Off-Grid Load Calculator

Each row is one maker's printed figure for one model, not an average for that kind of appliance. For everything else on your list (lights, laptops, chargers, pumps, washers, fans, freezers), read the watts on the appliance's nameplate or measure the draw, and enter that figure above.

Compressor fridges and freezers cycle on and off, so their average draw sits below the draw while the compressor runs. Dometic prints the CFX3 35's average as amp-hours per hour at 32 C ambient, which is why the fridge row above is an average. Use a Kill-A-Watt meter over a full day to measure an AC appliance's actual consumption for the most accurate load audit.

1. List every electrical device. Walk through each room and write down everything that plugs in or runs on batteries you plan to charge. Do not forget phantom loads (devices that draw power while "off": TV standby, phone chargers left plugged in, clocks on microwaves). A 10W standby load left on all day is 10 x 24 = 240Wh/day.
2. Measure actual wattage. A nameplate gives a rating, and an appliance can draw less than that in normal use. A plug-in meter such as the Kill-A-Watt, placed between the appliance and the wall, gives you the true draw in real time, plus cumulative kWh over 24 hours. It shows what each appliance actually uses, so you size the system to measured loads.
3. Multiply watts by hours. For each appliance: actual watts × daily hours of use = daily Wh. For example, 60W for 8 hours is 60 x 8 = 480Wh, and 100W for 4 hours is 100 x 4 = 400Wh. Be honest about usage: underestimated hours shrink the whole system.
4. Allow for losses and margin. An inverter loses part of the energy it converts. Victron prints one maximum efficiency for each model and battery voltage of its 1200VA to 5000VA inverters, from 92 to 95% across the models, and Renogy prints "over 90% efficiency" for its 2000W inverter. Wiring resistance and battery charge and discharge losses take a further share. For a budget with no separate inverter efficiency figure, Victron's RV solar sizing article says to "add about 10-15% extra for any 230V appliances" to cover inverter losses. Do not add it here: this calculator takes your inverter efficiency as an input, and that input covers the inverter losses. A margin on top also covers appliances you forgot and the occasional high-use day.
5. Total everything. Sum all appliance Wh values plus your loss margin, leaving out the inverter share when a calculator takes your inverter efficiency, as this one does. This is your daily energy budget: the single number that drives your entire solar panel and battery sizing decision.

Your daily Wh total determines everything downstream: battery bank capacity, solar array size, charge controller rating, and inverter capacity. It also steers the system voltage. Current is power divided by voltage (I = P / V), so the same load draws half the current at 24V and a quarter of it at 48V. Lower current means thinner cables and less voltage drop, which is why larger systems move up to 24V or 48V. Rather than a Wh-per-day rule, compare the currents your own loads draw at each voltage.

The single best way to reduce your off-grid system cost is to reduce your loads before sizing the system. Swapping an old full-size fridge for an efficient DC compressor fridge, replacing incandescent bulbs with LEDs and using a laptop instead of a desktop PC each cut the daily total; measure the old and the new appliance to see by how much. A lower daily total lowers the bank and array sizes this calculator returns. Once your system is sized, the solar ROI calculator shows the payback period and 25-year savings.

Think about peak loads too, not just daily energy. A 1,500W microwave draws 1,500 / 12 = 125A from a 12V battery bank, 1,500 / 24 = 62.5A at 24V and 1,500 / 48 = 31.25A at 48V, before the inverter's own loss adds to it. The battery size for inverter calculator helps you ensure the bank has enough capacity to deliver that peak current without excessive voltage sag. A 24V or 48V system halves or quarters that current and allows thinner, cheaper cables.