How to Calculate Your Off-Grid Daily Energy Needs

The foundation of any off-grid system is one number: your actual daily energy usage in watt-hours. Get this number wrong and everything downstream fails: your battery bank is too small, your solar array cannot keep up, and you run out of power on the third cloudy day. Get it right and the rest of the system design falls into place.

Our off-grid load calculator automates this process. This guide explains the methodology so you understand what the numbers mean and can catch mistakes that a calculator cannot, like forgetting the well pump or underestimating winter lighting hours.

Common Appliance Power Draws

Start with the devices you plan to run. Power draw in watts tells you how fast energy is consumed; hours of daily use tells you the total energy budget in watt-hours (Wh). The watts and hours in this table are example ranges for illustration, not measured or printed figures, except the 12V compressor fridge row, which is Dometic's print for one model: read your own appliance's nameplate or measure it. Each Daily Wh range is the low watts times the low hours and the high watts times the high hours.

Notice the gap in fridge energy use. Dometic prints an energy consumption of 0.98Ah per hour at 12V DC for its CFX3 35 compressor fridge, at 32 C ambient and 4 C inside: 0.98 x 24 x 12 = 282 Wh a day. A standard household AC fridge running through an inverter comes to 800-2,400 Wh/day in the table's example ranges. The fridge choice alone can make or break a modest off-grid system.

Also notice the space heater at the bottom of the table. Electric resistance heating is the largest load in the table. A 1,500W heater running 8 hours consumes 1,500 x 8 = 12,000 Wh, more than three times the 3,802 Wh/day of the worked example's cabin below. Off-grid heating should come from propane, wood, or a heat pump, not electric resistance. Our electrical load calculator does not weigh loads against a system size: it gives the demand-factored service load of a dwelling by the optional method that municipal worksheets print, and takes no battery or panel size.

The Load Calculation Process

Follow these steps to build your daily energy budget. Write everything down: the list becomes your system design document.

1. List every device you will run. Walk through your space room by room. Include lighting, appliances, electronics, pumps, and anything that plugs in or wires to the battery bank. Do not forget "invisible" loads: modem, router, smoke detectors, CO detectors, security cameras, battery chargers for tools, and well pump controllers that draw standby power 24/7.
2. Record the wattage of each device. Check the nameplate label, the product manual, or measure with a Kill-A-Watt meter. Nameplate watts are maximum draw. Many devices use less during normal operation. A fridge labeled "200W" runs its compressor intermittently, so its average draw over 24 hours is much lower. For intermittent loads, use the duty-cycle adjusted average, not the peak. The Department of Energy provides average wattage estimates (page no longer available at time of checking) for common household appliances if you cannot find the nameplate.
3. Estimate daily hours of use for each device. Be honest, not optimistic. Lights that you think run "3-4 hours" may run 5-6 hours in winter when it gets dark early (example hours, not measured figures). The fridge runs 24/7 but the compressor cycles: its daily runtime depends on ambient temperature and how often you open the door, so measure it with a meter over a full day.
4. Multiply watts by hours to get daily watt-hours. This is the core calculation: Watts x Hours = Wh per day. Sum all devices to get your total daily energy budget. The two-bedroom cabin in the worked example below comes to 3,802 Wh/day without electric heating; a larger home with more appliances comes to more, by its own list.
5. Add a safety margin. Parasitic losses (inverter standby draw, wire losses, charge controller overhead) take a share of your battery energy before it reaches your devices; measure them on your own system rather than assume a figure. Battery round-trip efficiency costs more: Victron prints a round-trip energy efficiency of 92% for a LiFePO4 battery and 80% for the average lead-acid battery, a loss of 100 - 92 = 8% and 100 - 80 = 20%. This guide adds 20% to the total for a LiFePO4 system and 25% for lead-acid, an assumption for illustration, not a measured figure. The off-grid load calculator applies the inverter's efficiency separately, so this margin is for the battery, the wiring and the loads you forgot.
6. Adjust for seasonal variation. If your winter usage differs markedly from summer (more lighting, less refrigeration, possible electric blankets), calculate both a winter and summer daily budget. Size your battery bank and solar array for the harder season: usually winter, because days are shorter and loads are often higher.

Worked Example: Two-Bedroom Off-Grid Cabin

A couple in rural Vermont is building a year-round off-grid cabin. They plan to heat with a wood stove and cook with propane, so electric heating is not in the load calculation. Here is their appliance audit (its watts and hours are assumptions for illustration, not measured figures):

Kitchen: 12V DC fridge (50W avg, 10 hrs/day = 500 Wh), microwave (1,000W, 15 min/day = 250 Wh), coffee maker (900W, 10 min/day = 150 Wh).

