Build an off grid load list by writing down every device you plan to run, multiplying each one’s wattage by the hours it runs per day, and adding the results into a single daily watt hour total. That number drives every other decision in the system.
An off grid load list is the one document that turns guesswork into arithmetic. Skip it and you end up sizing panels against a feeling, which is how people arrive at a battery bank that dies at three in the morning. Get it right and the watt hour target you are aiming at stops being a mystery.
Quick Answer
List every load, note its running watts, estimate daily runtime in hours, and multiply. Sum the results for a daily watt hour figure. Add a margin of twenty to thirty percent for losses and growth, then size batteries, panels, and inverter against that padded number rather than the raw total. With the design settled, commissioning the system is the sequence that confirms it performs the way the load list assumed.
Key Takeaways
Watts describe how hard a device pulls. Watt hours describe how much energy it consumes over a day, and only the second number sizes a system.
Runtime estimates cause more error than wattage estimates. People know a fridge is roughly 100 watts and badly misjudge how many hours it actually runs.
Surge loads size the inverter. Daily energy sizes the battery bank and the array. These are two separate problems and one list has to answer both.
Step by Step: Building the Load List
Work through these steps in order. Each one feeds the next, and skipping ahead to panel counts before the list is complete is what produces undersized systems.
1. Write down every load, including the small ones. Lights, phone chargers, a router, a water pump, a laptop, a fan. Small constant loads add up faster than large occasional ones.
2. Find the running wattage for each device. Check the label, the manual, or the specification sheet. Labels usually list maximum draw rather than typical draw, so treat the plate figure as a ceiling.
3. Estimate daily runtime honestly. A refrigerator compressor cycles, so it might run eight hours out of twenty four rather than the full day. A coffee maker pulls hard for six minutes and nothing for the rest.
4. Multiply watts by hours for each row. A 60 watt device running four hours consumes 240 watt hours. Do this for every line before adding anything together.
5. Sum the column. The total is your daily energy demand in watt hours. This single number is what the rest of the system exists to deliver.
6. Note the surge draw separately. Motors and compressors pull several times their running wattage on startup. Record the largest surge alongside the total, because inverter sizing answers to peaks rather than to daily energy.
7. Add a margin. Twenty to thirty percent covers wiring losses, inverter inefficiency, battery round trip losses, and the loads you have not thought of yet. Size against the padded figure.
What Belongs on the List and What Gets Missed
The obvious loads land on every list. Lighting, a fridge, a laptop, and a phone charger appear without prompting. The ones that get forgotten share a trait, which is that they draw quietly and continuously.
Standby power tops that group. Televisions, microwaves, and anything with a clock or a remote receiver keep pulling a few watts around the clock. Five watts of standby across four devices costs almost 500 watt hours per day.
Inverter idle draw belongs on the list too. A large inverter left switched on consumes power simply by being awake, often twenty to fifty watts. Over a full day that alone can rival a refrigerator.
Water pumps are the classic omission at cabins. They run in short bursts, which makes them feel free, and they pull hard while running. Well pumps in particular need a surge figure recorded next to their running watts.
Seasonal loads deserve their own column rather than an average. A fan matters in July and a heater matters in January, and the worst month sets the system size. Averaging the year together hides the month that actually breaks the bank.
Anything with a heating element is the load to scrutinize hardest. Kettles, toasters, hair dryers, and space heaters convert electricity to heat at roughly one to one, so they consume enormous energy for short runtimes. Many off grid builds move those loads to propane instead.
Watts, Watt Hours, and Amp Hours in One Place
Three units appear on every off grid spec sheet and people mix them up constantly. Keeping them separate is most of the battle.
| Unit | What it measures | Where it shows up |
|---|---|---|
| Watts (W) | Rate of draw at a single moment | Device labels, inverter continuous ratings |
| Watt hours (Wh) | Energy consumed over time | Daily load totals, battery capacity, panel output |
| Amp hours (Ah) | Charge capacity at a stated voltage | Battery labels, which is why voltage must accompany the figure |
| Surge watts | Brief peak draw at startup | Motor and compressor specs, inverter peak ratings |
Amp hours cause the most confusion because the number means nothing on its own. A 100 amp hour battery at 12 volts holds 1,200 watt hours. The same 100 amp hour label at 24 volts holds 2,400 watt hours.
Converting is straightforward. Multiply amp hours by system voltage to get watt hours, or divide watt hours by voltage to go the other way. Do the conversion once and work in watt hours from then on.
