Watts = Volts X amps

You need to be a little careful – sizing a generator is not quite as simple as calculating the power requirement of a load. For example, in the case of a refrigerator (or furnace), you need to understand what that current rating is. In most instances, it is the maximum sustained current with the device functioning. But refrigerators and furnaces both contain motors, and motors have to start before they run. Typically, a motor will draw 6-8 times it’s normal full load current for the time required for the motor to accelerate from standstill to its full operating speed.

If you size the generator for the steady state load only, you may find that it will stall when you try to start a motor. A rule of thumb is that the generator rating should be 3-4 times the rating of a motor in order to assure that the motor will actually start on an isolated generator.

Also, it sounds like you are thinking about one generator for your refrigerator, and one for your furnace. With all respect, that’s dumb. You would be better off with one larger generator that is sized to handle the full load of both. For example, the loads you have identified are:
refrigerator – 6.5 amps, or 715 watts
furnace – 10.8 amps, or 1188 watts

If you purchase a single 5000 watt (5kW) generator, it can handle the full load of both (plus a few lights, a TV, computer and cell phone charger), plus it can handle starting of either the compressor motor in the refrigerator or blower motor in the furnace (it is unlikely that both will want to start at the same time).

By the way, I trust you understand that you cannot operate the generator inside the house? Seriously – that’s a very important safety consideration.

The formula to get power is P=I^2*R where I is current and R is the resistance. Another formula is P=V^2/R where V is the voltage and R is the resistance. Hope this helps.

Formula is p=I*I * R (W)
P= (V*V) /R
Then find V

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A 1000 Watt Generator can support a total of 110 volts at 9 amps. The formula is Watts = Volts x Amps. If I were you, I would just purchase one larger generator around 5000 watts. Another idea is to purchase a small bank of solar panels and batteries. Use the panels to charge the batteries for when the power goes out.