Living area: LED lighting (4 fixtures x 12W x 6 hrs = 288 Wh), Wi-Fi router (12W x 24 hrs = 288 Wh), 2 laptops (50W each x 4 hrs = 400 Wh), phone charging (15W x 4 hrs = 60 Wh).

Bedroom/bath: LED lighting (2 fixtures x 10W x 3 hrs = 60 Wh), bathroom exhaust fan (30W x 1 hr = 30 Wh).

Utility: Well pump (750W x 1.5 hrs = 1,125 Wh), washing machine (400W, 1 hr a load, 3 loads/week averaged: 400 x 3 / 7 = 171 Wh/day), circulation pump for wood stove hydronic loop (60W x 8 hrs winter = 480 Wh).

Total daily load: 500 + 250 + 150 + 288 + 288 + 400 + 60 + 60 + 30 + 1,125 + 171 + 480 = 3,802 Wh. With the guide's 20% safety margin for a LiFePO4 system (step 5's assumption): 3,802 x 1.2 = 4,562 Wh/day.

This is a realistic number for a cabin without electric heating or air conditioning. The well pump is the largest single load: if they had municipal water, the total drops to 3,802 - 1,125 = 2,677 Wh before margin. The microwave and coffee maker are high-wattage but short-duration, so their daily impact is moderate. As outlined in our solar battery bank sizing guide, you can convert this daily energy figure directly into battery and panel requirements.

From Energy Budget to System Size

Battery bank sizing starts with autonomy: how many days of stored energy you need without sun. This example takes 2.5 days, an assumption for illustration, not a measured figure. The Vermont cabin needs 4,562 Wh/day x 2.5 days = 11,405 Wh of stored energy. The cabin's twelve loads are more than the off-grid load calculator's eight appliance rows, so enter the 4,562 Wh as one row (4,562W for 1 hour); its other inputs stay at the calculator's defaults unless this guide names them. For a 48V LiFePO4 bank at 90% DoD (an assumption for illustration, not a measured figure), 2.5 days of autonomy and the calculator's default 92% inverter efficiency, it sizes the bank at 269Ah. Its Days of Autonomy field takes whole days (1 to 10, in steps of 1), so on the page enter 2 or 3: at 2 days it returns 215Ah and at 3 days 323Ah, and the 2.5-day 269Ah lies between them. It sizes the bank at 51.2V, the nominal voltage of a 48V LiFePO4 bank, and divides by the inverter's efficiency as well as the depth of discharge, so its figure is not 11,405 Wh simply divided by 48V and by the depth of discharge. Two 48V 150Ah LiFePO4 batteries in parallel provide 2 x 150 = 300Ah, 300 - 269 = 31Ah above the 2.5-day figure; the 3-day 323Ah is 323 - 300 = 23Ah more than the pair gives. Use the solar battery count calculator to run your own numbers.

Solar array sizing depends on your location's peak sun hours, which our solar panel output guide explains in depth. NASA POWER's 2001-2020 climatology for Burlington, Vermont, prints 3.88 kWh per square meter a day over the year on a panel tilted at the latitude, 2.42 in January and 1.90 in December (one kWh per square meter a day is one peak sun hour). At 2.42 peak sun hours and the cabin's 4,562 Wh/day, the off-grid load calculator sizes the array at 2,576W: it divides the load by the 92% inverter efficiency and by the sun hours times its 14% system loss (0.86), its 98% charge controller factor and its 0.94375 cell-temperature factor. Its Peak Sun Hours input starts at 2, so December's 1.90 is below it; the same arithmetic at 1.90 gives 4,562 / 0.92 / (1.90 x 0.86 x 0.98 x 0.94375) = 3,281W. In 400W panels (a panel size taken for illustration), January's 2,576W is 2,576 / 400 = 6.44, so 7 panels (2,800W), and December's 3,281W is 3,281 / 400 = 8.2, so 9 panels (3,600W).

Inverter sizing is driven by peak load, not daily energy. The largest loads that can run together are the well pump (750W), the microwave (1,000W) and the living-area lighting (4 x 12W = 48W). Entered as the only three loads in the off-grid load calculator, they give a peak load of 1,798W, and the calculator sizes the inverter at 2,300W: it adds 25% to the peak and rounds up to the next 100W. Renogy's inverter sizing guide says to "add a margin of 20% to 25% power" once you have the total power of all appliances and the peak power of motor-based appliances. A well pump's motor also draws a starting surge above its running watts; no figure for it is printed here, so check the pump maker's starting figure against the inverter's surge rating.