Device labels sometimes list amps instead of watts. Multiply amps by the supply voltage, usually 120 in North America, to get running wattage. A 3 amp appliance on a 120 volt circuit draws 360 watts.
Turning the Load List Into System Size
The padded daily total feeds three separate calculations. Each one answers a different question and none of them substitutes for the others.
Battery capacity comes first. Decide how many days of autonomy you want without sun, multiply the daily total by that figure, then divide by your usable depth of discharge. Lithium chemistries allow deeper discharge than lead acid, which changes the answer substantially and shapes how large the bank has to be.
Array size comes second. Divide the daily watt hour total by the peak sun hours your location receives in the worst month you plan to use the system. That gives the array wattage you need, and a sizing calculator handles the arithmetic once you have the load figure.
Inverter size comes third and ignores the daily total entirely. Add up the loads that might run simultaneously for the continuous rating. Check the largest surge against the inverter’s peak rating, then confirm with a dedicated inverter calculation before buying.
Charge controller sizing follows from the array, not from the loads. It answers to panel current and voltage rather than to household demand. That is a separate calculation with its own inputs.
Where Load Lists Go Wrong
Using label wattage as running wattage
Nameplate ratings state maximum draw under full load, which most appliances rarely reach. A microwave labeled 1,200 watts may be describing cooking output rather than input. Using label figures everywhere inflates the total and produces an expensive oversized system.
Guessing refrigerator runtime
Fridges are the single most misjudged load on any list. The compressor cycles on and off, so daily consumption depends on ambient temperature, door openings, and insulation quality. Manufacturer annual kilowatt hour ratings divided by 365 give a far better daily figure than a runtime guess, and DC models built for off grid use behave differently again.
Forgetting that inefficiency stacks
Energy leaks at every conversion. Batteries lose some on the round trip, inverters lose some turning DC into AC, and wiring loses some as heat. A twenty to thirty percent margin absorbs the stack, which is why the padded number rather than the raw one drives sizing.
Building the list for the average day
Systems fail on the bad day, not the typical one. A cloudy stretch in the worst month with the heaviest loads running is the scenario worth sizing against. Averaging across the year quietly designs a system that works most of the time.
Treating power tools like household loads
Tools run intermittently and surge hard, so their daily watt hours look small while their peak draw looks alarming. That combination stresses the inverter without touching the battery much, and running tools off grid becomes an inverter question rather than an energy question.
Related Reading
Useful next steps: the appliances that suit off grid living helps prune the list before you size anything, cabin scale system planning works through a complete example, and efficient appliances for solar homes covers the cheapest way to shrink a total, which is running less in the first place.
Frequently Asked Questions
How do I build an off grid load list if I do not know my appliance wattages?
Start with the label on each device, which lists either watts or amps. Multiply amps by supply voltage when only amps appear. For anything unlabeled, a plug in energy meter measures actual consumption over several days and gives a far better figure than any published estimate.
What margin should I add to the daily total?
Twenty to thirty percent is the common working range. That covers inverter inefficiency, battery round trip losses, wiring losses, and the loads you forgot. Builds that expect to add equipment later often use the higher end, since expanding an array costs more than oversizing it once.
Do I need to include phantom and standby loads?
Yes, and they matter more than their small wattage suggests. A device drawing five watts continuously consumes 120 watt hours per day, roughly what a laptop uses in several hours of work. Four such devices approach half a kilowatt hour daily before anything gets switched on deliberately.
How do surge watts change the load list?
Surge figures do not affect daily energy at all, because the peak lasts a second or two. They size the inverter instead. Record the largest single surge on the list and check it against the inverter’s peak rating, since a motor that cannot start will trip the system repeatedly.
Should I list amp hours or watt hours?
Work in watt hours. Amp hours depend on system voltage, so the same number means different amounts of energy at 12, 24, and 48 volts. Converting everything to watt hours early keeps the arithmetic consistent when battery or system voltage changes later.
How many days of autonomy should the battery bank cover?
Two to three days suits most builds with a reasonable array and a generator available. Sites with long cloudy stretches and no backup often plan for four or more. More autonomy means a larger bank, so the answer trades cost against how much a dark week actually costs you.
Can a solar generator replace a full load list?
No, though it simplifies the hardware side. A portable unit still has a fixed capacity and a fixed inverter rating, so you need the daily total and the surge figure to know whether it fits. Checking what a solar generator will actually run starts from the same list.
