DRAFT
* Watts - a unit of power (in the moment). Usually printed/embossed on the appliance. 100w, for example. * Watt-hours (Wh)- energy (power over a given amount of time). A 100w load running for 30 minutes consumes 50Wh (100w x 0.5 hours = 50 Watt-hours) * current - typically measured in Amps1), this will dictate the size of wires and fuses. Amps = Watts / volts.2) * inverter losses - the inefficiency (heat) involved in converting 12vdc to 120vac, for example. Unless you know otherwise from published specs, assume your inverter losses are 20%. This means a 100w AC load will actually demand 120w from the system after losses.
We will use an average bank voltage of 13.0v for these examples, and inverter losses of 20%. A special attempt will be made to explain the power demands vs energy demands of each appliance.
We will see that for small appliances battery capacity is more important. For large appliances battery throughput is critical.
A 2qt crock is typically 90w on HI, so a max of 108w (8.3A demanded from the 12v system) It is a simple resistance heater that runs well off an inexpensive modified sine wave inverter. In terms of power (wattage) it will run off the smallest common lead and lithium batteries (100Ah) or a 270Wh power station.3)
During a three-hour run it will 324Wh of energy (3 hours x 108w). This would require a 648Wh (54Ah) lead battery capacity4) or a 405h lithium power station5). Note: if solar is contributing the size of the battery bank can be smaller.
The inverter can be a cheap MSW 100w.
An induction cooktop is typically 1500w on HI, so a max of 1800w (138A) demanded from the 12v system) It requires a more expensive pure sine wave inverter. In terms of power (wattage) it will take ~400Ah of AGM or 4,454Wh (348Ah) of lithium to run it.
During a 20-minute run it will use 900Wh of energy (.3 hours x 1800w). This would require a 1800Wh (150Ah) lead battery6) capacity or a 1,125Wh (88Ah) of lithium. Note: if solar is contributing the size of the battery bank can be smaller.
The inverter will be an expensive 2000w pure sine wave inverter.
| Device | Rated W | Inverter cost | lithium battery cost | total system cost |
|---|---|---|---|---|
| crockpot | 90w | $50 | $200 | $250 |
| induction cooktop | 1500w | $250 | $1,250 | $1,500 |
In between those extremes would be rice cookers (~300w constant) and the 3qt Instant Pots (~700w intermittent after pressurization).
If one already has a high-power system for other reasons it makes more sense to cook with big appliances. It may make less sense to build a high-power system to run big cooking appliances.