Off-Grid Load Calculator

Every off-grid system starts here. List what you run, how long you run it and whether it is AC or DC, and this returns the watt-hours your battery bank actually has to deliver each day, the amp-hours that comes to at your system voltage, and the peak load your inverter has to survive. AC loads are charged the inverter's conversion loss; DC loads are not.

Appliance Watts Hours/day Qty AC/DC Remove

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How It Works

The formula

Each row contributes watts times quantity times hours. The two totals are then treated differently, because an AC load has to come through the inverter and a DC load does not:

Row Wh = Watts × Quantity × Hours per day
Bank Wh/day = (AC Wh ÷ Inverter eff. + DC Wh) ÷ Wiring eff.
Ah/day = Bank Wh/day ÷ System voltage

Each row on its own is just watts times hours, which the watt hour calculator does for a single appliance in Wh, kWh and amp-hours. The answer here is watt-hours at the battery, not watt-hours at the socket. That distinction is where most self-built systems lose their margin: adding up appliance nameplates gives a number 8% to 15% below what the bank actually has to hold, and the shortfall only shows up in the second week of poor weather.

Worked example: a campervan

The loads the calculator opens with, on a 12 V system, a 90% inverter and 97% wiring:

Load W × qty h/day Type Wh/day
LED lights154DC60
Compressor fridge4510DC450
Water pump600.5DC30
Roof fan256DC150
Laptop604AC240
Phone chargers ×2203AC60
  • DC total = 60 + 450 + 30 + 150 = 690 Wh
  • AC total = 240 + 60 = 300 Wh, which costs the bank 300 ÷ 0.90 = 333 Wh
  • Drawn from the bank = (333 + 690) ÷ 0.97 = 1,055 Wh, or 1.05 kWh a day
  • At 12 V that is 88 Ah a day
  • Peak if everything ran at once = 145 W DC + 80 W AC = 225 W

The appliances themselves only asked for 990 Wh. The extra 65 Wh is the conversion and wiring overhead, and it is real energy the array has to replace every day.

What to do with the answer

The daily kWh figure feeds the battery bank size calculator and the solar system size calculator, in that order: the bank carries you through the nights and the bad days, and the array replaces what the bank lost. The peak watts figure feeds the inverter sizing calculator, which is a completely separate question and is answered by the largest instantaneous draw rather than by the daily total.

Duty Cycle: the Number People Get Wrong

Very few appliances draw their rated power for the hours they are switched on. Anything with a thermostat cycles, and the hours column has to carry the running time rather than the connected time. A fridge plugged in for 24 hours might only run its compressor for 10. Entering 24 in the hours column for a 45 W fridge overstates its consumption by more than a whole kWh a day, which is often larger than the entire rest of the van.

Appliance Rated draw Hours to enter Why
12V compressor fridge 40 to 60 W 8 to 12 Compressor cycles; more in hot weather, less in winter
Chest freezer 80 to 120 W 6 to 10 Better insulated than a fridge, so a lower duty cycle
Air conditioner 700 to 1,500 W 4 to 8 Cycles against the thermostat, not continuous
Water pump 50 to 90 W 0.2 to 1 Runs only while a tap is open or pressure drops
Electric kettle 1,500 to 2,200 W 0.1 to 0.25 Huge draw, tiny duration; matters for the inverter, not the bank
Laptop 30 to 90 W Actual use Charger rating is a maximum, not a draw; measure if it matters
Starlink / router 40 to 75 W Hours powered Genuinely continuous, and usually the second-biggest load
Diesel heater 10 to 40 W running Hours of use Glow plug draws 100 W plus, but only for two minutes per start

If a single load is more than about a quarter of your total, measure it rather than estimating it. A plug-in energy meter for an AC load, or a shunt-based battery monitor for a DC one, will settle in a week what a spreadsheet argues about for months.

Frequently Asked Questions

How do I calculate my off-grid power needs?

List every appliance with its running wattage, how many hours a day it actually runs, and how many of them you have. Multiply those three together for each one and add the results up to get watt-hours per day. Then divide the AC portion by your inverter efficiency, because AC loads cost the battery more than their nameplate, and divide the whole thing by a wiring efficiency of about 0.97. What comes out is the energy your bank has to deliver daily, which is the number every other off-grid calculation depends on.

Why are AC and DC loads counted differently?

A DC load runs straight off the battery at battery voltage, so a 45 W DC fridge takes 45 W from the bank. An AC load has to pass through the inverter, which converts DC to AC at roughly 85 to 93% efficiency, so a 45 W AC appliance takes about 50 W from the bank. Over a full day that gap adds up, and on systems that run most loads on AC it is worth 10% of the whole bank. Wiring anything that comes in a 12 V version, particularly fridges, lights and fans, directly to DC removes that loss entirely.

Should I use the nameplate wattage on the appliance?

Only as a starting point. Nameplate ratings are maximum draw under worst-case conditions, and many appliances never reach them: a 90 W laptop charger might average 35 W, and anything thermostatically controlled draws its rated power only while the compressor or element is on. Use the rated figure in the watts column but put the real running time in the hours column, and measure anything that dominates the total.

What margin should I add to the total?

Between 10 and 20% on top of a carefully itemised list, and more if the list was quick. The commonest reasons real consumption exceeds the estimate are loads nobody wrote down, phantom draws from anything left plugged in, and hotter weather making the fridge work harder. Adding margin here is far cheaper than adding it later, because both the bank and the array scale off this one number.

Does the peak load figure size my inverter?

It is the starting point, but it assumes everything runs at once, which is usually pessimistic in a house and roughly right in a van. Judge which loads genuinely coincide, then add headroom and check the surge requirement separately, because motors and compressors draw two to seven times their running current for the first second. The inverter sizing calculator handles both parts.

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