Power Consumption Calculator: Daily Watt-Hours and Amp-Hours
Daily watt-hours = watts x hours switched on x duty cycle. A 55W compressor fridge running a 40 percent duty cycle over 24 hours uses 528 watt-hours, which is 41 amp-hours at 12V, and covering that one appliance alone takes about 156W of solar in 4.5 peak sun hours or a 100Ah LiFePO4 battery for two days with no charging.
Every planning question in a 12V build reduces to one number: how many amp-hours a day does this thing cost. Once you have it, the battery bank, the solar array and the charging strategy all follow. This calculator takes one appliance at a time and shows the same consumption three ways: as energy, as the battery capacity that stores it, and as the solar array that replaces it. Seeing all three at once is what usually settles a purchase decision.
Power consumption calculator
Enter the load once and read it three ways. Duty cycle is the fraction of the time the load genuinely draws power while it is switched on, which is 100 percent for a light and around 40 percent for a fridge on a mild day.
- Average current while running
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- Per week
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- Inverter loss included
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Why is duty cycle more important than watts?
Duty cycle is the fraction of the time a load actually draws power while it is switched on. Nameplate watts describe the peak, and for anything that cycles, the peak is not what empties a battery.
A compressor fridge is the clearest case. Its published draw might be 55W, but the compressor is not running continuously. It runs until the cabinet reaches temperature, stops, and starts again when the temperature drifts back up. On a mild day that is around 40 percent of the time. In a hot vehicle parked in the sun it can be 70 percent, and in an unusually cool cabin it can drop to 25 percent. The same appliance, the same setting, and a spread of nearly three to one in daily consumption.
The practical consequence is that a build sized from nameplate figures is either wildly overspecified or, more dangerously, sized from a summer figure and deployed in a heatwave. The fridge draw by ambient temperature chart gives duty cycle against ambient so you can pick a figure that matches where you actually travel rather than an average of everywhere.
Other loads that cycle in the same way: diesel heaters, which run a high draw glow plug on every start and then settle to a low fan draw; laptop chargers, which taper as the battery fills; water pumps, which run for seconds at a time; and anything thermostatic. Loads that do not cycle at all include lights, fans on a fixed speed, and satellite terminals, which is exactly why a satellite terminal at a modest 30W quietly becomes one of the largest items on a load list.
What does everything actually cost per day?
Published draw figures, typical duty cycles and the resulting daily amp-hours at 12.8V. AC rows include an 88 percent inverter conversion but not the inverter standby draw, which is listed as its own row because it belongs to the inverter rather than to any appliance.
| Load | Watts | Duty cycle | Hours on | Ah per day | Supply |
|---|---|---|---|---|---|
| Compressor fridge, 45 to 55 qt, 75F ambient | 55 | 35 to 45% | 24 | 35 to 46 | 12V |
| Compressor fridge, same unit, 95F ambient | 55 | 55 to 70% | 24 | 57 to 72 | 12V |
| Roof vent fan, low | 4 | 100% | 8 | 2.5 | 12V |
| Roof vent fan, high | 36 | 100% | 4 | 11.3 | 12V |
| LED interior lighting, 6 pucks | 9 | 100% | 4 | 2.8 | 12V |
| LED awning strip, 20 ft | 24 | 100% | 3 | 5.6 | 12V |
| Water pump, on demand | 90 | 100% | 0.25 | 1.8 | 12V |
| Laptop, 12V USB-C PD socket | 60 | 70% | 6 | 19.7 | 12V |
| Laptop, through a 2000W inverter | 60 | 70% | 6 | 22.4 | AC |
| Phone and tablet charging | 20 | 80% | 3 | 3.8 | 12V |
| Satellite internet terminal | 30 | 100% | 8 | 18.8 | 12V |
| Diesel heater, running average | 18 | 100% | 8 | 11.3 | 12V |
| Diesel heater, glow plug on start | 110 | 100% | 0.1 | 0.9 | 12V |
| CPAP, no humidifier | 30 | 100% | 7 | 16.4 | 12V |
| CPAP with heated humidifier | 75 | 60% | 7 | 24.6 | 12V |
| Induction burner, medium setting | 1200 | 100% | 0.33 | 35.1 | AC |
| Electric kettle, one boil | 1200 | 100% | 0.05 | 5.3 | AC |
| Microwave, 900W cooking power | 1400 | 100% | 0.1 | 12.4 | AC |
| Inverter idle, left switched on | 15 | 100% | 24 | 28.1 | AC |
| Camera and drone battery charging | 45 | 80% | 2 | 6.4 | AC |
| TV or monitor | 60 | 100% | 2 | 10.7 | AC |
| 12V air compressor, airing up four tyres | 480 | 100% | 0.25 | 9.4 | 12V |
Three rows in that table are worth arguing about. The two fridge rows are the same appliance at two ambient temperatures, and the difference between them is larger than most people's entire lighting budget. A better insulated unit such as the Dometic CFX2 45 Litre Portable Refrigerator and Freezer holds temperature with a lower duty cycle in heat, which is a real reduction in daily amp-hours rather than a specification detail, and over a long trip it is worth more than the purchase price gap.
