way is to install a kWh meter on the heater's power line and leave it running for as long as possible. Thankfully, such meters are often available for loan from libraries and utility companies, so there's little cost except for the installation (which may require an electrician if the heater is directly wired).

There's one more thing we need to cover here: new builds. If you're planning an installation for a new construction, you won't have any fuel bills. In this case, your best bet is to contact an experienced professional – preferably someone who does heat-load analysis for homeowners – and get some rough figures upon which you can base estimates. These estimates are likely to be expressed in Btu per heating degree day per square foot.