Reference System Configurations

These rows run the off-grid load calculator at one daily load each, with every other input at the calculator's defaults: 5.18 peak sun hours (the PVWatts annual print for Kansas City, MO), 2 days of autonomy, a LiFePO4 bank at 80% depth of discharge and a 92% efficient inverter, at the system voltage each row names. Enter a row's daily load in the calculator's appliance fields (Appliance 1 Watts and Appliance 1 Hours/Day, through Appliance 8): 1,500W for 1 hour for the first row, and 5,000W for 4 hours for the last, since each Watts field takes up to 5,000W. Enter your own site's sun hours and your own inputs for your build. The inverter is sized from the peak load, not the daily energy, so it has no column here: see the inverter paragraph above.

The jump from "small cabin" to "full-size home" is stark. Air conditioning, electric water heating, and multiple large appliances push daily loads toward the last row's 20,000 Wh, where the calculator's bank is 1,061Ah at 48V and its array 5,276W. Shifting heating, water heating, and cooking to propane removes the three largest electrical loads and brings the system into a manageable size and budget.

Seasonal Load Variations and How to Plan for Them

Off-grid energy demand swings dramatically between seasons, and it swings in the wrong direction. Winter brings higher loads (more lighting hours, circulation pumps, electric blankets) at the exact time solar production drops. Summer brings air conditioning or fan loads at the exact time batteries run hottest and lose some capacity. A system sized for a mild October day will fail in January and again in August.

Here is a seasonal load profile for a cabin like the Vermont one in the worked example above. The lighting row is the worked example's 68W of fixtures (4 x 12W + 2 x 10W) at 4 and 8 hours; the other rows' figures are assumptions for illustration, not measured figures; its totals therefore differ from the worked example's 3,802 Wh.

Winter loads run 20% higher than summer in this example, and this cabin does not use electric heating. Cabins with electric space heaters or heat tape on water lines see a far larger winter rise. At the same time, NASA POWER's Burlington, Vermont climatology prints 4.88 kWh per square meter a day in June and 1.90 in December on a panel tilted at the latitude: December's sun is 1.90 / 4.88 = 39% of June's while demand climbs.

The practical answer is to size for the hardest month. Calculate your December daily load, then divide by December peak sun hours with system losses. That gives you a solar array big enough to survive winter without running a generator every night. In summer, the extra production keeps batteries full by midday and extends their lifespan through shallow cycling.

If your winter loads include a seasonal device (a circulation pump that only runs November through March, or a stock tank heater for livestock), build two separate load sheets. Label them "Summer baseline" and "Winter peak." The battery bank sizing calculator takes a daily energy usage, autonomy days, battery chemistry, system voltage, depth of discharge and inverter efficiency and returns the bank size in amp-hours: run it once for each sheet and compare the two banks.

One seasonal trap that catches first-time off-gridders: propane consumption rises in winter too. If you heat with propane and your propane supply runs low, the temptation is to plug in a space heater "just for a few hours." A 1,500W space heater running 6 hours eats 1,500 x 6 = 9,000 Wh, more than double the cabin's 3,802 Wh daily load before margin. Build enough propane storage or firewood supply to avoid falling back on electric resistance heating. Our propane generator runtime guide covers fuel planning in detail.

Common Mistakes That Blow Up Off-Grid Budgets

Forgetting phantom loads. A Wi-Fi router, modem, smoke detectors, and clock displays can draw 30-80W combined (an example range, not a measured figure), 24 hours a day. That is 30 x 24 = 720 to 80 x 24 = 1,920 Wh/day, 19-50% of the worked example's 3,802 Wh. Audit every plug and hardwired device.

Using nameplate watts instead of measured watts. A label that says "200W" is the peak draw, not the average. A fridge cycles its compressor, a laptop adjusts CPU power, a ceiling fan has speed settings. Measure actual draw with a Kill-A-Watt meter for at least 24 hours to get the real average. Our amps draw calculator takes a load's power in watts, the voltage, single or three phase and the power factor, and returns the current draw in amps; for a cycling appliance, enter the measured average watts.

Ignoring seasonal changes. The cabin in the seasonal table above needs 3,322 Wh/day in summer and 3,994 Wh/day in winter, from longer lighting hours, the wood stove circulation pump and the electric blanket, while the winter sun gives less. If you size the system for summer and move in during January, you will be running a generator every night.

Underestimating the well pump. The worked example's 1/2 HP submersible well pump draws 750W (an assumption for illustration, not a measured figure) and runs every time someone turns on a faucet, flushes a toilet, or waters the garden. If multiple fixtures run simultaneously, the pump cycles more frequently. At 1.5 hours a day it uses 750 x 1.5 = 1,125 Wh, more with irrigation. Pressure tank size affects cycling frequency: a larger tank means fewer pump starts and less total runtime.

The most reliable off-grid systems are designed by people who measured their loads honestly, added margin, and sized for winter. The solar panel and battery sizing calculator sizes a panel array and a battery bank from a daily usage; its output is only as accurate as the appliance list and usage estimates behind that daily usage.