The two laptop rows are the same laptop, once from a 12V USB-C Panel Mount Socket, 65W PD and QC3.0 (83W total) and once through an inverter. The difference is 2.7 amp-hours a day, which is modest. The inverter idle row is the one that matters: leaving a Renogy Pro 2000W Pure Sine Wave Inverter 12V to 120V switched on all day to serve that laptop costs 28 amp-hours whether the laptop is plugged in or not. Switch the inverter off and the whole line disappears. The inverter sizing calculator covers that trade in full.
How do you turn consumption into a purchase?
Two conversions, and the calculator does both. The battery figure is daily amp-hours multiplied by the days you want to coast, divided by the usable depth of discharge of the chemistry, which is 85 percent for LiFePO4. The solar figure is daily watt-hours divided by peak sun hours, divided by the 0.75 real-world derate that separates panel nameplate from delivered energy.
| Load | Ah per day | LiFePO4 for 2 days | Solar at 4.5 sun hours | Cost of the battery route | Cost of the solar route |
|---|---|---|---|---|---|
| Fridge at 75F | 41 | 97 Ah | 156 W | $300 to $400 | $120 to $200 |
| Fridge at 95F | 65 | 153 Ah | 247 W | $450 to $700 | $190 to $310 |
| Roof fan, 6 hours on high | 17 | 40 Ah | 64 W | $130 to $180 | $50 to $90 |
| Laptop, 6 hours, USB-C | 20 | 47 Ah | 75 W | $150 to $210 | $60 to $100 |
| Satellite terminal, 8 hours | 19 | 45 Ah | 71 W | $140 to $200 | $55 to $95 |
| Inverter left on all day | 28 | 66 Ah | 107 W | $200 to $290 | $80 to $140 |
| Diesel heater, 8 hours | 11 | 26 Ah | 43 W | $90 to $120 | $35 to $60 |
Read the last two columns together. On a straight per amp-hour basis, solar is consistently cheaper than battery, which is the arithmetic behind the usual advice to buy panel before you buy capacity. The catch is that solar only pays out when the sun is up, which is exactly what the battery is for. Both columns exist because a real build needs both, and the balance between them is set by how you travel rather than by which is cheaper. Prices are typical street ranges and change constantly, so treat them as proportions rather than quotes.
Take the daily figure across to the battery bank calculator to size the whole system rather than one appliance, and to the solar array calculator to see what a given roof can put back each day.
How do you check the number against reality?
Published figures are a starting point. Measurement is the answer, and it is cheap.
A AstroAI Digital Clamp Meter, AC/DC Current and Voltage (4000 count) clamps around the positive lead and reads current without breaking the circuit, which is the fastest way to find what a single appliance genuinely draws right now. That is how people discover that a cheap inverter idles at 22W rather than the 8W in the manual, or that an accessory somebody wired in years ago is pulling 1.5A around the clock.
