to tolerate.
An MPPT (Maximum Power Point Tracking) solar controller can also improve energy harvesting compared with simpler PWM controllers, particularly where panel voltage is significantly higher than battery voltage.
For genuine winter off-grid touring, it's sensible to consider solar as part of a wider charging strategy that may also include alternator charging via a suitable DC-DC charger, mains charging when available, or another backup source.
5. Don't Forget Your Battery Reserve
Solar doesn't just need to replace your average daily consumption. Ideally, your battery should also provide enough reserve to cope with periods of poor weather.
Suppose your motorhome consumes around 80Ah per day and you want to remain off-grid for two days without meaningful solar input.
You'd need approximately:
80Ah × 2 days = 160Ah of usable energy
That figure would need to be increased to account for the battery technology and your preferred depth of discharge.
This is why battery and solar capacity shouldn't be designed independently. A large battery without enough generation may eventually run flat, while a large solar array feeding a very small battery may have limited benefit once the battery reaches full charge.
💡 Pro Tip: Watch Out for Phantom Loads
Even when everything appears to be switched off, some motorhomes have continuous standby loads.
Gas alarms, control systems, entertainment equipment, USB chargers, Wi-Fi equipment and inverter standby consumption can all draw power around the clock. The actual figure varies considerably between vehicles, so don't assume that every motorhome has a fixed 3–5A parasitic load.
The easiest way to discover your own hidden consumption is to use a battery monitor or suitable DC current measurement device.
And remember how quickly a small continuous load adds up:
1A × 24 hours = 24Ah per day
At approximately 12V, that's around:
24Ah × 12V ≈ 288Wh per day
A continuous 3A load would therefore consume approximately 72Ah—or around 864Wh—in 24 hours.
That's potentially almost as much energy as the entire illustrative daily consumption in this article.
A battery monitor such as the Victron SmartShunt can make these invisible loads much easier to identify by showing battery current, voltage and accumulated energy use.
6. The Golden Rule: Measure, Don't Guess
These calculations are useful for planning, but your motorhome's actual energy consumption is what ultimately matters.
Two seemingly identical vehicles can have completely different energy requirements. One owner might use a small compressor fridge and a few LED lights, while another might run a television, laptop, Starlink or other communications equipment, an inverter, electric cooking equipment and heating controls.
If you're serious about extended off-grid touring, monitor your battery for several normal days and record:
- Your average daily Ah consumption
- Your highest daily consumption
- How much solar energy you actually recover
- Your battery's state of charge
- Your overnight consumption
- Any significant continuous standby loads
Once you have those figures, sizing your battery and solar array becomes much less of a guessing game.
The Bottom Line
For our illustrative 940Wh daily load, the simple calculation gives approximately 78Ah per day at a nominal 12V.
From there, you can work backwards:
Daily energy use → usable battery capacity → required solar generation → backup charging strategy
The goal isn't necessarily to install the biggest battery or the most solar panels you can fit on the roof. It's to build a system that matches how you actually travel, the seasons in which you travel and how much independence you want from campsite hook-ups.
Get those numbers right, and that remote clifftop pitch becomes considerably less stressful, because you can enjoy the sunset without wondering whether the battery will survive until breakfast.
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