For daily totals rather than instants, a shunt-based monitor such as the Victron SmartShunt 500A Battery Monitor counts every amp-hour in and out of the bank, so you can read a real day's consumption instead of modelling one. It also shows the history, which is how a parasitic draw gets found. The battery monitor roundup covers the options, and fitting one is the single change that converts a plan on this page into a gauge you can read.
One thing measurement will not do for you is set the fuse. Average consumption and peak current are unrelated: a water pump averaging under 2 amp-hours a day still pulls 8A while running, so it gets 12 AWG cable and a 15A fuse sized for the peak. Everything on this page is arithmetic drawn from published manufacturer specifications, offered as researched guidance rather than an electrical certification, and any lithium installation should be inspected by a qualified installer before it carries load.
Where to go next
Frequently asked questions
How do I calculate daily power consumption in amp-hours?
Multiply watts by the hours it runs and by the duty cycle to get watt-hours, then divide by 12.8 to convert to amp-hours at 12V. A 55W fridge switched on for 24 hours at a 40 percent duty cycle uses 528 watt-hours, which is 41 amp-hours. Duty cycle is the part people skip, and on a fridge it changes the answer by a factor of two between a cool day and a hot one.
What is duty cycle and why does it matter more than watts?
Duty cycle is the fraction of the time a load actually draws power while it is switched on. A fridge compressor cycles, a heater cycles, a laptop charger tapers as the battery fills. Nameplate watts describe the peak, not the average, so a 55W fridge left on for 24 hours does not use 1,320 watt-hours. It uses 400 to 900 depending on ambient temperature, and that spread is the whole planning problem.
How much does a 12V fridge use per day?
Roughly 35 to 46 amp-hours a day for a 45 to 55 quart compressor unit at a comfortable 75F ambient, and 57 to 72 amp-hours at 95F. Insulation quality, how often the lid opens, whether the unit sits in sun or shade and whether the contents went in pre-chilled all move the figure. It is the largest single continuous load in most builds, which is why battery sizing usually starts with it.
Does an inverter change the consumption figure?
Yes, twice. It adds a conversion loss of roughly 10 to 15 percent on everything that passes through it, so a 240 watt-hour laptop charge costs about 273 watt-hours at the battery. It also draws 8 to 25 watts continuously just to stay switched on, which over 24 hours is 15 to 47 amp-hours. That standby figure is frequently larger than the load the inverter was left on for.
How do I measure what my gear actually draws?
A DC clamp meter around the positive lead gives you the instant draw without breaking the circuit, which is the fastest way to check a single appliance. For daily totals, a shunt-based battery monitor counts every amp-hour in and out of the bank, so you can read consumption over a real day rather than modelling it. Published figures are a starting point and measurement is the answer.
What is the cheapest way to cut consumption?
Switch the inverter off when nothing needs it, which typically saves 15 to 40 amp-hours a day on its own. Then move the laptop to a 12V USB-C socket to skip the conversion loss entirely. Then give the fridge shade, ventilation clearance and pre-chilled contents, which can cut its draw by a quarter. All three cost almost nothing and together they usually beat adding a solar panel.
Electrical safety: 12V is low voltage but not low energy. A lithium house battery can push several hundred amps into a short circuit, and undersized or unfused cable is one of the most common causes of vehicle fires in DIY builds. Every circuit must be fused at the source of power, within a few inches of the battery terminal, at or below the ampacity of the smallest conductor it protects. Size cable for voltage drop as well as ampacity, torque terminals to specification, and have a lithium installation inspected by a qualified installer before it carries load. The figures on this page are researched guidance, not a substitute for ABYC E-11 or your component manufacturer's installation manual.
How we choose: we compare published manufacturer specifications, standards documents including ABYC E-11, and verified owner reviews. We do not test gear in person. Vehicle payload, roof load and charging limits vary by model and year, so confirm yours against the door jamb sticker and the owner's manual rather than any number published here.
Totalling your own build weight against your payload? The Overland Build & Power Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.